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Toxicant Disposition and Metabolism
Jan Chambers
Center for Environmental Health Sciences
College of Veterinary Medicine
[email protected]
• Disposition
Absorption—passage across membrane.
Distribution—circulation in blood stream.
Excretion—elimination from body.
• Metabolism
– Enzyme-mediated change in chemical structure.
– Change in size, configuration, polarity, reactivity.
• Portals of entry
– Digestive tract (oral route of exposure).
– Respiratory tract (respiratory route of exposure).
– Skin (dermal route of exposure).
• Mechanisms of entry
– Diffusion across membranes (kinetic energy).
– Filtration across membranes (hydrostatic energy).
– Active transport (specific carrier protein, metabolic
– Endocytosis (pinocytosis, phagocytosis) (receptors,
metabolic energy).
Digestive Tract
Brush Border
Respiratory System
• Majority of toxicants diffuse through membranes.
• Majority of toxicants/xenobiotics of biological
interest (e.g., drugs) are lipophilic.
• Therefore, a general requirement for toxicant
absorption is lipophilicity (non-polar, noncharged).
• Toxicant size is generally less relevant than
• Once absorbed, circulate in the blood
• Serum is aqueous medium.
• Lipophilic toxicants are not readily
dissolved or suspended in the serum.
• Bound to serum proteins (e.g., albumin).
• Must exit blood and cross membranes to
reach biological targets.
serum protein Æ serum Æ capillary
membrane Æ interstitial fluid Æ [cell
membrane Æ cytosol Æ (organelle
membrane Æ interior of organelle)] Æ
target molecule (i.e., receptor, enzyme,
channel, DNA).
Circulatory System
Hepatic Portal Vein and Enterohepatic
• Storage sites:
– fat stores and fatty tissues (e.g., liver) for
lipophilic toxicants; partition into fat; e.g.,
polychlorinated biphenyls (PCB’s).
– bone for divalent cations resembling calcium;
active transport using calcium transporter; e.g.,
• Stored toxicant is biologically inactive until
• Major routes: kidney (urine), digestive tract
• Minor routes: respiratory tract, tears, sweat.
• Urine and feces are aqueous media.
• Lipophilic toxicants cannot partition into urine or
feces; therefore, cannot be excreted—
bioaccumulation [(e.g., PCB’s , organochlorine
insecticides, persistent organic pollutants
Enterohepatic Circulation
• Chemical alteration to structure.
• Enzyme-mediated (enzyme is protein, chemical
catalyst, lowers activation energy for reaction).
• Outcome: changes physicochemical
characteristics of toxicant:
– Ability to be stored or excreted (half-life; potential for
– Reactivity with targets (toxic potential; bioactivation or
In general: lipophilic (readily absorbed/stored)
Æ hydrophilic (readily excreted).
Enzyme Reaction Schematic
Reaction Pathways
Categories of Biotransformation Reactions
• Phase 1: adds or uncovers a reactive group.
– Makes more polar; more likely to be excreted, but may
not be truly water soluble.
– More chemically reactive; possibly more toxic.
– More likely to undergo Phase 2 metabolism.
• Phase 2: adds an endogenous ligand.
– Usually makes more polar and usually water soluble.
– Usually ligand interacts with reactive group, so
decreases toxicity.
– May act on parent toxicant or Phase 1 metabolite.
Locations of Metabolism
• Most active tissue: liver.
• Moderately active tissues: kidney, skin, intestine.
• Therefore, oral route of exposure leads to greater
toxicant metabolism than respiratory or dermal
– If toxicant is bioactivated, oral route leads to greater
toxicity than other routes.
– If toxicant is detoxified, oral route leads to lesser
toxicity than other routes
Phase 1 Reactions
• Oxidation.
– Monooxygenases:
• Cytochromes P450 (CYP; P450).
• Flavin-containing monooxygenases (FMO).
– Dehydrogenases:
• Alcohol dehydrogenase.
• Aldehyde dehydrogenase.
• Hydrolysis.
– Hydrolases (esterases, amidases).
– Hydratases..
• Other.
Cytochromes P450
• Enzyme family, with broad substrate specificities.
• Most significant of all toxicant oxidation reactions.
• Adds one atom of molecular oxygen to substrate,
other atom becomes a reactive oxygen species (with
potential for oxidative damage within the cell).
• Very important in detoxication of many toxicants.
• Most important for bioactivations:
– carcinogens (e.g., polycyclic aromatic hydrocarbons,
PAH’s; benzene; vinyl chloride; aflatoxin).
– neurotoxicants (e.g., organophosphate insecticide oxons).
– hepatotoxicants (e.g., carbon tetrachloride).
P450 Reaction Cycle
P450: Epoxidation
P450: N-Oxidation
P450: Desulfuration/Dearylation
P450: O-Demethylation
• Addition of water to break a bond and
perhaps the molecule.
• Hydrolases (e.g., esterases, amidases): split
molecule into two metabolites.
• Hydratases: hydrates a bond, but molecule
remains intact.
• Usually detoxications.
Organophosphate Hydrolysis
Epoxide Hydration
Other: DDT Dehydrochlorinase
Phase 2 Reactions
• Conjugation reactions, adding an endogenous
ligand to a reactive moiety.
– Makes more water-soluble and usually detoxifies.
• Sulfate.
• Glucuronic acid (a sugar).
• Glutathione (a peptide).
– Makes less water-soluble and more readily absorbed.
• Metal methylation; e.g., inorganic to methyl mercury.
Sulfate Conjugation
Glucuronide Conjugation
Glutathione (GSH) Conjugation
Reaction Pathways
Levels of Enzymes
• Vary with age.
• Vary with sex.
• Inhibition—drug interactions, insecticide
• Induction—alcohol, PAH’s, PCB’s, drugs.
• Lipophilic toxicants can get in, but don’t
• Phase 1 metabolites more likely to leave but
may be highly toxic reactive metabolites.
• Phase 2 metabolites readily excreted.