title: 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 degrees C) postmicrosomal fraction. The reaction catalyzed by MA was optimum at pH 7.0 and at 17-23 degrees C, whereas the PMA reaction was optimum at pH 8.0-8.5 and at 35-40 degrees C. Apparent Km values of approximately 2.6 X 10(-6) M (for ST) and 6.6 X 10(-7) M (for OMST) were determined for PMA and MA, respectively.
mesh_terms: D016604:Aflatoxin B1; D000348:Aflatoxins / Q000096:biosynthesis*; D001230:Aspergillus / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D009243:NAD / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D013241:Sterigmatocystin / Q000378:metabolism*; D013347:Subcellular Fractions / Q000201:enzymology; D013816:Thermodynamics; D014966:Xanthenes / Q000378:metabolism*
chemical_list: D000348:Aflatoxins; D014966:Xanthenes; D009243:NAD; D013241:Sterigmatocystin; D009249:NADP; D016604:Aflatoxin B1