Molecular Mechanism of Phase I and Phase II Drug‐Metabolizing Enzymes: Implications for Detoxification
Introduction
Almost all drugs that undergo metabolic transformation are converted to metabolite, which are more polar than the parent compound. Biotransformation not only promotes drug elimination but also often results in inactivation of pharmacological activity, thereby changing the overall biological properties of the drug (Wilkinson, 2001). However, a number of relatively inert xenobiotics, which are foreign to living organisms (Greek; xenos and biot, for “stranger to life”), has been found to be converted by enzymes in the body to extremely toxic products. DMEs, which can be involved in complex metabolic networks of the body, are classed as phase I or phase II enzymes. Phase I enzymes often catalyze oxidation, reduction, and hydrolysis reactions. Most phase II enzymes catalyze conjugation reactions: UDP‐glucuronosyltransferases (UGTs), sulfotransferases (SULTs), or glutathione S‐transferases (GSTs). Drugs are often metabolized by sequential reactions involving phase I and II enzymes. The time taken for a drug to be cleared from the body is a function of both the rate of biotransformation and excretion. The major role of P450s is to introduce polar functional groups into nonpolar molecules. Conjugating enzymes catalyze the addition of more polar functional groups to the drug, such as glucuronate, sulfate, glutathione, or amino acids. NAD(P)H: quinone:oxidoreductase (also called DT‐diaphorase), methyltransferase, and acetyltransferase are also classed as phase II enzymes. Sequential biotransformations by both the P450R/P450 system and UGTs constitute one of the major routes for drug metabolism. These sequential reactions might be facilitated by the close spatial association of the enzymes on the endoplasmic reticulum (ER). The metabolites generated by phase I and II reactions are excreted from the body with the aid of membrane efflux pumps that include the multidrug resistance‐associated proteins (phase III reactions).
The P450R/P450 system and UGTs play a vital role in the metabolism and detoxification of xenobiotic (exogenous) and endobiotic (endogenous) compounds. The P450 and UGT superfamily of genes encode proteins that metabolize a diverse set of substrates. P450 proteins are encoded by distinct genes, whereas the P450R protein is encoded by a single gene. Thus, P450R can donate electrons to all microsomal P450s. In contrast, UGT proteins are encoded by both distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions (Gibbs et al., 2004). The combinatory pharmacogenetics of these enzymes is an increasingly important research field in clinical medicine (Wilke et al., 2005).
Section snippets
Phase I: NADPH‐Cytochrome P450 Reductase (P450R)/Cytochrome P450 (P450) System
Phase I enzymes (functionalization reactions), which mediate oxidation, reduction, or hydrolysis reactions, introduce a functional group (‐OH, ‐SH, ‐NH2, or ‐COOH) into the drug molecule. The P450R/P450 is a major oxidative enzyme system (Gibson and Skett, 2001). The P450‐containing microsomal electron transfer system catalyzes the oxidation of drugs and xenobiotics. Although there are numerous functional P450 genes in mammals (e.g., 84 in rat, 87 in mouse, 57 in human) (Gibbs et al., 2004),
Phase II: UDP‐Glucuronosyltransferase (UGT)
Phase II conjugation reactions lead to the formation of a covalent linkage between a functional group either on the parent compound or on one introduced as a result of a phase I reaction. UDP‐glucuronosyltransferases (UGTs) are the major class of enzymes that catalyze phase II reactions, such as the transfer of glucuronic acid (GlcUA) of UDP‐glucuronic acid (UDP‐GlcUA) to a functional group (‐OH, ‐SH, ‐NH2, C‐C, or ‐COOH) of a xenobiotic and endobiotic. The xenobiotic may be a drug molecule or
Subcellular Localization of the P450R/P450 System and UGT
The cellular membrane consists of a bilayer of amphipathic lipid molecules, made up of a polar head group and a nonpolar hydrocarbon tail. The hydrophobic tails are buried within the bilayer whereas the hydrophilic head groups are exposed at the surface. All eukaryotic cells have an ER, which comprise a system of membranes that enclose a space, or lumen (Palade, 1956). The space or lumen is physically separated from the cytosol by the ER membrane, which mediates selective transfer of molecules
Concluding Remarks
It is interesting to consider the coordinate structure/function of the phase I and phase II DMEs in the ER membrane (Fig. 13). The P450s and UGTs display broad overlapping substrate specificities, which together constitute a general enzyme system within the organism. The DMEs including P450 and UGT have a highly plastic active site, which can accommodate a wide variety of substrates. Furthermore, P450R can donate electrons to 50 microsomal P450s. The microsomal electron transport systems
Acknowledgments
The author thanks Dr. Jung‐Ja Kim, Department of Biochemistry, Medical College of Wisconsin, USA, and Dr. Peter Mackenzie, Department of Clinical Pharmacology, Flinders University School of Medicine, Flinders Medical Center, Bedford Park, Australia, and Drs. Yoshitsugu Shiro, Shingo Nagano, Hiroshi Sugimoto, Akihiro Kikuchi, and Atsuko Kohara of Biometal Science Laboratory, RIKEN Harima Institute/SPring 8 for useful discussions and suggestions.
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