Cytochrome P450 enzymes in drug metabolism and chemical toxicology: An introduction

Abstract

Cytochrome P450 (P450) enzymes include a family of related enzymes that are involved in metabolism of vitamins, steroids, fatty acids, and other chemicals. This review presents a brief historical overview of the discovery and characterization of P450 enzymes extending from intermediary metabolism to the fields of drug metabolism and toxicology. Introductions to P450 enzyme structure and function are also presented. The goals of this review are to 1) provide an introduction to a few of the many aspects of P450 research relating to humans, 2) introduce additional ways of thinking about metabolism, 3) provide some basic examples of P450 enzymology, and 4) provide applications to topics widely taught in undergraduate courses in biochemistry.Keywords: Cytochrome P450 enzyme, drug metabolism, chemical toxicity, P450 mechanism.Cytochrome P450 (P450) 1 enzymes are a family of heme-containing proteins found from bacteria to human. In mammals, the enzymes are membrane-bound (usually in the endoplasmic reticulum, although the seven in Families 11, 24, and 27 are found in mitochondria; Table I). P450s serve a variety of functions including steroid, fatty acid, eicosanoid, and vitamin A and D metabolism as well as metabolism of foreign compounds including natural products, pharmaceuticals, and carcinogens. P450s are found in every tissue except skeletal muscle and red blood cells [1]. With the completion of the human genome sequence project, there appear to be 57 distinct P450 genes in humans [2]. Of these, about 13 are "orphan" P450s for which the metabolic function is not yet known [3].There is wide interest in P450s due to applications in a variety of fields including biochemistry, biotechnology, chemistry, environmental sciences, enzymology, microbiology, physiology, pharmacology, plant sciences, and toxicology. Indeed, in the last decade alone, there were more than 1000 publications per year relating to P450s. This review will highlight the roles of human P450s in drug metabolism and chemical toxicity and applications to undergraduate studies. References to more in-depth reviews are also provided. DISCOVERY AND PURIFICATIONThe history of P450 can be traced back to questions related to the metabolism of steroids, drugs, and carcinogens in the 1940s. Early studies of difficult alkyl oxidations of amino acids and fatty acids, such as oxidation at the terminal methyl group of a fatty acid, allowed for the isolation of cell-free extracts capable of performing NAD(P)Hdependent oxidation reactions [4]. The "mixed-function oxidase" stoichiometry.that is now recognized as the basic reaction catalyzed by a P450 was not immediately obvious. In particular, the requirement for a reduced pyridine nucleotide to achieve an overall oxidation was surprising at the time. The concept of mixed-function oxidation was developed by O. Hayaishi et al. in Japan [5] and H. Mason in the U.S [6]. Subsequently, such reactions were shown to occur with steroids and xenobiotics, particularly drugs (J. Axelrod and B. Brodie et al.) [7,8] and carcinogen...

HPLC determination of caffeine and paraxanthine in urine: An assay for cytochrome P450 1A2 activity

“…Xenobiotics such as pharmaceuticals, food additives, and carcinogens are also known substrates for the P450 family of enzymes [1,2]. Named for their characteristic absorbance of light at 450 nm when reduced in the presence of carbon monoxide, P450s consist of a family of 57 members in humans and are present in organisms from bacteria to man [2].…”

“…Products also form from dealkylation, epoxidation, heteroatom oxygenation and dealkylation, group migration, oxidation of olefins and acetylenes, and heme inactivation reactions and many other atypical reactions. An introduction to P450 enzymes in the context of modern metabolism was recently presented in this journal [2]. The focus of this paper is to introduce instructors and students to an experimental investigation of P450 metabolism in humans, specifically metabolism of caffeine by P450 1A2.…”

“…Caffeine is a naturally occurring, nonpolar plant alkaloid. It has been identified as an appropriate probe for measuring the activity of P450 1A2 because of its rapid and complete absorption throughout the total body water, its low plasma protein binding, its complete biotransformation in the liver, and negligible renal excretion [2,19,28,29]. In fact, many pharmaceuticals have been analyzed for drug specific phenotyping of P450 1A2 alongside caffeine [30].…”

Effect of total body iron on metabolic dysfunction among U.S. females in the Nhanes 2003-2010, aged 12-49.

“…Iron is also vital for the enzymatic activity in xenobiotic (e.g., plant alkaloids, fungal toxins) and medication metabolism. All cytochrome P450 (CYP450) enzymes contain iron in their active site where the redox potential of iron is used to catalyze the oxidation (i.e., activation) of most medications (141)(142)(143)(144). These enzymes are expressed in every tissue except skeletal muscle and erythrocytes (142).…”

“…All cytochrome P450 (CYP450) enzymes contain iron in their active site where the redox potential of iron is used to catalyze the oxidation (i.e., activation) of most medications (141)(142)(143)(144). These enzymes are expressed in every tissue except skeletal muscle and erythrocytes (142). The reactions catalyzed by CYP450 enzymes begin with the abstraction of hydrogen from NADPH by cytochrome P450 reductase (142).…”