Modification of aflatoxin B₁ biotransformation in vitro and DNA binding in vivo by dietary broccoli in rats

Abstract

Cruciferous vegetables have been shown to have anticarcinogenic effects in animals but biochemical mechanisms have not been completely elucidated. The effects of dietary broccoli on in vivo DNA binding of the hepatocarcinogen aflatoxin B₁ (AFB) and in vitro formation of the putative carcinogenic intermediate, AFB‐8,9‐epoxide, as well as detoxification of the epoxide by conjugation with glutathione (CSH), were examined in this study. Animals were fed a purified diet, a purified diet plus 25% freeze‐dried broccoli, or standard rodent chow for 3 wk. In vivo binding of AFB to hepatic DNA was determined. Biotransformation of AFB in vitro (microsomal oxidation to AFB‐8,9‐epoxide, as well as hydroxylated metabolites, and cytosolic GSH conjugation of AFB‐8,9‐epoxide generated in situ) was measured by an HPLC method that allows specific and direct determination of AFB metabolites and thus, the rates of their formation. Microsomal mixed‐function oxidase and epoxide hydrolase activities and cytosolic glutathione S‐transferase activities were also measured with commonly used surrogate substrates. The rate of cytosolic conjugation of AFB‐8,9‐epoxide was increased 2.8‐fold by the broccoli diet and 2.2‐fold by the chow diet. These changes were not fully reflected by increases in activity with surrogate substrates. The chow diet did not affect epoxide hydrolase activity nor glutathione S‐transferase activity toward 3,4‐dichloronitrobenzene or benzo[alpyrene 4,5‐oxide, whereas these activities were significantly increased by the broccoli diet. Microsomal formation of AFB‐8,9‐epoxide was unaffected by the dietary treatments, whereas formation of aflatoxin M₁ was increased. The chow diet, but not the broccoli diet, increased the amount of aflatoxin Q₁ formed from AFB. Binding of AFB to DNA in vivo was significantly lower in the broccoli group but not in the chow‐fed animals. These results indicate that broccoli contains substances that cause a reduction in the binding of AFB metabolites to DNA, possibly through the induction of glutathione S‐transferase(s). Broccoli and rodent chow differ in their constituents that increase levels of xenobiotic biotransformation enzymes relative to a purified diet. The results also indicate the limitations of reliance on measurements of biotransformation pathways using surrogate substrates instead of carcinogenic compounds of interest.