The ‘drug-metabolizing enzymes’ comprise those enzymes which facilitate the biotransformation of drugs and other xenobiotics, generally rendering the lipophilic substrates more polar and hence more readily excretable. They are most abundant in mammalian liver, but also occur, at much lower activities, in the gastrointestinal tract, kidney, lung, and many other tissues. They have been classified into phase 1 enzymes (mixed-function oxidases, oxidoreductases, reductases, esterases), which modify the lipophilic xenobiotic substrates by oxidative, reductive or hydrolytic catabolism with the creation of functional groups, and phase 2 enzymes (conjugases), which transfer endogenous moieties (glucuronate, sulphate, acetate, glycine, glutathione, etc.) to the xenobiotics or their metabolites, thereby making them more polar and more readily eliminated from the cell (Parke, 1982~) . The mixed-function oxidases are the most extensively studied of these different enzymes, and comprise a NADPH-dependent flavoprotein (FAD, FMN), cytochrome P-450 reductase, and the terminal oxygen-activating/transferring enzyme, cytochrome P-450. This is an ubiquitous system, found in microorganisms, yeasts, higher animals and plants, and in many different tissues in mammals. In yeasts and higher organisms it is membrane-bound (endoplasmic reticulum and mitochondria) and generally functions to insert oxygen into xenobiotic and certain endogenous substrates (cholesterol, steroids, fatty acids). This enzyme system may also catalyse the reduction of xenobiotics (nitro and azo compounds), effect reductive dehalogenation (carbon tetrachloride, halothane, etc.) with formation of free radicals, and form ligand and covalent complexes with certain reaction products (radicals, carbenes, etc.). This complex-formation may result in the activation of oxygen by cytochrome P-450 to form superoxide anion and peroxide, without insertion of oxygen into organic substrates (Parke, 1982~) . The mixed-function oxidases also effect the oxygenation of numerous endogenous substrates and are involved in the o-hydroxylation of fatty acids, the biosynthesis and catabolism of cholesterol, steroid hormones, and the conversion of arachidonate into prostaglandins (Kupfer, 1980). The mixed-function oxidases, once thought to be a single, multi-functional enzyme system have, during the last decade, been solubilized, and shown by reconstitution to comprise a diverse three component system, namely cytochrome P-450, the flavoprotein reductase, and a phospholipid (phosphatidylcholine) electron coupler. The cytochrome P-450 fraction has been fractionated and it is now considered that tissues may contain several different cytochrome P-450 species with different, but overlapping, substrate specificities (Lu & West, 1980).