Arylhydroxamic acid N,O-acyltransferase substrates. Acetyl transfer and electrophile generating activity of N-hydroxy-N-(4-alkenyl-, and 4-cyclohexylphenyl)acetamides

Article abstract of DOI:10.1021/jm00348a005

Arylhydroxamic acid N,O-acyltransferase (AHAT) is an enzyme system that is capable of converting many N-arylhydroxamic acids into reactive electrophilic species. As part of an investigation into the influence of the structure of the aryl group upon the ability of N-arylhydroxamic acids to serve as substrates for AHAT, a series of N-hydroxy-N-(4-alkyl-, 4-alkenyl-, and 4-cyclohexylphenyl) acetamides was prepared and evaluated in vitro with partially purified rat and hamster hepatic AHAT. The nature of the 4-substituent markedly influenced the ability of the hydroxamic acids to serve as acetyl donors in the AHAT-catalyzed transacetylation of 4-aminoazobenzene (AAB). As the length of the 4-substituent was increased from methyl to pentyl, the compounds became increasingly more effective substrates. The compounds containing vinyl, propenyl, and 2-methylpropenyl 4-substituents were more effective acetyl donors than the corresponding compounds containing saturated 4-substituents. The three most effective AHAT substrates in the AAB transacetylation assay were N-hydroxy-N-(4-pentylphenyl)- (7), N-hydroxy-N-(4-propenylphenyl)- (10), and N-hydroxy-N-[4-(2-methylpropenyl)phenyl]acetamide (11), each of which was approximately as active as the standard compound, N-hydroxy-4-acetamidobiphenyl (1), with rat hepatic AHAT and approximately 60% as active as 1 with hamster hepatic AHAT. Both 1 and N-hydroxyl-N-(4-cyclohexylphenyl)acetamide (8) were activated by hamster hepatic AHAT to yield electrophilic intermediates that formed adducts with 2-mercaptoethanol. The 2-mercaptoethanol adducts were characterized by mass spectrometry and were identified as 4-phenyl-2-[(2-hydroxyethyl)thio]aniline (22) and 4-cyclohexyl-2-[(2-hydroxyethyl)thio]aniline (21). The structure of compounds 21 and 22 were confirmed by an unambiguous chemical synthesis. Both compounds 1 and 8 irreversibly inactivated hamster hepatic AHAT by a time-dependent process. The results of the inactivation experiments confirmed that 1 inactivates AHAT primarily via a suicide substrate mechanism and revealed that 8 inactivates the enzyme by a process consisting primarily of a pathway in which electrophiles are released into the medium and subsequently react with nucleophiles present on AHAT.