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 constructedthat co-express cDNAs coding for the human cytochrome P-450enzymes CYP1A1 or CYP1A2 in combination with human NADPH-cytochromeP-450 reductase (oxidoreductase). Microsomal fractions preparedfrom the strains were able to efficiently activate various drugsto Salmonella mutagens. These experiments demonstrated thata functional interaction occurred between the respective humanenzymes in the yeast microsomes. For every drug tested, themicrosomes containing CYP enzymes and oxidoreductase were 2-to 4-fold better in activation than the corresponding microsomesthat contained CYP alone. Interestingly, co-expression of CYP1A2with oxidoreductase resulted in a decrease of 7-ethoxyresorufin-O-deethylaseactivity, a problem which is related to this specific substrate.Using the microsomes, it was demonstrated that aflatoxin B₁,was activated to a mutagen not only by CYP1A2 but also by CYP1A1.In contrast, benzo[a]pyrene was exclusively activated by CYP1A1whereas CYP1A2 was inactive. The drug 3-amino-1-methyl-5H-pyrido[4,3-b]indole(Trp-P-2) was activated by CYP1A2 and to a lesser extent byCYP1A1. A strong substrate specificity was observed with thetwo structurally related heterocyclic 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 activated efficiently by both CYP enzymes, whereasMeIQ was only activated by CYP1A2 and not by CYP1A1. The factthat microsomes from vector transformed control strains wereunable to activate any of the drugs studied underlines the suitabilityof these microsomes for metabolic studies. Moreover, the presenceof suitable marker genes in the yeast strains will enable usto study mitotic recombination and gene conversion events inducedby drugs that require metabolic activation.