Enzymatic Interconversion of Aflatoxin B₁ and Aflatoxicol by Rhizopus sp.

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We found that an enzyme extracted from mycelia of Rhizopus sp. had potent interconversion activities of aflatoxin B₁ (AF B₁) and aflatoxicol A (AFL-A). The enzyme was suggested to be an NADP-dependent oxidoreductase, since both the oxidative conversion of AFL-A to AF B₁ and the reductive conversion of AF B₁ to AFL-A were found with completely the same patterns in fractions obtained by gel permeation and ion exchange chromatography. The enzyme had an apparent molecular weight of 40000 in gel permeation chromatography on Sephacryl S-200HR. The optimum pH for conversion of AF B₁ to AFL-A was around 6.0 to 7.0, while that of AFL-A to AF B₁ was around 7.5 to 9.0, at 35°C. The optimum temperature was 35°C for both conversions of AF B₁ to AFL-A and of AFL-A to AF B₁. Furthermore, only AFL-A, the (S)-isomer was produced from AF B₁ by catalysis by NADP-dependent oxidoreductase. However, we were unable to purify the enzyme to homogeneity by ammonium sulfate saturation, by chromatography on Sephacryl S-200HR, DEAE-Sepharose Fast Flow and 2', 5'ADP-Sepharose 4B.