Although drug biotransformation in vivo can occur by spon-which serves as the terminal oxidase. In fact, the microsomal taneous, noncatalyzed chemical reactions, most transformations membrane harbors multiple forms of this hemoprotein, and are catalyzed by specific cellular enzymes. At the subcellular level, this multiplicity is increased by repeated administration of or these enzymes may be located in the endoplasmic reticulum, exposure to exogenous chemicals (see text that follows). The mitochondria, cytosol, lysosomes, or even the nuclear envelope or name cytochrome P450 (abbreviated as P450 or CYP) is derived from the spectral properties of this hemoprotein. In its reduced plasma membrane.
(ferrous) form, it binds carbon monoxide to give a complex that absorbs light maximally at 450 nm. The relative abundance MICROSOMAL MIXED FUNCTION of P450s, compared with that of the reductase in the liver, OXIDASE SYSTEM & PHASE I REACTIONS contributes to making P450 heme reduction a rate-limiting step in hepatic drug oxidations. Many drug-metabolizing enzymes are located in the lipophilic Microsomal drug oxidations require P450, P450 reductase, endoplasmic reticulum membranes of the liver and other tissues. NADPH, and molecular oxygen. A simplified scheme of the When these lamellar membranes are isolated by homogenization oxidative cycle is presented in Figure 4–3. Briefly, oxidized (Fe+3)
FIGURE 4–3 Cytochrome P450 cycle in drug oxidations. e–, electron; RH, parent drug; ROH, oxidized metabolite.
P450 combines with a drug substrate to form a binary complex (step 1). NADPH donates an electron to the flavoprotein P450 reductase, which in turn reduces the oxidized P450-drug complex (step 2). A second electron is introduced from NADPH via the same P450 reductase, which serves to reduce molecular oxygen and to form an “activated oxygen”–P450–substrate complex (step 3). This complex in turn transfers activated oxygen to the drug substrate to form the oxidized product (step 4).
The potent oxidizing properties of this activated oxygen permit oxidation of a large number of substrates. Substrate specificity is very low for this enzyme complex. High lipid solubility is the only common structural feature of the wide variety of structurally unrelated drugs and chemicals that serve as substrates in this system (Table 4–1). However, compared with many other enzymes including phase II enzymes, P450s are remarkably sluggish catalysts, and their drug biotransformation reactions are slow.
Gene arrays combined with immunoblotting analyses of microsomal preparations, as well as the use of relatively selective functional markers and selective P450 inhibitors, have identified numerous P450 isoforms (CYP: 1A2, 2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4, 3A5, 4A11, and 7) in the human liver. Of these, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1, and CYP3A4 appear to be the most important forms, accounting for approximately 15%, 4%, 1%, 20%, 5%, 10%, and 30%, respectively, of the total human liver P450 content. Together, they are responsible for catalyzing the bulk of the hepatic drug and xenobiotic metabolism (Table 4–2, Figure 4–4).