Toxicant Disposition and Metabolism. Jan Chambers Center for Environmental Health Sciences College of Veterinary Medicine

Transcription

1 Toxicant Disposition and Metabolism Jan Chambers Center for Environmental Health Sciences College of Veterinary Medicine

2 Definitions Disposition Absorption passage across membrane. Distribution circulation in blood stream. Storage sequestration. Excretion elimination from body. Metabolism Enzyme-mediated change in chemical structure. Change in size, configuration, polarity, reactivity.

3 Disposition

4 Absorption 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 energy). Endocytosis (pinocytosis, phagocytosis) (receptors, metabolic energy).

5 Membrane

6 Cell

7 Digestive Tract

8 Brush Border

9 Respiratory System

10 Alveolus

11 Skin

12 Skin

13 Absorption--Summary 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 lipophilicity.

14 Distribution Once absorbed, circulate in the blood stream. 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.

15 Distribution serum protein serum capillary membrane interstitial fluid [cell membrane cytosol (organelle membrane interior of organelle)] target molecule (i.e., receptor, enzyme, channel, DNA).

16 Circulatory System

17 Hepatic Portal Vein and Enterohepatic Circulation

18 Storage 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., lead. Stored toxicant is biologically inactive until mobilized.

19 Excretion Major routes: kidney (urine), digestive tract (feces). 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 (POP s)].

20 Kidney

21 Nephron

22 Enterohepatic Circulation

23 Metabolism

24 Metabolism/Biotransformation 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 bioaccumulation). Reactivity with targets (toxic potential; bioactivation or detoxication). In general: lipophilic (readily absorbed/stored) hydrophilic (readily excreted).

25 Enzyme Reaction Schematic

26 Reaction Pathways

27 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.

28 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 routes. 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

29 Phase 1 Reactions Oxidation. Monooxygenases: Cytochromes P450 (CYP; P450). Flavin-containing monooxygenases (FMO). Dehydrogenases: Alcohol dehydrogenase. Aldehyde dehydrogenase. Hydrolysis. Hydrolases (esterases, amidases). Hydratases.. Other.

30 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).

31 P450 Reaction Cycle

32 P450: Epoxidation

33 P450: N-Oxidation

34 P450: Desulfuration/Dearylation

35 P450: O-Demethylation

36 Hydrolysis 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.

37 Organophosphate Hydrolysis

38 Epoxide Hydration

39 Other: DDT Dehydrochlorinase

40 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.

41 Sulfate Conjugation

42 Glucuronide Conjugation

43 Glutathione (GSH) Conjugation

44 Reaction Pathways

45 Levels of Enzymes Vary with age. Vary with sex. Inhibition drug interactions, insecticide synergists. Induction alcohol, PAH s, PCB s, drugs.

46 SUMMARY Lipophilic toxicants can get in, but don t leave. Phase 1 metabolites more likely to leave but may be highly toxic reactive metabolites. Phase 2 metabolites readily excreted.