Modulation of aflatoxin B 1 biotransformation in rabbit pulmonary and hepatic microsomes

Abstract

Aflatoxin B 1 (AFB 1) is a carcinogenic mycotoxin that requires activation to the corresponding 8,9-epoxide for activity. In addition to being present in foodstuffs, AFB 1 can contaminate respirable grain dusts and thus the respiratory system is a potential target for carcinogenesis. In the present study, we have investigated the role of polycyclic aromatic hydrocarbon-inducible forms of cytochrome P-450 in the pulmonary and hepatic microsomal activation ([ 3H]AFB 1-DNA binding) and detoxification ([ 3H]AFM 1 and [ 3H]AFQ 1 formation) of [ 3H]AFB 1. In rabbit lung microsomes, the apparent V max for [ 3H]AFM 1 formation was increased significantly when values were expressed per mg microsomal protein or per nmol P-450 present. In liver microsomes, the apparent V max for DNA binding and [ 3H]AFM 1 formation were increased by β-naphthoflavone (BNF) treatment (to 2.3 and 3.3 times control, respectively) when expressed per mg protein, but when expressed per nmol P-450, only AFM 1 formation was significantly increased. The apparent K m values for both these reactions were unaffected. The apparent V max for [ 3H]AFQ 1 formation was not affected by BNF treatment, but the apparent K m was increased to 4.5 times control. Boiling of microsomes or omitting the NADPH-generating system decreased DNA binding, AFM 1 formation and AFQ 1 formation by 89–97%, while addition of 1.0 mM SKF-525A inhibited these reactions by 46–57%. Addition of 1.0 mM α-naphthoflavone (ANF) had no effect on the biotransformation of [ 3H]AFB 1 in lung microsomes of control rabbits, but significantly decreased AFM 1 formation (by 31%) in lung microsomes from BNF-treated animals (other reactions were unaffected). In liver microsomes from BNF treated rabbits, 1.0 mM ANF inhibited DNA binding of [ 3H]AFB 1 by 68%, while there was no effect in control microsomes. ANF significantly inhibited AFM 1 formation in liver microsomes from both control and BNF-treated animals (by 87–97% and 67–78% at 1.0 mM and 2.0 μM, respectively), but had no effect on AFQ 1 formation in liver microsomes from animals in either treatment group. These results indicate an important role for the 1A subclass of P-450 isozymes in the biotransformation of AFB 1 to AFM 1 in rabbit lung and liver, and a minor role in AFB 1 activation in liver.