Biotransformation and Elimination of Toxicants pptx

1
Biotransformation and Elimination
of Toxicants
Principles of Environmental Toxicology
Instructor: Gregory Möller, Ph.D.
University of Idaho
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Learning Objectives
• Explain the role of biotransformation in toxicokinetics.
• Describe how biotransformation facilitates elimination
of toxicants.
• Distinguish between Phase I
and Phase II reactions.
• Define bioactivation
or toxication.
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Learning Objectives, 2
• Identify tissues and factors
involved in biotransformation.
• Summarize the role of elimination in
toxicokinetics.
• Describe processes occurring in
the kidney, liver and lung
related to the elimination
of toxicants.
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Metabolism
• Sum of biochemical rxns occurring to a

molecule within the body.
– Anabolism - “build-up”
– Catabolism - “break-down”
• Occurs in the cytoplasm or
at specific organelles within
the cell.
• Storage affects the body’s
ability to biotransform and
eliminate.
– Bone, lipid.
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Biotransformation
• Process that changes substances from hydrophobic
to hydrophilic to aid in elimination (grease to salt).
– Hydrophilic molecules are less able to cross cellular
membranes, hence fluid filterable (kidneys).
– Major elimination routes are
feces (biliary) and urine.
– Biological half-life, T
½
allows comparison of
rates of removal.
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Biotransformation Reactions
• Grouped as Phase I (functional group modification)
and Phase II (conjugation).
• Goals
– Produce water soluble metabolites.

– Activate natural/endogenous compounds
for normal function.
• Some compounds undergo
bioactivation
.
– The biotransformed metabolite
is more toxic than the original
compound.
2
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Results of Biotransformation
• Increase toxicity via a toxic metabolite.
• Decrease toxicity via metabolism of a toxic
parent compound.
• No effect on toxicity.
• Present to metabolize
endogenous compounds.
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Major Categories/Reactions
Phase I
Phase II
Elimination
oxidation
reduction
hydrolysis
conjugation
synthesis
polar

very
polar
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Enzymes of Biotransformation
• Oxidation (most important).
– Add O, remove H, increase valence.
– Cytochrome P-450, MFO, alcohol dehydrogenase,
oxidases, others.
• Reduction (less important).
– Remove O, add H, decrease valence.
– Reductases.
• Hydrolysis.
– Add water.
– Esterases, phosphtases, others.
Phase I Enzymes
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Phase I Reactions
NH2R
S
R
2
R
1
R
1
R
2
C

O
R
2
R
1
C-O
O
R
1
R
2
CS
RCH2OH
R
1
R
2
CH
OH
SO
R
2
R
1
NHOHR
R
1
CH2OOH
RCHO
R

1
R
2
CO
R
2
HO
N-oxidation
S-oxidation
Carbonyl reduction
Ester Hydrolysis
Desulfuration
Dehydrogenation
+
Hughes
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Enzymes of Biotransformation, 2
• Conjugation reactions.
• Enzymes (tranferases) + cofactor.
– Enzyme catalyzes.
– Cofactor donates group.
– Glucuronic acid, glutathione, sulfate,
acetyl group, methyl group.
– Tends to increase
molecular size and
polarity for excretion.
Phase II Enzymes
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PII Cofactors: GSH
H
N
O
HS
O
N
H
NH
2
HO
O
OH
O
Glutathione
3
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PII Cofactors: Acetyl-CoA
Acetyl Coenzyme A
N
N
N
NH
2
N
O
O
P
OH

O
O
O
P
O
HO
HO
O
NH
NH
O
S
HO
O
OH
P
HO
O
O
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PII Cofactors: PAPS
OH
O
O
N
N
N
N
NH

2
OSO
O
OH
PO
O
OH
P
OH
OOH
3’-Phosphoadenosine
5”-phosphosulfate
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PII Cofactors: UDPGA
O
HO
HO
H
H
H
H
2
C
H
N
H
N
O
O

OHP
O
O
HO
P
O
O
O
H
HO
H
HO
H
OH
H
H
-
O
O
Uridine-5’-
diphosphoglucuronic acid
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Benzene Metabolism
OH
ST
O
OH
P450
PAPS

O
OSO
3
OH
OH
Glutathione
Epoxidation
G
SH
GST
Toxic Epoxide
Phenol
Glucuronide
UDP-GT
Epoxide
Hydratase
Dihydrodiol
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Aniline
NH
2
P450
H
N
OH
Phase II
Amine
N-hydroxylation
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De-Alkylation
N
P450
H
N
HC
O
+
Phase II
Dimethyl-propyl-amine
Methyl-propyl-amine
Acetaldehyde
4
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Free Radical Generation
C
Cl
Cl
Cl Cl
N
A
DH
P450
Reducatase
C
Cl
Cl
Cl

To
x
ic F
r
ee Radical
GSH
Tet
r
achlo
r
o-methane
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Case Study: Fluorocitrate and Kangaroos
• Fluorocitrate found in legume
pasture plants
of Western Australia.
– Gastrolobium and Oxylobium.
• Highly lethal (TD 1 mg/1080 kg).
– Leaf concentrations can be 2.6 g/kg.
• The rat and gray kangaroo
of Western Australia have
evolved resistance.
– In vivo defluorination w/ glutathione.
– Other kangaroos from areas
w/o these plants are not tolerant.
WACALM
Harborne
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Rodenticide: Fluoroacetic Acid
OH
O
F
Co
A
SH
FCCOSCoA
H
H
Fluoroacetate Fluoroacetyl CoA
Sodium Fluoroacetate
Compound 1080
rodenticide
predator control
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Fluorocitrate Metabolite
HO
O
OOH
O
AcCoA
FAcCoA
OH
HO
O
OH
O
O

HO
F
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Krebs Cycle
OH
HO
O
OH
O
O
HO
F
AcCoA
FAcCoA
H
2
O
HO
O
OOH
O
HO
O
OH
O
O
HO
Aconitase
(Fluo

r
o)Cit
r
ate
Oxaloacetate
Cis-aconitate
Mitochondrial
energy production
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Deoxynivalenol, Vomitoxin
O
HO
HO
O
O
O
HO
HO
OH
O
CH
2
OH
Fusarium trichothecene
mycotoxin found on
corn and barley
5
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Aflatoxin B
1
O
O
O
O
O
H
H
O
O
O
O
O
O
H
H
O
OH
B
1
Q
1
= hepatic metabolite
Aspergillus mycotoxin
found on corn, peanuts
and cottonseed
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Benzo[a]pyrene

OR
R = sulphate or glucuronic acid
• Polycyclic aromatic
hydrocarbon.
• Environmental carcinogen.
• Cell cultures from rodents,
fish and humans
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Heavy Metal Toxicity - Pb
• Absorbed via Ca channels as divalent ion.
• Capable of reacting with a variety of binding sites.
– Protein precipitation.
• Specific toxic effect depends on rxns with ligands
that are essential for the living system.
• Metal ligands are formed with
sulfhydryl groups, as well as
amino, phosphate, imidazole,
and hydroxyl groups of enzymes
and essential proteins.
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Heavy Metal Toxicity - Pb, 2
• Sensitivity of a system and degree of
interference determines clinical effects.
– Digestion/respiration → absorption.
–Liver → detoxication.
–Kidney → excretion.
• Antidotes are competing ligands.
N

NO
OH
O
OH
O
HO
O
HO
EDTA
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Heavy Metal Toxicity - Pb, 3
• Metallic lead absorbed most efficiently
by the respiratory tract.
• 10% of ingested lead is absorbed.
– Small intestine.
– Lead salts are soluble in gastric juices; absorbed.
• Plasma to blood cells – erythrocytes.
• After oral ingestion:
– 60% bone (also hair, teeth).
– 25% liver (hepatocytes).
– 4% kidney (renal tubules).
– 3% intestinal wall.
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Heavy Metal Toxicity - Pb, 4
• Some endpoints.
– Sulfhydral enzyme inhibition.
– K transport in RBC inhibited
• Anemia.

– Porphyrinuria.
• Excreted chiefly in
feces and urine.
• Chelating agents:
–Ca -EDTA.
– Penicillamine.
– Dimercaptrol (BAL).
C
H2
CH
C
H2
HO
SH
SH
2,3-Dimercapto-propan-1-ol
6
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Case Study: Elevated PbB Associated with
Illicitly Distilled Alcohol, Alabama 1991
• The use of automobile radiators containing
lead-soldered parts in the illicit distillation of
alcohol (i.e., "moonshine") is an important
source of lead poisoning among persons in
some rural Alabama counties.
• In 1991, eight persons were diagnosed with
elevated blood lead levels (BLLs) at a local
hospital.
• 9 patients had been evaluated for alcohol-

related medical conditions at the hospital.
Manifestations included generalized tonic-clonic
seizures (six), microcytic anemia (five)
(hematocrit mean: 32.1%), encephalopathy
(two), upper extremity weakness (one), and
abdominal colic (one). BLLs ranged from 16
ug/dL to 259 ug/dL (median: 67 ug/dL).
MMWR (1992) 41(17);294-295
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Case Study: “Moonshine” Lead Toxicity
• Seven patients required hospitalization for
48 hours or longer (range: 2-18 days). Three
of these received chelation therapy; initial
BLLs were 67, 228, and 259 ug/dL. One
patient, whose BLL was 67 ug/dL, died
during hospitalization from alcohol-
withdrawal syndrome complicated by
aspiration pneumonia.
• Patients reported moonshine ingestion
ranging from 0.2 L per day to 1.5 L per day.
• The lead contents of specimens of
moonshine confiscated from two radiator-
containing stills in the county in 1991 were
7400 ug/L and 9700 ug/L, compared with
nondetectable amounts (less than 1.0 ug/L)
in municipal water from the county.
• Consumption of 0.5 L per day of moonshine
containing 9700 ug/L lead would result in a
steady state BLL of approximately 190

ug/dL.
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Elimination of Toxicants
• Urinary.
• Fecal.
• Respiratory.
• Other:
–Saliva.
–Sweat.
– Milk (transfer to child).
– Nails, Hair, Skin.
– Cerebrospinal fluid.
Hughes
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Kidney
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Renal Macrostructure
Renal cortex
Renal medula
Ureter
Bovine
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Renal Filtration Microstructure
7
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Renal Histology
Tubules
Glomerulus
Microscopic
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Urinary Excretion
• Glomerular filtration
• Tubular secretion
• Tubular reabsorption
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Fecal Excretion
• Excretion in bile to intestine.
– Active transport of toxicant parent and metabolites.
– Highly soluble Phase II metabolites (large, ionized)
• Excretion into the lumen of the GI tract.
– Direct diffusion from capillaries.
NLM
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Exhaled Air
• Gas phase xenobiotics.
• Passive diffusion from blood
to alveolus via concentration
gradient.
– The total alveolar epithelial
surface area within an average
adult human lung has been
estimated to be as large as

100-140 m
2
.
Gray's Anatomy 1918