Among members of the mouse cytochrome P450 2A family, P450 2A5 is the best catalyst of aflatoxin B1 (AFB1) oxidation to its 8, 9-epoxide (Pelkonen, P., Lang, M., Wild, CP, Negishi, M., and Juvonen, RO (1994) Eur. J. Pharmacol., Environ. Toxicol. Pharmacol. Sect. 292, 67- 73). Here we studied the role of amino acid residues 209 and 365 of the P450 2A5 in the metabolism and toxicity of AFB1 using recombinant yeasts. The two sites have previously ...