Cleveland T E, Bhatnagar D
United States Department of Agriculture, Southern Regional Research Center, New Orleans, LA 70179.
Can J Microbiol. 1987 Dec;33(12):1108-12. doi: 10.1139/m87-193.
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.
已确定寄生曲霉菌丝体中存在的两种酶活性各自的个体反应要求，这两种酶共同催化柄曲霉素（ST）转化为黄曲霉毒素B1（AFB1）。一种微粒体后活性（PMA）催化ST转化为O - 甲基柄曲霉素（OMST），一种微粒体活性（MA）催化OMST转化为AFB1。在S - 腺苷甲硫氨酸存在下，PMA受到两到三倍的刺激。添加NADPH可促进MA达到最大值；当在反应混合物中单独使用FAD、FMN、NAD或NADH作为辅因子时，未检测到这种活性。在微粒体部分分离过程中，大量（62%）的MA活性丧失，但通过用热处理（100℃）的微粒体后部分进行重组，该活性完全恢复。MA催化的反应在pH 7.0和17 - 23℃时最佳，而PMA反应在pH 8.0 - 8.5和35 - 40℃时最佳。分别测定了PMA和MA的表观Km值，约为2.6×10⁻⁶ M（对于ST）和6.6×10⁻⁷ M（对于OMST）。
