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Metabolism of Xenobiotics

ENVR/TOXC 442 Fall 2012. Metabolism of Xenobiotics. I. General Overview Aug 21, 2012 L.M. Ball Rosenau 158 lmball@unc.edu. Metabolism : Chemical reactions carried out by and in living systems Breakdown of organic matter (eg food) to release energy ( catabolism )

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Metabolism of Xenobiotics

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  1. ENVR/TOXC 442 Fall 2012 Metabolism of Xenobiotics I. General Overview Aug 21, 2012 L.M. Ball Rosenau 158 lmball@unc.edu

  2. Metabolism: Chemical reactions carried out by and in living systems Breakdown of organic matter (eg food) to release energy (catabolism) Construction of cell components (eg carbohydrates, proteins, lipids, nucleic acids, other macromolecules) using energy (anabolism) Carried out by enzymes (+ co-factors) Essential to life No metabolism = no life Xenobiotic Substance foreign (xenos = foreign) to life (bios) Chemical found in a living system which is not “naturally” present in that organism. Drugs (Drug metabolism) Environmental pollutants Not produced by organism Not useful to organism Metabolism carried out by enzymes (+ co-factors) Metabolism serves to eliminate xenobiotics Fundamental to toxicology

  3. Why? • Importance in Pharmacology/Therapeutics • Importance in Toxicology • Importance in Environmental Protection • Need to consider properties (therapeutic and/or toxic) of metabolites as well as those of the parent compound • Need to know • What metabolites are formed • Where they are formed • Kinetics of formation • Kinetics of elimination

  4. International Society for Study of Xenobiotics (ISSX) www.issx.org Xenobiotica Drug Metabolism and Disposition (ASPET) Chemical Research in Toxicology (ACS) Drug Metabolism Reviews (ISSX) Current Drug Metabolism Drug Metabolism Letters

  5. Phase I – Phase II • Phase I: Chemical modifications that introduce or uncover functional groups on a xenobiotic that provide sites for Phase II metabolism • Phase II: Synthetic reaction of a xenobiotic (or of a Phase I metabolite of a xenobiotic) with an endogenous substance that results in introduction of polar, ionizable groups to enhance water solubility and hence excretion

  6. Identification/quantitation of metabolites • Extraction from biological matrix • Separation/purification • Analytical techniques • Chromatography • Thin-layer, gas, liquid, high-pressure liquid…. • Spectrometry • Mass, nuclear magnetic resonance, UV-vis, fluorescence • Radiolabelled parent compound

  7. Systems studied • Whole animals (humans, rats) • Isolated perfused organs • Reconstructed organs/tissues • Cells (primary isolates, cultures) • Subcellular fractions • S9 • Microsomes • Cytosol • Purified enzymes • Cells engineered to over/under express specific enzymes • Organisms with genes for specific enzymes knocked out

  8. Overview: Uptake and distribution(ADME) • Major Portals of Entry • Lung • Intestinal Tract • Skin • Major Sites of Metabolism • Liver • Lung • Intestinal Tract • Kidneys • Skin • Target organs • Major Routes of Elimination • Urine • Feces • Exhaled air • Pharmacokinetic descriptors

  9. Passage across membranes • Passive diffusion (Fick’s Law) • Flux is proportional to concentration gradient J = -D * ΔC/Δx • Transporters (can go against concentration gradient) • Metabolism generates a concentration gradient

  10. TCDD 2,3,7,8-tetrachlorodibenzo-p-dioxin http://www.dioxin2012.org

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