GENOTOXICITY OF AFLATOXIN B-1 - EVIDENCE FOR A RECOMBINATION-MEDIATEDMECHANISM IN SACCHAROMYCES-CEREVISIAE

The potent liver carcinogen aflatoxin B-1 (AFB1) is metabolized by cyt ochrome P450 to the mutagenic epoxide. We have observed that activated AFB1 also strongly induced mitotic recombination in the yeast Sacchar omyces cerevisiae. To compare the recombinogenicity of AFB1 to its mut agenicity, three metabolically competent S. cerevisiae strains have be en constructed. The frequencies of induced recombinants resulting from gene conversion or chromosomal translocations were determined by diff erent prototrophic selections using two strains, whereas the inducibil ity of forward mutations was determined by the frequency of drug resis tance in the third strain. Human cytochrome P4501A1- (CYP1A) and NADPH -cytochrome P450-oxidoreductase cDNAs were expressed in the strains to ensure intracellular metabolism to the epoxide. Exposure of the strai ns to AFB1 resulted in a 139- and 24-fold increase in the translocatio n and gene conversion frequencies, respectively, whereas the mutation frequency was increased only 3-fold. In contrast, benzo[a]pyrene-7,8-d ihydrodiol and ethyl methanesulfonate induced mutation and mitotic rec ombination to similar degrees. We conclude that AFB1 exerted a strong recombinogenic, but only a weak mutagenic, effect. The recombinogenici ty of AFB1 in yeast may indicate a mechanism for the high proportion o f loss of heterozygosity that has been detected in AFB1-related human liver cancers.