PRINCIPLES OF TOXICOLOGY - Biology East Borneo

62 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATION3.1 SITES OF BIOTRANSFORMATIONXenobiotic metabolism occurs in all organs and tissues in the body. Because many of the chemicalsmetabolized can have deleterious effects on the body, xenobiotic metabolism can be considered adefense mechanism that hastens the elimination of a toxic chemical and thus terminates the exposure.When viewed as a defense mechanism, it is not surprising that the exposure is best terminated at thepoint of exposure. These are the so-called portals of entry (shown as sites of absorption [a] forxenobiotics [X] in Figure 3.1), and constitute mainly the skin, lung, and intestinal mucosa. While drugmetabolizing enzymes are present in all these tissues (Table 3.2), and at relatively high activity in some,particularly intestine and lung, the liver is by far the most important tissue for xenobiotic metabolism(site [m] in Figure 3.1).Although it is not the first tissue of the body to be exposed to chemicals, the liver receives the entirechemical load absorbed from the gastrointestinal tract, which is the predominant portal of entry formost xenobiotics (Figure 3.1). The xenobiotic metabolizing enzymes are present in high concentrationsand the organ itself has large bulk, approximately 5 percent of the total body weight. Xenobioticsabsorbed from the lungs and skin can also quickly move to the liver for metabolism. Once in the liver,the highly vascular nature of the tissue and the intimate contact between blood and hepatocytes, whichcontain the xenobiotic metabolizing enzymes, allows for the rapid diffusion of chemicals in andmetabolites out (Figure 3.5).Although not a portal of entry, the kidney is an organ where xenobiotics are likely to beconcentrated during the excretion process, and this may be the reason for the relatively high levelof xenobiotic metabolizing enzymes in this tissue. Although the data presented in Table 3.2 arefrom laboratory animals, there is little evidence to contraindicate the existence of a similardistribution pattern in humans.Within the liver, hepatocytes or parenchymal cells are the major site of drug biotransformation,and within these cells it is the endoplasmic reticulum, which occupies about 15 percent of thehepatocyte volume and contains 20 percent of the hepatocyte protein, which houses the bulk ofthe critical drug metabolizing enzyme activity. (The nonparenchymal cells, including endothelialand Kupffer cells, constitute 35 percent of liver cell number but only contribute 5–10 percent ofliver mass. Their drug metabolizing enzyme activities are typically less than 20 percent of that inhepatocytes).When liver is carefully homogenized, fragments of the endoplasmic reticulum are converted tomicrosomes (an artifact of cell disruption). The drug-metabolizing enzymes located in the endoplasmicreticulum are often referred to as microsomal enzymes, and it is often stated that chemicals aremetabolized by liver microsomes. Enriched microsomal fractions are usually obtained by differentialsedimentation, either as a suspension with cytoplasm (10,000g supernatant) or as a sediment free ofcytosol (105,000g precipitate) (Table 3.3).Many important xenobiotic metabolizing enzymes reside in the cytoplasm and microsomal fractions(Figures 3.3 and 3.6).Oxidations and glucuronidations are the most common reactions occurring in microsomes. Theterminal oxidase responsible for many of the oxidations, cytochrome P450, represents about 5 percentof the microsomal protein under normal conditions; more if induction has occurred (see text below).Other flavoproteins necessary for cytochrome P450 function and epoxide hydrolase, an enzymeimportant in the further metabolism of epoxides formed by cytochrome P450–dependent oxidation,are also conveniently located in the endoplasmic reticulum (Figure 3.6). Microsomal metabolism intissues other than liver is seldom quantitatively important in overall drug elimination, but localgeneration of active metabolites may be important in drug-induced tissue damage, carcinogenesis, andother effects. Enzymes located in the cytoplasm of the hepatocyte catalyze a wide variety of both phaseI and phase II reactions. Dehydrogenases and esterases are examples of phase I enzymes foundpredominantly in the cytosol. The sulfotransferase and glutathione transferase enzymes also depictedin Figure 3.6 serve as examples of phase II enzymes that are similarly located.