Metabolism of Aflatoxin B₁ in the Isolated Nuclei of Rat Liver

Abstract

The nuclear biotransformation of aflatoxin B₁ in vitro was observed with regard to inducer specificity, pH dependency, time course, kinetics, inhibitor sensitivity, and nuclear localization, and these data were compared with those from the microsomal transformation of aflatoxin B₁. The nuclei and microsomes are capable of metabolizing aflatoxin B₁ into aflatoxin M₁, aflatoxin Q₁, and two unidentified fluorescent compounds in the presence of fortified NADPH generating system. Pretreatments of rats by 3-methylcholanthrene or polychorinated biphenyl enhanced both the nuclear and microsomal C-9α-hydroxylation of aflatoxin B₁ into aflatoxin M₁ and phenobarbital or polychlorinated biphenyl induced aflatoxin Q₁ production. The optimal pHs for aflatoxin M₁ and Q₁ were 8.3 and 7.4, respectively, both in the nuclei and microsomes. Kinetic analysis revealed the K_m of aflatoxin M₁ formation in methylcholanthrene-induced nuclei was 9.4×10^-5M, and this value was very close to that obtained with the microsomes. Inhibitor experiments revealed a high sensitivity of aflatoxin M₁ formation to 7, 8-benzoflavone and a low sensitivity of aflatoxin Q₁ to SKF 525 A. These findings and data on the detergent treatment of nuclei suggest that the nuclear cytochrome P-448 system, induced by 3-methyl-cholanthrene and localized in the outer membrane, catalyzes the aflatoxin M₁ formation, and the cytochrome P-450 system induced by phenobarbital biotransforms aflatoxin B₁ into aflatoxin Q₁.
Pretreatment of rats by phenobarbital was found to induce microsomal degradation or detoxication of aflatoxin B₁ into water-soluble metabolites, and no such an induction was observed in the nuclei.