Mechanism-based inactivation of N-arylhydroxamic acid N,O-acyltransferase by 7-substituted-N-hydroxy-2-acetamidofluorenes

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Abstract
N-Arylhydroxamic acid N,O-acyltransferase (AHAT) catalyzes the transfer of the N-acetyl group from N-aryl-hydroxamic acids [some of which are mutagens and carcinogens] to arylamines. In the absence of an arylamine acceptor, AHAT catalyzes the conversion of N-arylhydroxamic acids to reactive electrophilic intermediates that become irreversibly bound to cellular nucleophiles, including those present on AHAT itself. As part of an investigation of the AHAT-catalyzed bioactivation process, a series of 7-substituted analogs of N-hydroxy-2-acetamidofluorene was synthesized and evaluated in vitro as substrates and inactivators of a partially purified hamster hepatic AHAT preparation. All of the compounds functioned as acetyl donors in the AHAT-catalyzed transacetylation of 4-aminoazobenzene (AAB) and all of them were inactivators of AHAT. The inactivation process exhibited apparent 1st-order kinetics, and the 7-methoxy compound exhibited the largest inactivation rate constant. Quantitative structure-activity analysis provided support for the concept that positively charged species are involved in the inactivation of AHAT by this series of compounds. Results of experiments in which nucleophilic trapping agents such as glutathione, Cys, Met, guanosine, phosphate and tRNA were included in incubation mixtures with AHAT and the N-arylhydroxamic acids indicated that electrophiles which diffuse away from the enzyme active site participate in the inactivation process.

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