Tài liệu Báo cáo khoa học: 1,5-Diamino-2-pentyne is both a substrate and inactivator of plant copper amine oxidases ppt

1,5-Diamino-2-pentyne is both a substrate and inactivator of plant
copper amine oxidases
Zbyne
ˇ
k Lamplot
1
, Marek S
ˇ
ebela
1
, Michal Malon
ˇ
2
, Rene
´
Lenobel
2
, Karel Lemr
3
, Jan Havlis
ˇ
4
, Pavel Pec
ˇ
1
,
Chunhua Qiao
5
and Lawrence M. Sayre
5
1

Department of Biochemistry,
2
Laboratory of Growth Regulators, and
3
Department of Analytical Chemistry, Faculty of Science,
Palacky
´
University, Olomouc, Czech Republic;
4
Department of Analytical Chemistry, Faculty of Science, Masaryk University, Brno,
Czech Republic;
5
Department of Chemistry, Case Western Reserve University, Cleveland, OH, USA
1,5-Diamino-2-pentyne (DAPY) was found to be a weak
substrate of grass pea (Lathyrus sativus, GPAO) and sainfoin
(Onobrychis viciifolia, OVAO) amine oxidases. Prolonged
incubations, however, resulted in irreversible inhibition of
both e nzymes. For GPAO and OVAO, rates of inactivation
of 0.1–0.3 min
)1
were determined, the apparent K
I
values
(half-maximal inactivation) were of the order of 10
)5
M
.
DAPY was found to be a mech anism-based inhibitor of the
enzymes because the substrate cadaverine significantly pre-
vented irreversible inhibition. The N

1
-methyl and N
5
-methyl
analogsofDAPYweretestedwithGPAOandwereweaker
inactivators (especially the N
5
-methyl) than DAPY. Pro-
longed incubations of GPAO or OVAO with DAPY resul-
ted i n the appear ance of a yellow–brown c hromophore
(k
max
¼ 310–325 nm depending on the w orking buffer) .
Excitation at 310 nm was associated with emitted fluores-
cence with a maximum at 445 nm, suggestive of extended
conjugation. After dialysis, the color intensity was substan-
tially decreased, indicating the f ormation of a low molecular
mass secondary product of turnover. The compound pro-
vided positive reactions with ninhydrin, 2-aminobenzalde-
hyde and Kovacs’ reagents, suggesting the presence of an
amino group and a nitrogen-con taining heterocyclic struc-
ture. The secondary product was separated chromato-
graphically and was found not to irreversibly inhibit GPAO.
MS indicated an exact molecular mass (177.14 Da) and
molecular formula (C
10
H
15
N
3

). Electrospray ionization- and
MALDI-MS/MS analyses yielded fragment mass patterns
consistent with the structure of a dihydropyridine derivative
of DAPY. Finally, N-(2,3-dihydropyridinyl)-1,5-diamino-2-
pentyne was identified b y means of
1
H- and
13
C-NMR
experiments. This structure suggests a lysine modification
chemistry that could be responsible for the observed inacti-
vation.
Keywords: a mine oxidase; diamine; mechanism-based inhi-
bition; nuclear magnetic resonance; oxidation.
Copper-containing amine oxidases (CAOs, EC 1.4.3.6) play
a crucial role in the metabolism of primary amines. These
enzymes a re widely distributed in natu re [1]. In micro-
organisms, CAOs have a nutritional role in the utilization of
primary amines as the sole nitrogen and carbon source. In
mammals and plants, CAOs appear to be tissue specific, and
are implicated in wound healing, detoxification, cell growth,
signaling and apoptosis [1]. The oxidative deamination of
amine substrates catalyzed by CAOs yields the correspond-
ing aldehydes with the concomitant production of hydrogen
peroxide and ammonia [2].
The reaction proceeds through a transamination mech-
anism mediated by a n active site cofactor topaquinone. The
cofactor is derived from the po st-translational self-process-
ing of a specific tyrosine residue that requires both active site
copper and molecular o xygen [2]. T he k ey step i n the

oxidative deamination is conversion of the initial substrate
Schiff base (quinoimine) to a product Schiff base (quino-
aldimine) facilitated by Ca proton abs traction via a
conserved aspartate residue acting as a general base at the
active site [3]. This step is followed by hydrolytic release of
the aldehyde product and the reduced cofactor is finally
reoxidized by molecular oxygen with the release of H
2
O
2
and NH
4
+
. The reduced topaquinone exists in two forms.
The first is an aminoresorcinol derivative coexisting with
Cu(II), which is in equilibrium with the second form, Cu(I)-
semiquinolamine radical [3]. The r ole o f copper i n the
reoxidation step h as not been sufficiently elucidated for
Correspondence to M. S
ˇ
ebela, Department of Biochemistry, Faculty of
Science, Palacky´ University, S
ˇ
lechtitelu˚ 11, CZ-783 71 Olomouc,
Czech Republic. Fax: + 420 5856 34933; Tel.: + 420 5856 34927;
E-mail:
Abbreviations: ABA, 2-aminobenzaldehyde; ACA, 6-aminocaproic
acid; BEA, 2-bromoethylamine; CAO, copper-containing amine
oxidase; DABY, 1,4-diamino-2-butyne; DAPY, 1,5-diamino-2-pen-
tyne; DDD, 3,5-diacetyl-2,6-dimethyl-1,4-dihydropyridine; DMAB,

4-(dimethylamino)benzaldehyde; DMAC, 4-(dimethylamino)cinna-
maldehyde; ESI, electrospray ionization; GPAO, grass pea (Lathyrus
sativus) amine oxidase; HABA, 2-(4-hydroxyphenylazo)benzoic acid;
IT, ion trap; LSAO, lentil (Lens esculenta) amine oxidase; MALDI,
matrix-assisted laser desorption/ionization; OVAO, sainfoin
(Onobrychis viciifolia) amine oxidase; PSAO, pea (Pisum sativum)
seedling amine oxidase; PSD, post source decay; Q, quadrupole;
TNBS, 2,4,6-trinitrobenzenesulfonic acid.
Enzyme: copper-containing amine oxidase (EC 1.4.3.6).
Note: A website is available at p
(Received 24 August 2004, revised 23 September 2004,
accepted 13 October 2004)
Eur. J. Biochem. 271, 4696–4708 (2004) Ó FEBS 2004 doi:10.1111/j.1432-1033.2004.04434.x
most of the CAOs. For the amine oxidase from Hansenula
polymorpha, it seems likely that cofactor reoxidation
involves electron transfer from substrate-reduced topa-
quinone to oxygen that is bound at a site separate from
copper [2].
Plant CAOs prefer diamine substrates like putrescine and
cadaverine, hence they are also called diamine oxidases [4].
Inhibitors of these enzymes have recently been reviewed [5].
Among them, a special place is reserved for mechanism-
based inhibitors, which undergo turnover-dependent con-
version to electrophilic products capable of covalent binding
to an active-site nucleophile resulting in inactivation. Two
reported strategies in designing such inhibitors are the
incorporation of either halogen or unsaturation at the
b-position of amine substrates, examples being 2-bromo-
ethylamine (BEA) [6] and 1,4-diamino-2-butyne (DABY)
[7], respectively. Namely, the inactivating effect of DABY

(as a putrescine analog) on plant CAOs has been studied in
detail at the molecular level [8–10]. Various b-unsaturated
compounds were tested in the reaction with bovine plasma
CAO [10–13]. Propargylic and chloroallylic diamines were
highly potent inhibitors of the enzyme, more so than simple
allylic diamines [11–13]. A recent study showed that the
homopropargyl amine, 1-amino-3-butyne, is also a potent
inactivator of certain CAOs [14]. For this reason, and
because it is an analog of cadaverine (pentane-1,5-diamine),
the best known substrate o f plant CAOs [4], it seemed
important to determine the potential inactivating properties
of the higher DABY homolog, 1,5-diamino-2-pentyne
(DAPY). The unsymmetrical DAPY comprises both a
propargyl and homopropargyl amine.
DAPY was synthesized and tested as a substrate of two
plant CAOs. DAPY acts as a mechanism-based inhibitor of
the enzymes, causing their modification with the concom-
itant inactivation. However, in comparison with the effect of
DABY previously published [8], the modification extent is
considerably decreased. Only a few amino acid side c hains
seem to be modified as a result of the reaction. A major part
of DAPY oxidation product, aminopentynal, after the
conjugate addition of an unreacted DAPY molecule, is
converted to a free nitrogenous heterocyclic compound,
whose dihydropyridine-derived structure was determined
using various analytical methods.
Materials and methods
Chemicals
The previously unreported DAPY dihydrochloride was
synthesized from the known 1,5-dichloro-2-pentyne [15] by

displacement of the activated propargylic chloride with
methanolic ammonia in a pressure bottle [11], tert-butoxy-
carbonyl protection of t he introduced primary amine group,
displacement of the less reactive homopropargylic chloride
with methanolic ammonia in a pressure bottle (accompan-
ied by an elimination side reaction), and finally HCl-
mediated deprotection. Elemental analysis showed 33.38%
C, 7.41% H and 14.01% N (calculated 33.01, 7.08 ,
and 13.18 %, respectively). Melting point: 177– 179 °C;
13
C-NMR spectrum (deuterium oxide): d 17.0, 29.4, 38.0,
74.3 and 82.8 p.p.m.; electrospray ionization ion trap mass
spectrometry (ESI-IT-MS) and MS/MS: a single quasimo-
lecular peak of m/z 99.1 providing fragment peaks of m/z
82.0 and 70.0. N
5
-Methyl-1,5-diamino-2-pentyne dihydro-
chloride was prepared as for DAPY, substituting metha-
nolic methylamine in the penultimate step. N
1
-Methyl-1,5-
diamino-2-pentyne dihydrochloride was prepared by reac-
tion of 1,5-dichloro-2-pentyne with aqueous methylamine,
and then with methanolic ammonia. Synthetic details and
characterization of these analogs are given elsewhere [16].
3,5-Diacetyl-2,6-dimethyl-1,4-dihydropyridine (DDD)
was prepared using the Hantzsch synthesis [17]. 2-Amino-
benzaldehyde (ABA), bicinchoninic acid solution (Cat. No.
B9643), 4-(dimethylamino)benzaldehyde (DMAB), 4-(di-
methylamino)cinnamaldehyde (DMAC), 3-hydroxypyri-

dine, pyrrole and 2,4,6-trinitrobenzenesulfonic acid
(TNBS) solution (5%, w/v) were from Sigma (St. Louis,
MO, USA). Deuterium oxide (D
2
O, 99.96%) and d
4
-
methanol (CD
3
OD, 99.95%) were from Aldrich (Milwau-
kee, WI, USA). 6-Aminocaproic acid (ACA) and NADH
were supplied by Fluka (Buchs, Switzerland). 2-(4-Hy-
droxyphenylazo)benzoic acid (HABA) was from Bruker
Daltonik GmbH (Bremen, Germany). All other chemicals
were of analytical purity grade.
Enzymes
Plant diamine oxidases from grass pea (Lathyrus sativus,
GPAO) and sainfoin (Onobrychis viciifolia,OVAO)seed-
lings were prepared in homogeneous forms following
published protocols [18,19]. Specific activities assayed with
cadaverine as a substrate were 50 and 120 UÆmg
)1
, respect-
ively. Bovine liver catalase (2000 UÆmg
)1
) and horseradish
peroxidase (100 UÆmg
)1
) were commercial products from
Fluka. Protein content in enzyme samples was estimated

using a standard method with bicinchoninic acid [20].
Kinetic measurements
CAO assay was carried out following a previously published
protocol [9]. The guaiacol spectrophotometric method was
used, which is based on a coupled reaction of horseradish
peroxidase [21]. Kinetic parameters of tim e-dependent
inactivation of the enzymes by DAPY were evaluated
according to the literature on mechanism-based inhibition
[11,22]. Various 0.1
M
potassium phosphate buffersin the pH
range 5.0–8.0 were used in experiments performed to describe
the influence of pH on the inhibition potency of DAPY. To
assess the influence of ionic strength on the reaction, 0.1
M
Britton–Robinson buffer, pH 7.2, containing variable
potassium chloride was used. Rapid scanning of absorption
spectra of GPAO or OVAO mixed with DAPY under
admission of air was carried out by means of a DU-4500
spectrophotometer (Beckman, Fullerton, CA, USA) essen-
tially as described previously [9]. Aerobic scans at longer time
intervals (10–120 min) after mixing GPAO or OVAO with
DAPY (1 : 100) were carried out using a Lambda 11
spectrophotometer (Perkin–Elmer, Ueberlingen, Germany).
TLC of DAPY oxidation product
TLC of the GPAO-DAPY reaction mixture was carried out
using commercial TLC plastic sheets (4 · 8cm) with a
layer of Silica gel 60 F
254
(Merck, Darmstadt, Germany);

Ó FEBS 2004 DAPY inactivates plant amine oxidases (Eur. J. Biochem. 271) 4697
n-propanol/MeOH/saturated sodium acetate solution
(40 : 3 : 60 v/v/v) was used as a mobile phase. Primary,
secondary and tertiary amino groups were detected using
ninhydrin, sodium nitroprusside and Dragendorff’s rea-
gents, respectively. Aldehyde groups were detected using
Schiff’s reagent.
Spectrofluorimetry
AsolutionofDAPY(5m
M
)in20m
M
potassium phos-
phate buffer, pH 7.0, was oxidized by an excess of GPAO at
23 °C for 12 h. After that, the reaction mixture was filtered
using a centrifugal cartridge Microcon (Millipore, Bedford,
MA, USA), 0.5 mL, equipped with a 10 kDa cut-off filter.
The filtrate was used for spectrofluorimetry. Fluorescence
emission spectra of the DAPY oxidation product and
model compounds (DDD, 3-hydroxypyridine, NADH and
pyrrole) were obtained by means of an LS50B spectroflu-
orimeter (Perkin–Elmer, Boston, MA, USA). The oxidized
DAPY was measured with a fixed excitation at 310 n m.
Similarly, for the model compounds, the respective wave-
lengths of maximal absorption were taken as excitation
wavelengths.
Colorimetric trapping of DAPY oxidation product
For the various methods listed, absorption spectra were
recorded on Lambda 11 spectrophotometer against a blank
without DAPY. (a) Reaction with ABA [ 23]: DAPY

(2.5 m
M
) was oxidized by an excess of GPAO (500 nkat)
in 0.1
M
potassium phosphate buffer, pH 7.0, in the presence
of catalase (20 0 U) and 2.5 m
M
ABA. After 1 h of
incubation at 30 °C, 1 mL of 15% trichloroacetic acid was
added to the reaction mixture o f the total volume 5 mL; (b)
Reaction with ninhydrin [24]: an aliquot (1 mL) of GPAO/
DAPY mixture (5 l
M
GPAO, 4 m
M
DAPY; initial con-
centrations) in 0.1
M
potassium phosphate buffer, pH 7.0,
was taken out after 2 h of incubation at 23 °Candmixed
with the same volume of warm ninhydrin reagent [24]. This
was followed by the addition of acetic acid (1.5 mL). The
sample was kept in a boiling water bath for 30 min to
develop the color. It was then cooled and 2.5 mL of acetic
acid was added to make up the volume to 6 mL. (c) Reac-
tion with Kovacs’ reagent: an aliquot (1 mL) of GPAO/
DAPY mixture (2 l
M
GPAO, 0.2 m

M
DAPY; i nitial
concentrations) in 0.1
M
potassium phosphate buffer,
pH 7.0, was removed after 90 min of incubation at 23 °C
and mixed with 2 mL of the original Kovacs’ reagent
containing DMAB [7,8,25] (or its alternative contaning
DMAC [8]), incubated at 50 °C for 30 min and cooled on ice
bath. In an alternative experiment, the GPAO/DAPY
reaction mixture was first separated by ultrafiltration using
the Microc on c entrifugal cartridge as described above and
only the ultrafiltrate mixed with Kovacs’ reagent. Three
model compounds (DDD, pyrrole and NADH) were used to
compare spectral properties of their DMAB-adducts with
that of the DAPY oxidation product.
MS of DAPY reaction mixture
Samples for ESI-IT-MS were prepared by the oxidation of
5m
M
buffered DAPY solution with an excess of GPAO.
Two buffers were used to optimize results: 0.1
M
ammo-
nium bicarbonate, pH 7.8, and 0.1
M
Bistris/HCl, pH 7.0.
To evaluate the reactivity of the initial product aminoalde-
hyde, the reaction was also carried out in the presence of
5m

M
ACA as a trapping nucleophilic reagent. After
incubation at 30 °C for a sufficiently long time interval
(6–24 h), the reaction mixtures were separated by ultrafil-
tration using the Microcon centrifugal cartridge as given
above. The filtrate was properly diluted using methanol
before measurements. Mass spectra were obtained using an
iontrapmassspectrometerFinniganMATLCQ(Thermo
Electron Corp., San Jose, CA, USA) equipped with an ESI
interface. All samples were directly introduced to the
electrospray interface of the mass spectrometer by a syringe
at a flow rate of 5 lLÆmin
)1
. The ionization mode used
produced positively charged quasimolecular ions [M+H]
+
.
Parameters of the electrospray were as follows: source
voltage 5.6 kV, sheath gas flow 20 units, cone voltage
33.43 V, capillary temperature 250 °C.
Enzymatic microscale production of DAPY oxidation
product
The larger quantity of DAPY oxidation product required
for further characterization was prepared by a cyclic flux
of D APY solution through a hydroxyapatite column
(1 · 10 cm) containing immobilized GPAO and catalase.
After GPAO (10 mkat) and catalase (10 mka t) were loaded
in 10 m
M
ammonium bicarbonate, pH 7.8, the column was

washed with the same buffer. Then 50 mL of 5 m
M
DAPY
in 10 m
M
ammonium bicarbonate was left to circulate
through the column at 21 °C using a peristaltic pump at a
flow rate of 1 mLÆmin
)1
for 24 h. After stopping the cyclic
flux, the column was additionally washed with 20 mL of
10 m
M
ammonium bicarbonate, and the eluate w as added
to the solution of oxidized DAPY. The combined solution
was then filtered using an ultrafiltration cell (100 mL)
equipped with a 10 kDa cut-off filter (Amicon, Danvers,
MA, USA). Water was removed on a rotary vacuum
evaporator Rotavapor R-200 (Bu
¨
chi, Switzerland) at 70 °C.
The remaining solid was extracted by methanol (2 · 1mL),
the extract transferred to a test tube and the solvent
spontaneously evaporated at 21 °C. Alternatively, an excess
of GP AO was added to 10 mL of 20 m
M
DAPY solution
in 20 m
M
potassium phosphate buffer, pH 7.0, and the

resulting mixture incubated at 37 °C for 24 h. During that
time, GPA O was added twice more at 4-h intervals. Afte r
ultrafiltration, the sample was processed as above.
RP-HPLC separation of DAPY oxidation product
The isolated DAPY oxidation product was dissolved in
0.3% (v/v) triethylamine acetate, pH 7.0. It was then
separated by RP-HPLC on a Supelcosil LC
18
column,
25 cm · 4.6 mm i.d., 5 lm particles (Supelco, Bellefonte,
PA, USA) connected to a Gold Nouveau 125 NM HPLC
system equipped with a diode array detector Model 168
operating at 200–600 nm (Beckman, Fullerton, CA, USA).
The buffers used were as follows: A, 0.3% (v/v) triethyl-
amine acetate, pH 7 .0; B, 0.3% (v/v) triethylamine acetate,
pH 7.0, containing 60% (v/v) acetonitrile. Separations at
a flow rate of 1 mLÆmin
)1
were run isocratically in the
4698 Z. Lamplot et al. (Eur. J. Biochem. 271) Ó FEBS 2004
beginning (10 min), then with an increasing linear gradient
from 0 t o 100% B for 20 min and isocratically at 100% B
for a n a dditional 2 0 min. This w as followed with a
decreasing linear gradie nt from 100 to 0% B in 8 min and
a short final isocratic step to give the total time 60 min.
Fractions showing highest absorption at 310 n m were
pooled, frozen and lyophilized. For MALDI-MS and
ESI-MS analyses, the obtained solids were extracted by
methanol; for
1

H-NMR experiments the extraction w as
performed using D
2
O.
MS of HPLC-separated DAPY oxidation product
MALDI-TOF-MS and MALDI-PSD-TOF-MS (PSD,
post source decay) were carried out using an Axima CFR
mass spectrometer (Kratos Analytical, Manchester, UK)
equipped with a nitrogen laser wavelength of 337 n m. Peak
power was 6.0 mW: positive mode with pulsed extraction
was used. MALDI probes were prepared by mixing 0.5 lL
of a sample diluted by acetonitrile with 0.5 lL of saturated
HABA in the same solvent. Acquired spectra were
processed by K ratos A xima CFR software
KOMPACT
v. 2.1.1. Exact m ass measurements to determine the
elemental composition of the DAPY oxidation product
were performed using ESI-Q-TOF-MS on a Q-Tof
micro
TM
mass spectrometer (Micromass, Manchester,
UK). The collision-induced dissociation was used to get
MS/MS data. All samples were directly introduced to the
electrospray interface of the instrument by a syringe at a flow
rate of 5 lLÆmin
)1
. The ionization mode used produced
positively charged quasimolecular ions [M+H]
+
. Parame-

ters of the electrospray were as follows: source voltage
2.5 kV, cone voltage 15 V, source temperature 80 °C,
desolvation temperature 120 °C. Acquired spectra were
processed by
MASSLYNX
v. 4 software (Micromass).
NMR spectroscopy of DAPY oxidation product
First, GPAO-catalyzed oxidation of DAPY was carried
out in 20 m
M
D
2
O-potassium phosphate buffer, pD 7.0,
similarly to previous work performed with a CAO and
agmatine [26]. DAPY.2HCl (3 mg) was dissolved in 0.5 mL
of D
2
O. Control
1
H- an d
13
C-NMR spectra were recorded
at 27 °C on a Bruker AVANCE 300 MHz NMR spectro-
meter (Bruker Analytik, Rheinstetten, Germany), using
tetramethylsilane as internal standard. After this measure-
ment, the DAPY solution was pipetted into a test tube
containing a GPAO sample lyophilized from 0.5 m L of a
homogeneous GPAO (16 mgÆmL
)1
)in20m

M
potassium
phosphate buffer, pH 7.0. The mixture was shaken well on a
vortex and incubated at 30 °C for 12 h. The enzyme protein
was then removed using the Microcon centrifugal cartridge
as described above, and t he filtrate was used for recording
1
H- and
13
C-NMR spectra.
1
H-NMR spectra in D
2
Owere
also measured with the DAPY oxidation product extracted
from a lyophilizate obtained after RP-HPLC separation of
the GPAO/DAPY reac tion mixture.
Another NMR experiment was carried out as follows:
DAPY (5 m
M
)in2mLof20m
M
potassium phosphate
buffer, pH 7.0, was mixed with an excess of GPAO (5 mg,
added as a concentrated solution in the same buffer) and the
mixture was incubated at 30 °C for 12 h. After that, the
same amount of GPAO was added again and the incubation
proceeded for an a dditional 12 h. Before
1
H- and

13
C-NMR
measurements, the resulting solution was ultrafiltered as
described above and the filtrate was lyophilized. The NMR
sample was prepared by extracting the lyophilizate with
0.5 mL of CD
3
OD.
Determination of free primary amino groups
Primary amino groups in GPAO were determined by
modification of the established TNBS method [27,28]. A
sample of the enzyme (0.1 mL of a buffered solution
containing 10–20 mgÆmL
)1
) was added to 0.9 mL of 4%
(w/v) sodium bicarbonate, pH 8.5, in a test tube and mixed
using a vortex. Later, 0.5 mL of 0.01% TNBS was added
with mixing and incubation at 40 °C in the dark for 1 h.
ACA (1 mgÆmL
)1
) was used as a standard to construct the
corresponding calibration curve (10–50 lg). After incuba-
tion, all samples were measured at 345 nm against a blank
containing water instead of the enzyme. To determine free
amino groups in DAPY-reacted GPAO (100 : 1; 2 h of
incubation at 30 °C), the reacted enzyme was exhaustively
dialyzed aga inst 20 m
M
potassium phosphate, p H 7.0,
before an aliquot was processed as given above.

Chromatofocusing, quinone staining
Chromatofocusing was p erformed on a Mono P HR 5/20
column (Amersham Biosciences) connected to a BioLogic
Duo Flow liquid chromatograph (Bio-Rad, Hercules, CA,
USA). Loading buffer: 25 m
M
Tris/HCl, pH 8 .2; elution
buffer: Polybuffer 96 (Amersham Biosciences, 3 mL) was
mixed with Polybuffer 74 (Amersham Biosciences, 7 m L),
diluted with water, adjusted to pH 5.0 with acetic acid and
then filled to a final volume of 100 mL. All samples were
dialyzed against the loading buffer b efore separation.
Redox-cycling quinone staining on nitrocellulose membrane
was carried out as described previously [29].
Results
Kinetic measurements
The oxidative conversion of DAPY was studied using two
plant CAOs after these enzymes had been isolated from
grass pea and sainfoin seedlings. Initial rates measured with
2.5 m
M
DAPY showed that the compound is a weak
substrate. For GPAO, the initial rate reached 5% of the
value measured for putrescine at the same concentration.
For OVAO, the initial rate with DAPY was 10% towards
that of cadaverine as the best substrate for this enzyme.
However, because OVAO prefers cadaverine to putrescine
by a factor 2.7 (and such a property is unique among plant
CAOs) [19], this value may be recalculated as 27% towards
that of putrescine.

Longer incubations of both studied enzymes with DAPY
led to a significant decrease in their catalytic activity toward
ÔnormalÕ substrates. The inhibition was time- and concen-
tration-dependent and irreversible, as the activity could not
be restored by dilution or dialysis. Pseudo-first-order
inhibition kinetics were observed at 30 °CwithDAPY
concentrations ranging from 5 to 40 l
M
; Fig. 1 shows
Ó FEBS 2004 DAPY inactivates plant amine oxidases (Eur. J. Biochem. 271) 4699
semilogarithmic plots for GPAO, where the slope for each
regression line represents the observed rate constant k
obs
.
The kinetic constants describing the inactivation of GPAO
and OVAO were determined from the corresponding Kitz–
Wilson replots (1/k
obs
vs. 1/[DAPY]; see inset in Fig. 1 as an
example). From these plots k
inact
, the maximal rate of
inactivation, is 1/y ) intercept, and K
I
, the concentration
required for half-maximal inactivation, is )1/x ) intercept.
The determined values for GPAO were similar when
measured in 0.1
M
potassium phosphate or Bistris/HCl

buffers, pH 7.0 (Table 1). Comparat ively, for OVAO,
inactivation by DAPY is slower, but the K
I
is lower.
GPAO and OVAO (both 70 n
M
in 0.1
M
potassium
phosphate buffer, pH 7.0) were each individually incubated
with seven different concentrations of DAPY varying from
1to50l
M
at 30 °C for 1 h. Remaining activity was
determined by the ratio of the measured activity of the
inactivated enzyme to the control enzyme incubated without
DAPY. A plot of the remaining activity (%) v s. [DAPY]/
[GPAO] or [DAPY]/[OVAO] was constructed. Extrapola-
tion of the linear portion of the data at lower [DAPY] gave
the partition ratio (turnover number minus one). This ratio,
the number of molecules leading to product per each
inactivation event, was determined to be 120 for DAPY/
GPAO (Fig. 2) and 200 for DAPY/OVAO.
The inhibition strength of DAPY is dependent on pH.
GPAO (70 n
M
) was incubated with 50 l
M
DAPY in 0.1
potassium phosphate buffers of different pH values over the

range 5.0–8.0 at 30 °C for 1 h. The obtained remaining
activity values were then plotted against pH. DAPY showed
a maximal inhibition effect at pH 7.5. The extent of GPAO
inhibition by DAPY is also influenced by ionic strength.
The reaction was performed in 0.1
M
Britton–Robinson
buffer, pH 7.2, where ionic strength had been adjusted with
KCl to r each values from the range 0 .085–0.4. The
percentage of remaining a ctivity after 1 h of incubation of
the reaction mixture (70 n
M
GPAO, 50 l
M
DAPY) at
30 °C increases with increasing ionic strength (not shown).
Enzymes are protected against mechanism-based inhi-
bitors by their substrates and competitive inhibitors. These
compounds bind at the active site and compete with binding
of the inhibitor. Inactivation of the enzyme is therefore
slowed down. GPAO (2 l
M
) was incubated with 100 l
M
DAPY in the absence and presence of 1 m
M
cadaverine as a
substrate. At chosen time intervals, aliquots of the reaction
mixtures were taken out for activity assay. The protective
effect of cadaverine was s ignificant. For example, after

15 min of incu bation, the remaining activity was 15% in the
reaction mixture with cadaverine and only 8% without.
Due to the potential information that might be provided
about the mechanism of enzyme inactivation by DAPY, we
also determined the kinetics of inactivation of GPAO b y the
two p ossible N-monomethyl analogs o f DAPY. As shown
in Table 1, N
1
-methyl-DAPY and especially N
5
-methyl-
DAPY were weaker inactivators relative to DAPY itself.
Spectrophotometry and spectrofluorimetry, TLC
Substrates of CAOs are known to disturb the characteristic
absorption spectrum of the enzymes [4]. Under anaero-
biosis, the topaquinone cofactor maximum at 500 nm is
bleached after the substrate addition and replaced by a
complex spectrum of the Cu(I)-semiquinolamine radical
showing maxima at 360, 435 and 465 nm. This is supple-
mented with a peak at 315 nm that is thought to reflect the
Fig.1. EffectofincubationtimeoninactivationofGPAObyDAPY.
The semilogarithmic plot was constructed for the following DAPY
concentration s: 5 (j), 10 (m), 20 (r)and40l
M
(d). Activity was
measured with 70 n
M
enzyme in 0.1
M
potassium phosphate bu ffer,

pH 7.0, at 30 °C by means of the guaiacol spectrophotometric method
[21]. The inset shows the corresponding Kitz–Wilso n replot for the
determin at io n of k
inact
and K
I
values.
Table 1. Inactivation kinetics. Experiments were performed at 30 °Cin
0.1
M
potassium phosphate buffer, pH 7.0, except where noted .
Inhibitor/enzyme
k
inact
(min
)1
)
K
I
(l
M
)
t
1/2
at saturation
a
(min)
DAPY with GPAO
b
0.27 45 2.2

DAPY with GPAO 0.31 50 1.9
DAPY with OVAO 0.13 10 4.5
N
1
-Methyl-DAPY
with GPAO
0.11 45 6.3
N
5
-Methyl-DAPY
with GPAO
0.05 36 13.9
a
Time required for half of the enzyme to become inactivated in the
presence of saturating concentration of inhibitor.
b
In 0.1
M
Bistris/
HCl buffer, pH 7.0.
Fig. 2. Partition ratio plot for inactivation of G PAO by DAPY.
Residual GPAO ac tivities after 1 h of incubation with D APY were
plotted against the corresponding values of the concentration ratio
[DAPY]/[GPAO]. Activity was measured with 70 n
M
enzyme in 0.1
M
potassium phosphate buffer, pH 7.0, at 30 °C using the guaiacol
spectrophotometric method [21].
4700 Z. Lamplot et al. (Eur. J. Biochem. 271) Ó FEBS 2004

presence of the aminoresorcinol form of topaquinone (the
fully reduced cofactor), which is in equilibrium. Using
rapid-scanning techniques, the mentioned spectral features
are observable also in the presence of air [9].
As shown in Fig. 3 (upper), rapid scanning after the
aerobic addition of DAPY to a purified GPAO in 0.1
M
potassium phosphate buffer, pH 7.0, revealed the formation
of a spectrum identical to that of a substrate-reduced CAO.
In addition, the reaction of the studied plant CAOs with
DAPY gave rise to an oxidation product providing near
UV/visual absorption with a maximum at 310–315 nm,
which increased in intensity with temperature. Absorbances
measured after 90 min of incubation of GPAO/DAPY
reaction mixtures at 50 °C were almost three times higher
than those measured after the incubation at 37 °C. Figure 3
(lower), shows an increasing development of the product
within the first 1 0 min after mixing OVAO with DAPY in
0.1
M
potassium phosphate buffer, pH 7.0. The s ame
spectrum was also observable in the GPAO/DAPY reaction
mixture. In 0.1
M
Bistris/HCl buffer, pH 7.0, the product
absorption maximum was shifted to 325 nm (not shown).
GPAO and OVAO were fully inactivated by incubation
with an excess of DAPY and no activity could be recovered
by dialysis. The inactivated enzymes after dialysis were still
faint yellow due to a broad absorption below 330 nm, but

the color intensity was substantially decreased.
After separation of the enzyme protein by ultrafiltration,
the GPAO/DAPY reaction mixture exhibited a fl uorescence
emission spectrum with a m aximum at 445 nm (shoulders at
485 and 520 nm) when excited at 310 nm. For model
compounds, the following fluorescence characteristics were
obtained: DDD, solvent water, emission maximum at
503 nm (excitation at 410 nm); 3-hydroxypyridine, solvent
water, emission maximum at 460 nm (excitation at
310 nm); NADH, solvent water, emission maximum at
465 nm (excitation at 340 nm); pyrrole, solvent water,
emission maximum at 360 nm (excitation at 290 nm).
TLC experiments revealed the presence of a free primary
amino group in the DAPY oxidation product obtained by
the reaction of GPAO (positive ninhydrin spot, R
f
¼ 0.62);
DAPY itself showed R
f
¼ 0.33 in the same system. Staining
for t ertiary a mines ( Draggendorff’s reagent) was a lso
positive for the GPAO/DAPY reaction mixture (an orange
spot, R
f
¼ 0.62). Staining for aldehydes using Schiff’s
reagent was negative.
Colorimetric detections of DAPY oxidation product
Kovacs’ reagent containing DMAB (detects indoles and
pyrroles [25]) was previously used for the visualization of
pyrrole derivatives formed by the oxidation of DABY by

plant amine oxidases [7,8]. Reaction mixtures of the studied
CAOs with DAPY reacted positively with Kovacs’ reagent
after a period of incubation and provided a red soluble
adduct upon heating. The red color intensity increased with
increasing temperature in the range 30–50 °C. A typical
absorption spectrum of the adduct with a maximum at
520 nm (shoulder at 500 nm) is shown in Fig. 4 (upper).
Fig. 3. Spectrophotometric studies on the
reactions of GPAO and OVAO with DAPY.
(Upper) Difference ab sorption spectrum of
GPAO (20 l
M
) after the addition of DAPY
(final concentration 1 m
M
) in air-saturated
0.1
M
potassium phosphate buffer, pH 7.0.
The spectrum was recorded 2 s after mixing
the reactants at 30 °C, using the rapid-scan-
ning technique [9]. (Lo wer) Time-dep endent
development of the DAPY oxidation product
as observed in difference absorption spe ctra.
The spectra were recorded using a solution of
OVAO (20 l
M
) in air-saturated 0.1
M
potas-

sium phosphate buffer, pH 7.0, after adding
0.1
M
DAPY (1 m
M
final concentratio n) at
30 °C. Intervals between scans: 15 s, total
time: 10 min.
Ó FEBS 2004 DAPY inactivates plant amine oxidases (Eur. J. Biochem. 271) 4701
Replacing DMAB in Kov acs’ reag ent with DMAC led to a
shift of the adduct absorption maximum to 650 nm (not
shown). However, if the reaction mixture was dialyzed
before the addition of the reagent, the spectrum was almost
negligible (Fig. 4, upper). Three model compounds were
tested for this reaction. The synthesized DDD reacted with
Kovacs’ reagent to form a product with an absorption
maximum at 600 nm having a shoulder at 560 nm (Fig. 4,
lower). NADH also reacted wi th the reagent and pro vided a
spectrum with a single peak centered at 510 nm. DMAB-
reacted pyrrole provided a maximum at 565 nm with a
shoulder at 520 nm.
The use of ABA for the spectrophotometric activity assay
of plant CAOs was refined almost four decades ago [23].
Plant CAOs oxidize the diamines putrescine and cadaverine
to form the corresponding aminoaldehydes, which sponta-
neously cyclize to 1-pyrroline and 1-piperideine, respect-
ively. The latter cyclic imines condense with ABA to
generate the corresponding substituted dihydroquinazolin-
ium compounds. The DAPY oxidation product obtained
by the reaction of GPAO provided an adduct with ABA

characterized by an absorption maximum at 430 nm (not
shown).
The ninhydrin reagent described for activity assay of
CAOs by Naik et al. [24] was also tested to trap the DAPY
oxidation product in the reaction mixture with GPAO. The
same cyclic imines above (1-pyrroline and 1-piperideine)
react with ninhydrin in strongly acidic medium to form
colored compounds of unknown structure with absorption
maxima at 440 and 515 nm, respectively [30]. The DAPY
oxidation product displayed a broad absorption between
400 and 550 nm with a maximum at 465 nm after reaction
with ninhydrin (not shown).
MS of DAPY reaction mixture
Figure 5 (upper) shows an ESI-IT mass spectrum of the
GPAO/DAPY reaction mixture prepared using 0 .1
M
ammonium bicarbonate, pH 7.8. There are two major
peaks of the reaction product observable in the spectrum
with m/z 178.3 and 222.3. The former ion showed fragment
peaks with m/z 161.3, 149.2 and 135.2, the latter provided
peaks with m/z 205.2, 193.2 and 176.2 in the respective
MS/MS spectra. The peak with m/z 222.3 was not observed
when the r eaction was carried out in 0.1
M
Bistris/HCl
buffer, pH 7.0. Figure 5 (lower) shows a mass spectrum of
the GPAO/DAPY reaction mixture prepared in 0.1
M
ammonium bicarbonate containing ACA as a reagent for
trapping of the product aminoaldehyde, where several new

peaks appeared. ACA itself is represented by a peak with
m/z 132.1 (MS/MS: a clear fragment peak with m/z 114.1.)
There is one more peak visible with m/z 211.3 (MS/MS:
fragment peaks with m/z 193.3, 106.0 and 96.0), which
probably reflects an adduct of the reaction product with
ACA.
ESI-IT-MS of the low molecular mass fraction of the
GPAO/DAPY reaction mixture prepared in 20 m
M
potassium phosphate buffer, pD 7.0 (made in D
2
Ofor
the purpose o f NMR spectroscopic analysis) r evealed
isotopic peaks belonging to quasimolecular ions of the
reaction product. The highest intensity was observed for
apeakwithm/z 179.2, lower intensities were observed
for peaks in the following order: m/z 181.2, 180.2, 182.2
and 178.2. The peak with m/z 179.2 provided an MS/MS
spectrum showing fragments with m/z 162.2, 150.2 and
136.2 (not shown).
HPLC separation and MS analysis of DAPY oxidation
product
HPLC separation of the isolated DAPY oxidation product
from enzymatic microscale production was carried out
using a n instrumen t e quipped with a diode array detector.
Thus individual runs could be monitored continuously at
214, 240 and 310 nm. The buffer system used was chosen
according to that published for peptide separation from
tryptic digests [31].
Fig. 4. Reaction of the DAPY oxidation product and a dihydropyridine

model compound with 4-(dimethylamino)benzaldehyde. (Upper) An
aliquot (1 mL) of the GPAO/DAPY reaction mixture in 0.1
M
potassium phosphate buffer, pH 7.0, was mixed with 2 mL of Kovacs’
reagent, incubated at 50 °C for 30 min and finally cooled in an ice-
bath. The absorption spectrum was recorded against a blank con-
taining water instead of the reaction mixture; ( upper) reaction mixture
(lower) reaction mixture after removing protein by ultrafiltration. For
experimental details see Materials and methods. (Lower) A 1 mL
portion of 5 m
M
DDD was mixed with 2 mL of Kovacs’ reagent,
incubated at 50 °C for 30 min and finally cooled in an ice bath.
Absorption spectra were then recorded against a blank containing
water instead of the dihydropyridine.
4702 Z. Lamplot et al. (Eur. J. Biochem. 271) Ó FEBS 2004
Three peaks at elution times 3.0 min (1), 15.8 min (2) and
23.0 min (3) were collected and their composition analyzed
using ESI-MS and MALDI-MS. The largest peak absorb-
ing at 310 nm (peak 1) appeared to correspond to a single
chemical compound (m/z 178.1). The corresponding
ESI-IT-MS/MS spectrum is presented in Fig. 6, where
fragmentation peaks were observed with m/z 161.1, 149.1,
144.1, 135.1, 132.1, 120.1, 109.1, 95.0 and 82.0. After
lyophilization, a solid obtained from peak 1 did not produce
irreversible inhibition of the studied enzymes. There was one
more compound in peak 2 with m/z 257.1, whose MS/MS
spectrum provided peaks with m/z 240.1, 228.1, 214.3,
202.3, 176.2, 1 61.2, 149.2 and 133.0. Finally, peak 3
contained at least five compounds. In addition to those

with m/z 178.1 and 257.1 there were three more peaks with
m/z 334.3 (fragmentation: m/z 317.3, 305.2, 291.1, 253.2 and
240.1), 350.2 (fragmentation: m/z 333.2 and 307.1) and
431.3 (fragmentation: m/z 414.3, 388.3 and 337.1).
MALDI-TOF-MS of the separated peak 1 provided a
single compound with m/z 178.1; the same m/z value was
obtained by an ionization without using the HABA matrix.
MALDI-PSD-TOF-MS provided a f ragmentation pattern
consistent wit h th e ESI-IT- MS/MS experiments already
mentioned (data not shown).
ESI-Q-TOF-MS analysis of the HPLC peak 1 permitted
the determination of both exact m ass and elemental
composition of the DAPY oxidation product. An m/z
value of 178.14 was obtained, which matches a molecular
formula C
10
H
16
N
3
. Peaks in the corresponding MS/MS
spectrum provided the following m/z values and elemental
composition of ions: 161.11 (C
10
H
13
N
2
), 149.11 (C
9

H
13
N
2
),
135.09 (C
8
H
11
N
2
), 109.08 (C
6
H
9
N
2
), 95.06 (C
5
H
7
N
2
)and
82.06 (C
5
H
8
N).
Fig. 5. ESI-IT-MS analyses of GPAO/DAPY

reaction mixtures. (Upper) DAPY (5 m
M
)in
0.1
M
ammonium bicarbonate, pH 7.8, was
mixedwithanexcessofGPAOandincubated
at 30 °C for 24 h. After removing protein by
ultrafiltration, the reaction mixture was ana-
lyzedbyESI-IT-MSasdescribedinMaterials
and methods. (Lower) A combined solution of
DAPYandACA(each5m
M
)in0.1
M
ammonium bicarbonate, pH 7.8, was mixed
with an excess of GPAO and incubated at
30 °C for 24 h. After removing protein by
ultrafiltration, the reaction mixture was ana-
lyzedbyESI-IT-MSasdescribedinMaterials
and methods.
Fig. 6. MS/MS spectrum of DAPY oxidation product. The isolated
DAPY oxidation product from enzymatic microscale production was
dissolved in 0.3% (v/v) triethylamine acetate, pH 7.0, and separated by
RP-HPLC as described in Materials and methods. The 310 nm-peak
at an elution time 3.0 min was collected and analyzed by ESI-IT-MS
and MS/MS. The spectrum shown was recorded after collision-
induced fragmentation of the parent ion belonging to the DAPY
oxidation product (m/z 178.1).
Ó FEBS 2004 DAPY inactivates plant amine oxidases (Eur. J. Biochem. 271) 4703

NMR spectroscopy of DAPY oxidation product
For initial experiments, DAPY oxidation by GPAO was
performed in 20 m
M
potassium phosphate buffer made in
D
2
O (pD 7.0). The enzyme protein was removed by
ultrafiltration and the filtrate was directly measured. Several
signals with the following chemical shifts were observed in
the
13
C-NMR spectrum: d (p.p.m.) 17.0, 25.1, 29.1, 37.9,
62.5, 72.1, 74.3, 82.7, 123.0, 131.2 and 159.8. The corres-
ponding
1
H-NMR spectrum contained various signals in
the region 2.0–4.5 p.p.m., but these were largely obscured
by the residual water peak (4.5–5.0 p.p.m.). In a ddition, two
vinylic signals at d 5.7 and 7.8 were observed, but their
intensities were too small to ascertain their multiplicities
(not shown). Much better
1
H-NMR spectra were obtained
after the extraction of the ultrafiltered and lyophilized
GPAO/DAPY r eaction m ixture by CD
3
OD. Figure 7
shows such a spectrum, including the following signals: d
2.63–2.73 (m), 3.12 (q, J ¼ 6.95 Hz), 3.31 (p, J ¼ 1.65 Hz),

3.79 (t, J ¼ 2.20 Hz), 4.19 (t, J ¼ 2.20 Hz), 5.35 (d, J ¼
6.22 Hz), 7.82 (d , J ¼ 6.22 Hz). The spectrum is partially
obscured by two signals of residual methanol at d 3.3 and
4.6–5.1. There are also three complex signals that are quite
difficult to interpret, which are centered at d values of 2.80,
3.55 and 3.65.
13
C-NMR spectra measured with the GPAO/
DAPY mixture in CD
3
OD resembled those recorded in
D
2
O, but the obtained quality was lower.
1
H-NMR spectra
were also recorded in D
2
O using the solid obtained by
lyophilization of the peak 1 from the HPLC separation
mentioned above. However, NMR signal intensities were
insufficient due to the low concentration of compound. In
addition, these spectra were obscured by two peaks of
residual triethylamine from the elution buffer at d 1.2 and
3.2 (data not shown).
Other analyses
Lysine residues in plant CAOs are possible targets for
covalent binding of reactive electrophilic aminoaldehydes
formed by the turnover of acetylenic diamine substrates
[7–11]. The cDNA of GPAO subunit (without the signal

peptide) has been cloned and sequenced (D. Kopec
ˇ
ny´ ,
N. Houba-He
´
rin, H. G. Faulhammer & M. S
ˇ
ebela,
unpublished results; EMBL/GenBank accession number
AJ786401). T he translated protein sequence i s largely
similar to those two published for PSAO [32,33] and
comprises 38 lysines. GPAO and PSAO have very similar
peptide maps obtained by MALDI-MS experiments [34].
PSAO protein sequence of Koyanagi et al. [32], which is
deposited under accession number JC7251 (NCBI Protein
Databank) differs slightly from that of Tipping and
McPherson [33], accession number Q43077, in that whereas
the former sequence comprises 39 lysine residues, the latter
has only 38 lysine residues, as calculated for the mature
form of the protein. Although native PSAO (a dimer) is thus
expected to comprise 76–78 lysine residues, only 32 are
solvent-accessible [8]. We determined 38 accessible lysines
per dimer in the native GPAO using a modified protocol
with the TNBS reagent, and 36 lysines per dimer (average
values from repeated measurements) after reaction with the
DAPY.
Quinone-staining experiments [29] with the DAPY-inac-
tivated GPAO were positive and demonstrated that the
topaquinone cofactor was not modified in a redox-inactive
form (data not shown). Chromatofocusing experiments

performed according to that with DABY-inactivated GPAO
[8] revealed that the pI value of the DAPY-inactivated
GPAO was not dramatically changed. The native GPAO
is characterized by a pI of 7.2 [18]. After the reaction with
an excess of DAPY, the enzyme sample comprised more
species having isoelectric points of pI 6.8–7.5 (Fig. 8).
Therefore, the inactivation resulted in a heterogeneous
mixture of differently charged protein molecules.
Discussion
DAPY was synthesized as an analog of cadaverine
(pentane-1,5-diamine), which is known as the best substrate
Fig. 7.
1
H-NMR spectrum of DAPY oxidation product. DAPY (5 m
M
)in2mLof20m
M
potassium phosphate buffer, pH 7.0, was mixed with an
excess of GPAO (5 mg, added as a concentrated solution in the same buffer) and the mixture was incubated at 30 °C for 12 h. The same amount of
GPAO was added again and the incubation proceeded for an additional 12 h. The resulting solution was centrifuged to remove protein precipitate
and ultrafiltered, and the filtrate was lyophilized. The NMR sample was finally prepared by extracting the lyophilizate with 0.5 mL of CD
3
OD. The
insets shows a detailed view of the v inylic doubl et signals belonging to 2,3-dihydropyridine.
4704 Z. Lamplot et al. (Eur. J. Biochem. 271) Ó FEBS 2004
of plant CAOs [4]. Contrary to naturally occurring diam-
ines, the DAPY molecule contains a triple bond at the
b-andc-positions from the two primary amine termini. The
oxidative conversion of the compound by GPAO and
OVAO was demonstrated by measuring the production of

H
2
O
2
using spectrophotometry. Therefore, the enzymes are
able to undergo complete turnover [3]. However, DAPY
was oxidized more efficiently by OVAO than by GPAO.
Although, to date, OVAO has not been crystallized nor had
its structure solved, this observation might be explained in
terms of different arrangements of the active sites of the
enzymes resulting in the preference for C5-diamine sub-
strates by OVAO.
Similarly to the conversion of its lower homolog DABY
by PSAO [7], DAPY oxidation by the studied enzymes led
to their irreversible inhibition. The apparent inactivation
constants K
I
of 10
)5
M
are on the same order of magnitude
as those K
I
values previously described for BEA [6] and
DABY [7] in the reactions with lentil seedling amine oxidase
(LSAO) and PSAO, respectively. The obtained rates of
inactivation resembled for e xample that for BEA as
measured with LSAO [6], but they were lower than that
for DABY in the reaction with PSAO [7]. At the same time,
whereas the determined partition ratio values with GPAO

and OVAO were in the range observed for BEA (r ¼ 100)
and some other monohalogenated alkylamines (r < 500) in
the reactions with LSAO [6], they were significantly higher
than that for DABY and PSAO (r ¼ 17) [7]. From this
point of view, plant CAOs are more resistant to the
inactivation by DAPY than by DABY. Binding of DAPY
at the active site of GPAO is dependent on both pH and
ionic strength. In these features, the reaction does not differ
from those of typical plant CAO substrates like putrescine
or cadaverine. GPAO and OVAO inactivation caused by
the substrate DAPY fulfills the criteria of a mechanism-
based inhibition: it is time dependent, irreversible and can be
weakened in the presence of a normal substrate [5,7,11,22].
DAPY oxidations by GPAO and OVAO were accom-
panied by spectral changes. The typical absorption spec-
trum of a substrate-reduced CAO, which appeared after the
rapid addition of DAPY to either GPAO or OVAO
solutions, was in accordance with the substrate properties of
DAPY determined by the guaiacol spectrophotometric
assay. Presuming that DAPY oxidation follows the same
mechanism as for common substrates of plant CAOs, the
reaction should generate 5-amino-2-pentynal or 5-amino-3-
pentynal as a product aldehyde (DAPY is not a symmetric
molecule). Although no free aldehyde was detected by TLC
in the reaction mixture, indirect evidence for an amino-
pentynal turnover product was that an adduct formed
(m/z 211.3) when DAPY was enzymatically oxidized in the
presence of ACA. This adduct exhibited a MS/MS
fragmentation pattern similar to that of free ACA, showing
a loss of a water molecule from the carboxylic group ()18,

m/z 211.3 fi m/z 193.3).
There are detailed reports on the mechanism-based
inhibition of CAOs by DABY in the literature [7–11]. The
authors have shown that D ABY is oxidized to 4-amino-2-
butynal, which induces inactivation by adducting to a
nucleophile in the substrate c hann el. In addition, the
reaction brings about multiple surface labeling of the
enzyme, which probably occurs through solvent-accessible
nucleophilic residues [8]. DAPY oxidation appears to result
in much less extensive protein modification, as the number
of free primary amino groups in the enzyme did not change
dramatically. Chromatofocusing of DAPY-inactivated
GPAO revealed only a small change in the isoelectric point,
likely caused by the modification of a few amino acid
residues upon binding of aminopentynal. This binding
seems to be nonspecific, as the existence of some micro-
heterogeneity (at least two species with different pI values)
in the inactivated enzyme was confirmed. As in the case of
DABY, the cofactor topaquinone is not modified by the
reaction, as demonstrated by an unchanged quinone redox
staining.
Absorption spectroscopy demonstrated the formation of
a secondary product in the GPAO/DAPY reaction mixture
with k
max
at 310 nm and emitted fluorescence (k
max
at
445 nm) upon excitation at 310 nm, supportive of extended
conjugation. Dialysis of the reaction mixtures containing

GPAO or OVAO and DAPY, resulted in decoloration,
demonstrating that the chromophore generated is a free low
molecular mass compound. Several colorimetric assays
provided evidence for t he presence of a n itrogenous
heterocycle, in addition to a f ree amino group. The
GPAO/DAPY reaction mixture exhibited a positive reac-
tion with ABA and ninhydrin reagents, similar to that
observed for the cyclic imines 1-pyrroline and 1-piperideine
formed upon enzymative oxidation of putrescine and
cadaverine, respectively. In acidic medium, DMAB
and DMAC reacted with the DAPY oxidation product
(and also with the model compounds DDD and NADH) to
give markedly colored adducts. This probably occurs upon
binding of the r eagents at the a-position of the heterocycle
[8].
The rigidity of the triple bond in the presumed amino-
pentynal turnover product would prevent cyclization, but
this geometrical constraint would be relaxed by conjugate
addition of a nucleophile. Thus, as shown in Fig. 9, if a
molecule of unreacted DAPY is ad ded to either of the two
possible aminopentynal turnover products, cyclization to a
six-membered heterocycle and eventual formation of a
common resonance-stabilized 4-amino-2,3-dihydropyridine
would be predicted. The extended conjugation would be
consistent with the observed absorption and fluorescence
Fig. 8. Chromatofocusing of DAPY-inactivated GPAO. Chromatofo-
cusing was performed on a Mono P HR 5/20 column using a BioLogic
Duo Flow liquid chromatograph at a flow rate of 1 mLÆmin
)1
.The

loading buffer was 25 m
M
Tris/HCl, pH 8.2, and the elution buffer was
a diluted mixture of Polybuffer 96 and Polybuffer 74 adjusted to
pH 5.0 with acetic acid. All samples were dialyzed against the loading
buffer before separation. Approximate ly 5 mg of protein was loaded.
Ó FEBS 2004 DAPY inactivates plant amine oxidases (Eur. J. Biochem. 271) 4705
spectral properties. The final structure is also consistent with
MS and NMR data. In particular, the two coupled vinyl
doublets at 5.35 and 7.82 p.p.m. in the
1
H-NMR spectrum
(Fig. 7) a re consistent with the e lectron-rich C5 a nd
electron-deficient C6 positions of the 4-amino-2,3-dihydro-
pyridine. The
13
C-NMR spectrum additionally demonstra-
ted the presence of a triple bond (d 74.3 and 82.7 p.p.m) and
aliphatic carbon atoms (signals of d 20–40 p.p.m), consis-
tent with presence of an unoxidized DAPY moiety.
The theoretical molecular mass calculated for the predic-
ted N-(2,3-dihydropyridinyl)-1,5-diamino-2-pentyne was in
accordance with that determined by ESI-Q-TOF-MS
(177.14 Da) and the fragmentation pattern of the peak
with m/z 178.1 observed by MALDI-MS/ESI-MS. Either
the N
1
or N
5
amino group of unoxidized DAPY could add

to the initial aminopentynal turnover product. Although an
insufficient amount of the oxidation product was available
to perform the detailed two-dimensional NMR experiments
that would be needed to distinguish between the N
1
-andN
5
-
(2,3-dihydropyridin-4-yl)-1,5-diamino-2-pentyne isomers,
the observed ESI-IT-MS/MS fragmentation peaks ( Fig. 6 )
are most consistent with the former compound, as depicted
in Fig. 10. A small peak at m/z 144.1 (C
9
H
8
N
2
)requires
substantial dehydrogenation and is hard to reconcile with a
particular structure. It should be pointed out that only the
peaks with m/z 135.1 and 132.1 are consistent with only the
N
1
- and not the N
5
-isomer, and a small peak at m/z 120.1
(C
7
H
8

N
2
) is most readily reconciled with the N
5
-isome r.
Thus, we cannot exclude the possibility that the DAPY
oxidation product represents a mixture of mainly N
1
-(2,3-
dihydropyridin-4-yl)-1,5-diamino-2-pentyne, contaminated
with a small amount of the N
5
-isomer that coelutes during
RP-HPLC separation.
The MS experiments performed in the present study also
demonstrated that the aminopentynal product undergoes
further oligomerization reactions, which result in the
formation of compounds having higher molecular masses
(256–430 Da). For example, the peak with m/z 257.1
probably represents adduction of aminopentynal to both
amino groups of the same unoxidized DAPY molecule
to give N
1
,N
5
-bis-(2,3-dihydropyridin-4-yl)-1,5-diamino-2-
pentyne, and the peak with m/z 334.1 suggests even one
more aminopentynal coupling. Finally, i t should be pointed
out that the 4-amino-2,3-dihydropyridine core structure is
consistent with the molecular ion m/z 211.3 observed for the

ACA-trapped DAPY oxidation product, wherein ACA
rather than unoxidized DAPY would add to the initial
aminopentynal turnover product (see Fig. 9).
According t o the mechanism for formation of t he
chromophoric DAPY oxidation product (Fig. 9), the same
resonance-stabilized 4-amino-2,3-dihydropyridine moiety
would form if the initial aminopentynal turnover product
were trapped by an enzyme-based lysyl residue (Fig. 9). The
evidence for modification of at least some lysines during
incubation of GPAO with DAPY suggests that such
adduction could be responsible for the irreversible enzyme
inactivation observed. In this regard, t he i nactivation
mechanism would then be highly analogous to that
discerned for inactivation of PSAO or GPAO by DABY,
where the initial 4-amino-2-butynal product undergoes
conjugate addition by a channel lysyl residue, followed by
dehydrative cyclization to give a 3-aminopyrrole [8]. On the
basis of the highly apparent formation of the low molecular
mass DAPY oxidation product identified here, one might
speculate why t he analogous product, N-(pyrrole-3-yl)-1,4-
diamino-2-butyne, was not observed during plant CAO
metabolism of DABY. Although such product might have
actually been present, there are two reasons why it is
probably less apparent. The first is that DABY is a more
potent inhibitor than DAPY, so that higher concentrations
of the latter, amenable to formation of the observed
coupling product, were employed. The second is that the
aminopentynal turnover product from DAPY appears to
modify the pertinent substrate channel lysine with signifi-
cantly less efficien cy than does t he 4-amino-2-butynal

product from DABY, so that there is greater turnover
prior to inactivation.
To obtain clues as to the mechanism of inactivation of
GPAO by DAPY, we investigated the inhibitory potency of
the two possible mo no-N-methyl derivatives of DAPY with
Fig. 9. Mechanism of DAPY oxidation by GPAO. The scheme reflects
the summary results of kinetic, spectrophotometric, spectroflu orimet-
ric, MS and NMR experiments performed in this study. First, DAPY
is oxidized by the enzyme to either 5-amino-2-pentynal or 5-amino-3-
pentynal, which subsequently (after tautomerization in the latter c ase)
reacts with a second DAPY molecule (or ACA) acting as nucleophile.
After nucleophilic adduction, the two possible aldehydes undergo
cyclization to form ultimately the same reso nance s tabilized 4-amino-
2,3-dihydropyridine ring. Enzyme inactivation may follow the same
overall reaction except that the 4-amino-2,3-dihydropyridine is formed
on the enzyme by addu ction of the intial aminopentynal turnover
product to a lysine residue.
Fig. 10. MS/MS fragmentation scheme for DAPY oxidation product.
The scheme reflects the postulated c ollision-induced dissociation of
the parent DAPY oxidation product ion (m/z 178.1) assigned as
N
1
-(2,3-dihydropyridin-4-yl)-1,5-diamino-2-pentyne, according to the
observed MS/MS peaks obtained by E SI-IT-MS/MS (Fig. 6).
4706 Z. Lamplot et al. (Eur. J. Biochem. 271) Ó FEBS 2004
this enzyme. Topaquinone-dependent enzymes are mostly
not known to metabolize secondary amines. The finding
that one but not the other of the two N-methyl derivatives of
DAPY acted as an inactivator, would suggest that meta-
bolism leading to inactivation occurs only at the propar-

gylamine terminus or the homopropargylamine terminus.
Our finding that both derivatives act as inactivators o f
GPAO, albeit weaker than DAPY, suggests that enzyme
metabolism of DAPY leading to inactivation can occur at
either amino group, as shown in Fig. 9.
In conclusion, DAPY was found here to be both a
substrate and inactivator of plant CAOs. Prior to complete
enzyme inactivation, the e nzymes generate significant
amounts of two possible aminopentynal turnover products.
Either 5-amino-3-pentynal (after tautomerization) or
5-amino-2-pentynal can condense w ith a molecule of
unoxidized DAPY to give an adduct capable of cyclization
to the same 4-amino-2,3-dihydropyridine, which is appar-
ently resistant to hydrolysis on account of extended
resonance stabilization. The structure of the final product,
most likely N
1
- rather than N
5
-(2,3-dihydropyridin-4-yl)-
1,5-diamino-2-pentyne, was supported by spectrophoto-
metric, spectrofluorimetric, MS and NMR measurements.
Elucidation of the adduct structure suggests that enzyme
inactivation occurs through the same chemical mechanism,
but involving an enzyme-based lysyl residue, rather than a
second DAPY molecule, in adduct formation with the
initial aminopentynal turnover product (Fig. 9). Nonethe-
less, furthe r s tudies are needed to ascertain the true
molecular nature of enzyme modification lead ing to inac-
tivation by DAPY. Although DAPY is not such a powerful

inactivator in comparison with the previously studied
shorter analog DABY, the reaction with DAPY does not
result in as extensive labeling of the enzyme as was observed
for DABY. The significance of the presen t work resides also
in the possible use of plant CAOs for organic synthesis,
which has already been suggested [35]. Pure chemical
preparation of a dihydropyridine such as that characterized
here would be difficult.
Acknowledgements
This work was supported by grants MSM 153100010, MSM 153100013
and 14BI3 (Czech-Italian cooperation, a joint grant with the Ministry
of Foreign Affairs, Italy) from the Ministry of Education, Youth and
Sports, Czech Republic, a nd by grant GM 48812 from the National
Institutes of Health (to L.M.S.). Fluorescence spectra were recorded by
courtesy of Dr Martin Modriansky´ from the Institute of Medical
Chemistry and Biochemistry, Faculty of Medicin e, Palacky´ University.
Dr Ivo Fre
´
bort, Palacky´ University, is thanked for the initial impetus to
start with this research.
References
1. S
ˇ
ebela, M., Fre
´
bort, I., Petr
ˇ
ivalsky´ ,M.&Pec
ˇ
, P. (2002) Copper/

topa quinon e-contain ing amine oxidases – recent research devel-
opments. In Studies in Natural Products Chemistry,Vol.26.Bio-
active Natural Products, Part G. (Atta-ur-Rahman, ed.), pp.
1259–1299. Elsevier, Amsterdam.
2. Klinman, J.P. (2003) Th e multi-functional topa-quin one copper
amine oxida ses. Biochim. Biophys. Acta. 1647, 131–137.
3. Padiglia, A., Medda, R., Bellelli, A., Agostinelli, E., Morpurgo, L.,
Mondovı
`
, B., Finazzi-Agro
`
,A.&Floris,G.(2001)Thereductive
and oxidative half-reactions and the role of copper ions in plant
and mammalian copper-amine oxidases. Eur. J. Inorg. Chem. 1,
35–42.
4. Medda, R., Padiglia, A. & Floris, G. (1995) Plant copper-amine
oxidases. Phytoche mistry . 39 , 1–9.
5. Padiglia, A., Medda, R., Pedersen, J.Z., Lorrai, A., Pec
ˇ
,P.,
Fre
´
bort, I. & Floris, G. (1998) Inhibitors of plant copper amine
oxidases. J. Enzyme Inhib. 13, 311–325.
6. Medda, R., Padiglia, A., Pedersen, J.Z., Finazzi-Agro, A., Rotilio,
G. & Floris, G. (1997) Inhibition of copper amine oxidase by
haloamines: a killer product mechanism. Biochemistry. 36, 2595–
2602.
7. Pec
ˇ

,P.&Fre
´
bort, I. (1992 ) 1,4-D iamino- 2-butyne as the
mechanism-based pea diamine oxidase inhibitor. Eur. J. Biochem.
209, 661–665.
8. Fre
´
bort, I., S
ˇ
ebela, M., Svendsen, I., Hirota, S., Endo, M.,
Yamauchi, O., Bellelli, A., Lemr, K. & Pec
ˇ
, P. (2000) Molecular
mode of interaction of plant amine oxidase with the mechanism-
based inhibitor 2-butyne-1,4-diamine. Eur. J. Biochem. 26 7, 1423–
1433.
9. S
ˇ
ebela, M., Fre
´
bort, I., Lemr, K., Brauner, F. & P ec
ˇ
, P. (2000) A
study on the rea ctions of plant copper amine oxidase with C 3 and
C4 diamines. Arch. Biochem. Biophys. 384, 88–99.
10. Shepard,E.M.,Smith,J.,Elmore,B.O.,Kuchar,J.A.,Sayre,L.M.
& Dooley, D.M. (2002) Towards the development of selective
amine oxidase inhib itors. Mechanism-based inhibition of six
copper containing amine oxidases. Eur. J. Biochem. 269, 3645–
3658.

11.Jeon,H.B.,Lee,Y.,Qiao,C.,Huang,H.&Sayre,L.M.
(2003) Inhibition of bovine plasma amine oxidase by 1,4-diamino-
2-butenes and -2-butyn es. Bioorg.Med.Chem.11, 4631–4641.
12. Jeon, H.B. & Sayre, L.M. (2003) Highly potent propargylamine
and allylamine inhibitors of bovine plasma a mine oxidase. Bio-
chem. Biophys. Res. Commun. 304, 788–794.
13. Jeon, H.B., Sun, G. & Sayre, L.M. (2003) Inhibition of bovine
plasma amine oxidase by 4-aryloxy-2-butynamines and related
analogs. Biochim. Biophys. Acta. 1647, 343–354.
14. Qiao, C., Jeon, H.B. & Sayre, L.M. (2004) Selective inhibition of
bovine plasm a am ine oxi dase by homopropargylamine, a new
inactivator motif. J. Am. Chem. Soc. 126, 8038–8045.
15. Besace, Y., Marszak-Fleury, A. & Marszak, I. (1971) Delepine
reaction of acetylenic compounds. Bull.Soc.Chim.Fr.4, 1468–
1472 [in French].
16. Qiao, C. (2004) New copper amino oxidase probes: synthesis,
mechanism and enzymolo gy. PhD Thesis, Case Western Reserve
University, Cleveland, OH.
17. Torchy, S., Cordonnier, G., Barbry, D. & Van den Eynde, J.J.
(2002) Hydrogen transfer from Hantzsch 1,4-dihydropyridines t o
carbon-carbon double bonds un der microwave irradi ation.
Molecules. 7, 528–533.
18. S
ˇ
ebela, M., Luhova
´
,L.,Fre
´
bort, I., Faulhammer, H.G., Hirota,
S., Zajoncova

´
, L., Stuz
˘
ka,V.&Pec
ˇ
, P. (1998) Analysis of the
active sites of copper/topa quinone-containing amine oxidases
from Lathyrus odoratus and L. sativus seedlings. Phytochem. Anal.
9, 211–222.
19. Zajoncova
´
,L.,S
ˇ
ebela, M., Fre
´
bort, I., Faulhammer, H.G., Nav-
ra
´
til, M. & Pec
ˇ
, P. (1997) Quinoprotein amine oxidase from
sainfoin seedlings. Phytochemist ry. 45 , 239–242.
20. Smith, P.K., K rohn, R.I., Hermanson, G.T., Mallia, A.K.,
Gartner, F.H., Provenzano, M.D., Fujimoto, E.K., Goeke, N.M.,
Olson, B.J. & Klenk, D.C. (1985) Measurement of protein using
bicinchoninic acid. Anal. Biochem. 150, 76–85.
21. Fre
´
bort, I., Haviger, A. & Pec
ˇ

, P. (1989) Employment of guaiacol
for the determination of activities of enzymes generating hydrogen
peroxide and fo r the dete rmination of gluc ose in blo od and urine.
Biolo
´
gia (Bratislava) 44, 729–737.
Ó FEBS 2004 DAPY inactivates plant amine oxidases (Eur. J. Biochem. 271) 4707
22. Kent, U.M., Yanev, S. & Hollenberg, P.F. (1999) Mechanism-
based inactivation of cytochromes P450 2B1 and P450 2B6 by
n-propylxanthate. Chem. Res. Toxicol. 12, 317–322.
23. Machola
´
n, L. (1966) Beitrag zur spektrophotom etrischen Bes-
timmung der enzymatischen Oxydation von aliphatischen Diam-
inen. Collect. Czech. Chem. Commun. 31, 2167–2174 [in German].
24. Naik, B.I., Goswami, R.G. & Srivastava, K. (1981) A rapid and
sensitive assay of amine oxidase. Anal. Biochem. 111, 146–148.
25. Kovacs, N. (1928) Eine vereinfachte Methode zum Nachweis der
Indolbildung durch Bakterien. Z. Immunita
¨
tsforsch. 55, 311–315
[in German].
26. Ascenzi,P.,Fasano,M.,Marino,M.,Venturini,G.&Federico,
R. (2002) Agmatine oxidation by copper amine oxidase –
biosynthesis a nd b iochemical characterization of N-amidino-
2-hydroxypyrrolidine. Eur. J. Biochem. 269, 884–892.
27. Fields, R. (1971) The measurement of amino groups in proteins
and peptides. Biochem. J. 124, 581–590.
28. Habeeb, A.F.S.A. (1966) Determination of free amino groups in
proteins by trinitrobenzenesulfonic acid. Anal. Biochem. 14, 328–

336.
29. Paz, M.A., Flu
¨
ckiger, R., Boak, A., Kagan, H.M. & Gallop, P.M.
(1991) Specific detection of quinoproteins by redox-cycling stain-
ing. J. Biol. Chem. 266, 689–692.
30. Pec
ˇ
,P.&Pavlı
´
kova
´
, M. (1985) New colorimetric method for the
determination of diamine oxidase activity by ninhydrine. Biolo
´
gia
(Bratislava) 40, 1217–1225.
31. Mu,D.,Janes,S.M.,Smith,A.J.,Brown,D.E.,Dooley,D.M.&
Klinman, J.P. (1992) Tyrosine codon corresponds to topa quinone
attheactivesiteofcopperamineoxidases.J. Biol. Chem. 267,
7979–7982.
32. Koyanagi, T., Matsumura, K., Kuroda, S. & Tanizawa, K. (2000)
Molecular cloning and heterologous expression of pea seedling
copper amine o xid ase. Biosci. Biotechnol. Biochem. 64, 717–722.
33. Tipping, A.J. & McPherson, M.J. (1995) Cloning and molecular
analysis of the pea seedling copper amine oxidase. J. Biol. Chem.
270, 16939–16946.
34. S
ˇ
ebela, M., Kopec

ˇ
ny´ , D., Lamplot, Z., Havlis
ˇ
,J.,Thomas,H.&
Shevchenko, A. (2005) Thermostable b-cyclodextrin conjugates of
two similar plant amine oxidases and their p roperties. Biotechnol.
Appl. Biochem. 41,inpress.
35. Cragg, J.E., Herbert, R.B. & Kgaphola, M.M. (1990) Pea-seedling
diamine oxidase: applications in synthesis and evidence relating to
its mechanism of action. Tetrahedron Let t. 31, 6907–6910.
4708 Z. Lamplot et al. (Eur. J. Biochem. 271) Ó FEBS 2004