PRINCIPLES OF TOXICOLOGY - Biology East Borneo

72 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATIONPhase I; ReductionsReductive metabolism in the liver endoplasmic reticulum can occur through the mediation of bothhemoprotein (cytochrome P450) and flavoproteins. Reductions of azo and nitro groups are the mostcommonly encountered (Figure 3.10), but reduction of disulfides, sulfoxides, epoxides, and N-oxidescan also occur. In many instances, the products of reductive metabolism can be reoxidized underaerobic conditions.Phase II; GlucuronidationGlucuronidations are catalyzed by a group of closely related 55,000-dalton isozymes, termed UDPglucuronosyltransferases,located within the endoplasmic reticulum. They catalyze the transfer ofglucuronic acid from a uridinediphosphoglucuronic acid (UDPGA) cofactor to a carboxyl or hydroxyl(phenol), or less often an amine group on the xenobiotic (or phase I metabolite) (Figure 3.3). TheUDPGA is generated from the abundant carbohydrate supply in the liver as glucose-1-phosphate, andfollowing the reaction with UTP, the resultant UDP-glucose is oxidized. The formation of theglucuronide does not involve the acid group of glucuronic acid, so the conjugate retains acid and ionizedcharacter at physiological pH, providing dramatic enhancement of water solubility and excretabilityto the xenobiotic. Glucuronides are actively secreted into bile and in the proximal tubule of the kidney.Xenobiotics conjugated as glucuronides can be released as either a phase I metabolite or the originalmolecule by the action of glucuronidases of both mammalian and microbial origin.UDP-glucuronosyltransferases occur in multiple forms. The most common classification utilizedfor the enzymes responsible for the metabolism of xenobiotics are those (GT1) that conjugate planarphenols (e.g., 1-naphthol, 4-nitrophenol) and are induced by polycyclic hydrocarbon-like molecules(see Table 3.6) and those (GT2) that conjugate nonplanar phenols (e.g. morphine, chloramphenicol)and are induced by phenobarbital and similar compounds. There are other forms which appear to bemore selective for endogenous substrates, notably those for the 17 hydroxysteroids (testosterone), the3 hydroxysteroids (androsterone) and bilirubin. More recent studies using the powerful techniques ofmolecular biology have provided a more rational classification system, but to aid the reader inunderstanding the bulk of existing literature, the old system has been used in this chapter. Likecytochrome P450s, UDP-glucuronosyltransferases are often substrate selective rather than substratespecific, being able to metabolize a wide range of compounds poorly (e.g., 4-nitrophenol is conjugatedby almost all isozymes) while metabolizing substrates with particular characteristics very efficiently.Also like cytochrome P450s, more than one form may be induced by a xenobiotic inducing agent (bothbilirubin and testosterone as well as morphine conjugations are induced by phenobarbital).Phase II; SulfationSulfate conjugation is an important alternative to glucuronidation for phenolic compounds andoccasionally arylamines. Sulfate availability within the cell may be limited, so this conjugation pathwaydecreases in importance with higher xenobiotic or phenolic metabolite concentrations. The 3′-phosphoadenosine-5′-phosphosulfate(PAPS) cofactor from which the sulfate group is transferred isgenerated from ATP and inorganic sulfate. The sulfate can be derived from the sulfur containing aminoacids, cysteine and methionine. The enzymes catalyzing the sulfate conjugations are a family ofcytosolic 64,000-dalton enzymes, termed sulfotransferases, and are one of the exceptions to the majorgroups of drug metabolizing enzymes in that they appear to not be induced by xenobiotic compounds(see Table 3.6). The sulfates are completely ionized at physiological pH and easily eliminated. Muchlike glucuronides, enzymes exist (termed sulfatases) that can break the conjugate and return thexenobiotic, if it is phenolic, or the phase I metabolite of a xenobiotic, if it was oxidized or hydrolyzedto that functional group.