Molecular Pharmacology, Vol 10, 776-789, Copyright © 1974 by the American Society for Pharmacology and Experimental Therapeutics

Metabolism of Aflatoxin B₁ and Its Metabolism-Dependent and Independent Binding to Rat Hepatic Microsomes

¹ Department of Experimental Therapeutics and J. T. Grace, Jr., Cancer Drug Center, Roswell Park Memorial Institute, Buffalo, New York 14203, and Department of Biochemistry and Nutrition, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

The NADPH-mediated metabolism of aflatoxin B₁ by rat hepatic microsomes was studied by means of difference spectroscopy. A decrease in absorbance at 360 nm, due to the disappearance of aflatoxin B₁ during its metabolism, was caused by the formation of a metabolite having a spectral peak at 398 nm. From the solubility, spectral, and microsomal binding characteristics, this metabolite appears to be aflatoxin B_2a (aflatoxin hemiacetal). Difference spectroscopic studies on microsomes reisolated from incubation mixtures containing aflatoxin B₁ with or without NADPH showed that aflatoxin B₁ was bound to microsomes, giving a complex having a spectral peak at 360 nm; this binding did not require NADPH and the complex could be separated by gel filtration chromatography. A metabolite of aflatoxin B₁, the formation of which was NADPH-dependent, was bound to microsomes, yielding a complex having a spectral peak at 405 nm; this complex was not dissociated by gel chromatography on by treatment with trichloracetic acid or extraction with chloroform and acetone. SKF 525-A and L-cysteine inhibited the formation of this complex, the former by inhibiting the metabolic conversion of aflatoxin B₁ to aflatoxin B_2a and the latter by blocking the binding of the metabolite to microsomes. Aflatoxin B₁ and aflatoxin B_2a appeared to bind to different sites on the microsomes. A change in pH caused an alteration in the structure of aflatoxin B_2a and its extraction with chloroform. pH equilibria for this phenomenon were estimated to be 3.00 and 7.35. Based on these studies, it is proposed that aflatoxin B_2a, under alkaline conditions, cleaves to yield a dialdehyde derivative(s) which binds to microsomes, forming Schiff bases with free amino groups. These studies also indicate that, under certain conditions, rat hepatic microsome-mediated conversion of aflatoxin B₁ to a water-soluble metabolite, which appears to be aflatoxin B_2a, represents a major metabolic pathway.

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ACKNOWLEDGMENTS We gratefully acknowledge the valuable criticism provided by Drs. H. Schwartz, C. Dave, and F. Rosen during the preparation of this manuscript, and are also thankful to Miss L. Caballes for her assistance in radiochemical experiments and to Mr. L. Motycka for his skilled technical assistance. We also wish to express our deep appreciation and thanks to Dr. E. Mihich for his continued encouragement and interest in this work.