Phase II enzymes and bioactivation

Abstract

A colloquium entitled Phase II enzymes and bioactivation was held during the 10th International Symposium on Microsomes and Drug Oxidations in Toronto, Ont., on July 20, 1994. This colloquium was a tribute in recognition of the contributions by Dr. James R. Gillette in advancing our understanding of drug metabolism and chemical toxicity. A major focus of the colloquium was formation of conjugates such as those with glutathione (GSH) that may not lead to detoxification but to bioactivation. The GSH conjugates may be further metabolized to reactive species that cause toxicity. The nephrotoxicity of hydroquinone and bromobenzene is mediated via quinine–glutathione conjugates, and is manifested in cellular changes, including induction of the gadd-153 and hsp-70 mRNA. The formation of GSH conjugates is also involved in the bioactivation of the vicinal dihalopropane 1,2-dibromo-3-chloropropane; cytotoxic lesions are observed in the kidney and testes. The evidence indicates that conjugation is mediated by the GSH S-transferases. The symposium also covered aspects of the importance of conjugation in the pharmacokinetics of certain drugs. Conjugation reactions including sulfation are markedly influenced by the manner in which the liver processes the drug. Characteristics such as erythrocyte binding, as in the case of acetaminophen, become limiting factors in the conjugation reactions. Conjugation reactions can lead to a different outcome, such as acquired drug resistance. Conjugation of metallothioneins with the alkylating mustard drugs melphalan and chlorambucil can lead to the formation of protein adducts. Conjugation of reactive intermediates with these small molecular weight proteins may be considered as a phase II reaction and a mechanism of detoxification. A different pathway for the metabolism of xenobiotics is catalyzed by the carboxylesterases, a family of enzymes that is involved in hydrolysis of chemical compounds, generally leading to detoxification. Three rat esterases have been purified, cloned, and characterized. Two forms, hydrolase A and hydrolase B, are present in liver microsomes in a number of species, including the human. These are also detected in extrahepatic tissues. A third esterase, hydrolase S, is found in rat liver microsomes and rat serum, and may be a serum carboxylesterase secreted from the liver. A better knowledge of esterases will advance our understanding of pharmacokinetics and mechanisms of the effects of chemicals such as phenacetin and acetaminophen, two drugs that Dr. Gillette has worked with extensively. The data presented herein reflect the new and innovative approaches that have been adopted to investigate various aspects of chemical toxicity and drug metabolism. These data also indicate that significant insights are likely to come from integrated approaches utilizing established toxicological techniques together with those from other disciplines, including molecular biology and analytical chemistry.Key words: drug metabolism, drug disposition, toxicity, conjugation.