Individual reaction requirements of two enzyme activities, isolated from Aspergillus parasiticus, which together catalyze conversion of sterigmatocystin to aflatoxin B1

Abstract

Individual reaction requirements were determined for each of two enzyme activities present in Aspergillus parasiticus mycelia which together catalyze conversion of sterigmatocystin (ST) to aflatoxin B1 (AFB1). A postmicrosomal activity (PMA) catalyzed conversion of ST to O-methylsterigmatocystin (OMST) and a microsomal activity (MA) catalyzed conversion of OMST to AFB1. PMA was stimulated two- to three-fold in the presence of S-adenosylmethionine. Addition of NADPH promoted the maximum MA; this activity was not detected when FAD, FMN, NAD, or NADH were utilized individually as cofactors in reaction mixtures. A substantial amount (62%) of MA was lost during isolation of the microsomal fraction, but the activity was completely restored by reconstitution with a heat-treated (100 °C) postmicrosomal fraction. The reaction catalyzed by MA was optimum at pH 7.0 and at 17–23 °C, whereas the PMA reaction was optimum at pH 8.0–8.5 and at 35–40 °C. Apparent Km values of approximately 2.6 × 10−6 M (for ST) and 6.6 × 10−7 M (for OMST) were determined for PMA and MA, respectively.