HIGH PROMUTAGEN ACTIVATING CAPACITY OF YEAST MICROSOMES CONTAINING HUMAN CYTOCHROME-P-450 1A AND HUMAN NADPH-CYTOCHROME P-450 REDUCTASE

Yeast Saccharomyces cerevisiae strains have been constructed that co-e xpress cDNAs coding for the human cytochrome P-450 enzymes CYP1A1 or C YP1A2, in combination with human NADPH-cytochrome P-450 reductase (oxi doreductase). Microsomal fractions prepared from the strains were able to efficiently activate various drugs to Salmonella mutagens. These e xperiments demonstrated that a functional interaction occurred between the respective human enzymes in the yeast microsomes. For every drug tested, the microsomes containing CYP enzymes and oxidoreductase were 2- to ii-fold better in activation than the corresponding microsomes t hat contained CYP alone. Interestingly, co-expression of CYP1A2 with o xidoreductase resulted in a decrease of 7-ethoxyresorufin-O-deethylase activity, a problem which is related to this specific substrate. Usin g the microsomes, it was demonstrated that aflatoxin B-1 was activated to a mutagen not only by CYP1A2 but also by CYP1A1. In contrast, benz o[a]pyrene was exclusively activated by CYP1A1 whereas CYP1A2 was inac tive. The drug 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) was a ctivated by CYP1A2 and to a lesser extent by CYP1A1. A strong substrat e specificity was observed with the two structurally related heterocyc lic arylamines 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQ(x)). MeIQ(x) was a ctivated efficiently by both CYP enzymes, whereas MeIQ was only activa ted by CYP1A2, and not by CYP1A1. The fact that microsomes from vector transformed control strains were unable to activate any of the drugs studied underlines the suitability of these microsomes for metabolic s tudies. Moreover, the presence of suitable marker genes in the yeast s trains will enable us to study mitotic recombination and gene conversi on events induced by drugs that require metabolic activation.