Mechanism of Metabolic Activation of the Analgetic Bucetin to Bacterial Mutagens by Hamster Liver Microsomes

Bucetin (N-(β-hydroxybutyryl)-p-phenetidine) was found to be mutagenic to Salmonella typhimurium TA100 in the presence of liver 9000g supernatant fractions (S9) prepared from polychlorinated biphenyl (PCB)-treated hamsters and a reduced nicotinamide adenine dinucleotidephosphate (NADPH)-generating system. However, the analgetic was not mutagenic in the presence of NADPH-fortified S9 from PCB-treated rat liver. The mutagenic potency of bucetin was about a quarter of that of the structurally related analgetic, phenacetin. PCB-treated hamster liver microsomes fortified with NADPH activated bucetin to two direct-acting mutagens, N-hydroxyphenetidine and p-nitrosophenetole, through deacylation followed by N-hydroxylation. The nitroso compound arose from N-hydroxyphenetidine via autoxidation. N-(β-Hydroxybutyryl)-p-aminophenol, a major metabolite of bucetin under the conditions used, was not mutagenic to TA 100 either with or without NADPH-fortified S9 from PCB-treated or untreated rats or hamsters. N-Hydroxybucetin, which was about 70 times less mutagenic than N-hydroxyphenacetin in the presence of PCB-treated hamster S9, was not detected as a metabolite of bucetin from the NADPH-fortified reaction mixtures. Although no species difference was observed in p-phenetidine N-hydroxylation, the rate of bucetin deacylation was over 90 times higher in hamsters than in rats. The rate of microsomal deacylation of bucetin was much lower than that of phenacetin or N-butyryl-p-phenetidine. These results suggest that the species difference in bucetin mutagenicity is due to the difference in deacylating activity between rat and hamster liver microsomes, and also that the β-hydroxyl group in the butyryl side chain makes bucetin poorly hydrolyzable in microsomes, resulting in lower mutagenic activity as compared with phenacetin.