The differential mutagenicity of isoniazid in fluctuation assays and Salmonella plate tests.

The anti-tuberculostatic drug, isoniazid (INH) was evaluated for its mutagenic potential using Salmonella plate tests and fluctuation assays with various strains of bacteria, and different metabolic activation systems. In the Salmonella plate test INH proved to be a weak directly-acting base-substitution mutagen which was detoxified by S9-mix. S. typhimurium TA 1530 and TA 1535 were the sensitive strains, and this result confirmed some of the published data. In the present studies mutagenic activity was further diminished in the presence of larger concentrations of rat liver S9-mix. Furthermore, the reduction in mutagenic activity was observed with S9-mix derived from untreated, Aroclor 1254-treated or phenobarbitone/beta-naphthoflavone treated rats. In direct contrast, using the microtitre fluctuation assay, the mutagenic activity of INH was elevated in the presence of rat liver S9-mix, and continued to increase with increasing S9-concentration. This result was obtained irrespective of the S9-source. S. typhimurium strains TA 1530, TA 1535 and his G46, and E. coli strains TA 85, TA 86 and WP2 uvrA were all sensitive to the mutagenicity of INH after metabolic activation. The primary step in the metabolic activation of INH in the fluctuation test was mediated by a cytosolic enzyme, and the activity of dapsone as a competitive substrate implicated the involvement of an N-acetyl transferase. The rapid diffusion of the cytosolic enzyme into the basal agar layer, or the non-specific binding of the enzyme (or the active mutagenic INH metabolite) to components of the agar, may explain the contradictory data obtained in the Salmonella plate test. The modifying effects of agar on the distribution of drug metabolising enzymes within liver S9 fractions should be carefully considered when evaluating data from Salmonella plate tests.