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Metabolism of Xenobiotics

Robert K. Murray, MD, PhD

BIOMEDICAL IMPORTANCE

Increasingly, humans are subjected to exposure to various foreign chemicals (xenobiotics)—drugs, food additives, pollutants, etc. The situation is well summarized in the following quotation from Rachel Carson: “As crude a weapon as the cave man’s club, the chemical barrage has been hurled against the fabric of life.” Understanding how xenobiotics are handled at the cellular level is important in learning how to cope with the chemical onslaught, and thus helping to preserve the environment. For example, building on such information, attempts are being made to modify microorganisms by introducing genes that encode various enzymes involved in metabolizing specific xenobiotics to harmless products. These modified organisms will then be used to help dispose of various pollutants that contaminate the planet.

Knowledge of the metabolism of xenobiotics is basic to a rational understanding of pharmacology and therapeutics, pharmacy, toxicology, management of cancer, and drug addiction. All these areas involve administration of, or exposure to, xenobiotics.

HUMANS ENCOUNTER THOUSANDS OF XENOBIOTICS THAT MUST BE METABOLIZED BEFORE BEING EXCRETED

A xenobiotic (Gk xenos “stranger”) is a compound that is foreign to the body. The principal classes of xenobiotics of medical relevance are drugs, chemical carcinogens, and various compounds that have found their way into our environment by one route or another, such as polychlorinated biphenyls (PCBs) and certain insecticides. More than 200,000 manufactured environmental chemicals exist. Most of these compounds are subject to metabolism (chemical alteration) in the human body, with the liver being the main organ involved; occasionally, a xenobiotic may be excreted unchanged. At least 30 different types of enzymes catalyze reactions involved in xenobiotic metabolism; however, this chapter will only cover a selected group of them.

It is convenient to consider the metabolism of xenobiotics in two phases. In phase 1, the major reaction involved is hydroxylation, catalyzed mainly by members of a class of enzymes referred to as monooxygenases or cytochrome P450s. Hydroxylation may terminate the action of a drug, though this is not always the case. In addition to hydroxylation, these enzymes catalyze a wide range of reactions, including those involving deamination, dehalogenation, desulfuration, epoxidation, peroxygenation, and reduction. Reactions involving hydrolysis (eg, catalyzed by esterases) and certain other non-P450-catalyzed reactions also occur in phase 1.

In phase 2, the hydroxylated or other compounds produced in phase 1 are converted by specific enzymes to various polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids, or by methylation.

The overall purpose of the two phases of metabolism of xenobiotics is to increase their water solubility (polarity) and thus excretion from the body. Very hydrophobic xenobiotics would persist in adipose tissue almost indefinitely if they were not converted to more polar forms. In certain cases, phase 1 metabolic reactions convert xenobiotics from inactive to biologically active compounds. In these instances, the original xenobiotics are referred to as “prodrugs” or “procarcinogens.” In other cases, additional phase 1 reactions (eg, further hydroxylation reactions) convert the active compounds to less active or inactive forms prior to conjugation. In yet other cases, it is the conjugation reactions themselves that convert the active products of phase 1 reactions to less active or inactive species, which are subsequently excreted in the urine or bile. In a very few cases, conjugation may actually increase the biologic activity of a xenobiotic.

The term “detoxification” is sometimes used for many of the reactions involved in the metabolism of xenobiotics. However, the term is not always appropriate because, as mentioned above, in some cases the reactions to which xenobiotics are subject actually increase their biologic activity and toxicity.

ISOFORMS OF CYTOCHROME P450 HYDROXYLATE A MYRIAD OF XENOBIOTICS IN PHASE 1 OF THEIR METABOLISM

Hydroxylation is the chief reaction involved in phase 1. The responsible enzymes are called monooxygenases or cytochrome P450s. It is estimated that there are some 57 cytochrome P450 genes present in humans. The reaction catalyzed by a monooxygenase (cytochrome P450) is as follows:

RH above can represent a very wide variety of xenobiotics, including drugs, carcinogens, pesticides, petroleum products, and pollutants (such as a mixture of PCBs). In addition, endogenous compounds, such as certain steroids, eicosanoids, fatty acids, and retinoids, are also substrates. The substrates are generally lipophilic and are rendered more hydrophilic by hydroxylation.

Cytochrome P450 is considered the most versatile biocatalyst known. The actual reaction mechanism is complex and has been briefly described previously (Figure 12–6). It has been shown by the use of ¹⁸O₂ that one atom of oxygen enters R-OH and one atom enters water. This dual fate of the oxygen accounts for the former naming of monooxygenases as “mixed-function oxidases.” The reaction catalyzed by cytochrome P450 can also be represented as follows:

Cytochrome P450 is so named because the enzyme was discovered when it was noted that preparations of microsomes that had been chemically reduced and then exposed to carbon monoxide exhibited a distinct peak at 450 nm. Microsomes contain fragments of the endoplasmic reticulum, where much of the P450 content of cells is located (see below). Among reasons that this enzyme is important is the fact that approximately 50% of the common drugs humans ingest are metabolized by isoforms of cytochrome P450; these enzymes also act on various carcinogens and pollutants. The major cytochrome P450s in drug metabolism are members of the CYP1, CYP2, and CYP3 families (see below).

Isoforms of Cytochrome P450 Make Up a Superfamily of Heme-Containing Enzymes

The following are important points concerning cytochrome P450s.

1. Because of the large number of isoforms (about 150) that have been discovered, it became important to have a systematic nomenclature for isoforms of P450 and for their genes. This is now available and in wide use and is based on structural homology. The abbreviated root symbol CYP denotes a cytochrome P450. This is followed by an Arabic number designating the family; cytochrome P450s are included in the same family if they exhibit 40% or more amino acid sequence identity. The Arabic number is followed by a capital letter indicating the subfamily, if two or more members exist; P450s are in the same subfamily if they exhibit greater than 55% sequence identity. The individual P450s are then arbitrarily assigned Arabic numerals. Thus, CYP1A1 denotes a cytochrome P450 that is a member of family 1 and subfamily A and is the first individual member of that subfamily. The nomenclature for the genes encoding cytochrome P450s is identical to that described above except that italics are used; thus, the gene encoding CYP1A1 is CYP1A1.

2. Like hemoglobin, they are hemoproteins.

3. They are widely distributed across species, including bacteria.

4. They are present in highest amount in liver cells and enterocytes but are probably present in all tissues. In liver and most other tissues, they are present mainly in the membranes of the smooth endoplasmic reticulum, which constitute part of the microsomal fraction when tissue is subjected to subcellular fractionation. In hepatic microsomes, cytochrome P450s can comprise as much as 20% of the total protein. P450s are found in most tissues, though often in low amounts compared with liver. In the adrenal, they are found in mitochondria as well as in the endoplasmic reticulum; the various hydroxylases present in that organ play an important role in cholesterol and steroid biosynthesis. The mitochondrial cytochrome P450 system differs from the microsomal system in that it uses an NADPH-linked flavoprotein, adrenodoxin reductase, and a nonheme iron-sulfur protein, adrenodoxin. In addition, the specific P450 isoforms involved in steroid biosynthesis are generally much more restricted in their substrate specificity.

5. At least six different species of cytochrome P450 are present in the endoplasmic reticulum of human liver, each with wide and somewhat overlapping substrate specificities and acting on both xenobiotics and endogenous compounds. The genes for many isoforms of P450 (from both humans and animals such as the rat) have been isolated and studied in detail in recent years. The combination of there being a number of different types and each having a relatively wide substrate specificity explains why the cytochrome P450 family can metabolize thousands of different chemicals.

6. NADPH, not NADH, is involved in the reaction mechanism of cytochrome P450. The enzyme that uses NADPH to yield the reduced cytochrome P450, shown at the left-hand side of the above equation, is called NADPH-cytochrome P450 reductase. Electrons are transferred from NADPH to NADPH-cytochrome P450 reductase and then to cytochrome P450. This leads to the reductive activation of molecular oxygen, and one atom of oxygen is subsequently inserted into the substrate. Cytochrome b₅, another hemoprotein found in the membranes of the smooth endoplasmic reticulum (Chapter 12), may be involved as an electron donor in some cases.

7. Lipids are also components of the cytochrome P450 system. The preferred lipid is phosphatidylcholine, which is the major lipid found in membranes of the endoplasmic reticulum.

8. Most isoforms of cytochrome P450 are inducible. For instance, the administration of phenobarbital or of many other drugs causes hypertrophy of the smooth endoplasmic reticulum and a three- to fourfold increase in the amount of cytochrome P450 within 4-5 days. The mechanism of induction has been studied extensively and in most cases involves increased transcription of mRNA

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