PHASE-II ENZYMES AND BIOACTIVATION

A colloquium entitled Phase II enzymes and bioactivation was held duri ng the 10th International Symposium on Microsomes and Drug Oxidations in Toronto, Ont., on July 20, 1994. This colloquium was a tribute in r ecognition of the contributions by Dr. James R. Gillette in advancing our understanding of drug metabolism and chemical toxicity. A major fo cus of the colloquium was formation of conjugates such as those with g lutathione (GSH) that may not lead to detoxification but to bioactivat ion. The GSH conjugates may be further metabolized to reactive species that cause toxicity. The nephrotoxicity of hydroquinone and bromobenz ene is mediated via quinone-glutathione conjugates, and is manifested in cellular changes, including induction of the gadd-153 and hsp-70 mR NA. The formation of GSH conjugates is also involved in the bioactivat ion of the vicinal dihalopropane 1,2-dibromo-3-chloropropane; cytotoxi c lesions are observed in the kidney and testes. The evidence indicate s that conjugation is mediated by the GSH S-transferases. The symposiu m also covered aspects of the importance of conjugation in the pharmac okinetics 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. C onjugation 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 smal l molecular weight proteins may be considered as a phase II reaction a nd a mechanism of detoxification. A different pathway for the metaboli sm of xenobiotics is catalyzed by the carboxylesterases, a family of e nzymes that is involved in hydrolysis of chemical compounds, generally leading to detoxification. Three rat esterases have been purified, cl oned, and characterized. Two forms, hydrolase A and hydrolase B, are p resent in liver microsomes in a number of species, including the human . These are also detected in extrahepatic tissues. A third esterase, h ydrolase 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 me chanisms of the effects of chemicals such as phenacetin and acetaminop hen, 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 d rug metabolism. These data also indicate that significant insights are likely to come from integrated approaches utilizing established toxic ological techniques together with those from other disciplines, includ ing molecular biology and analytical chemistry.