Metabolism of Xenobiotics I. General Overview Aug 25, 2011 L.M. Ball Rosenau 158 ENVR/TOXC 442 Fall 2011

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

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

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

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

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

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

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

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

TCDD