The metabolic oxidation of the ethynyl group in 4-ethynylbiphenyl in vitro.

1. The metabolism of 4-ethynylbiphenyl has been studied in vitro with subcellular fractions of normal and induced rat liver, and rat intestinal microflora (caecal contents). 2. Oxidation was NADPH-dependent, was inhibited by CO and stimulated by pretreatment with phenobarbitone or 3-methylcholanthrene. 3. Oxidation of the ethynyl group occurred in washed microsomal preparations, but not significantly in soluble fractions. Oxidation of the ethynyl group by a microsomal fraction preceded aromatic hydroxylation and no metabolites containing the intact ethynyl group were detected. 4. The major metabolite in liver fractions was biphenyl-4-ylacetic acid. This was the only product produced by a modified Udenfriend system. 5. Metabolism of 4-ethynylbiphenyl by rat caecal contents under anaerobic conditions produced very small amounts of 4-vinylbiphenyl. 6. In a modified Ames test with Salmonella typhimurium TA98, 4-ethynylbiphenyl gave a weak positive result that was doubled after 'activation' with an induced rat S9 fraction.