RESISTANCE TO AFLATOXIN-B1 IS ASSOCIATED WITH THE EXPRESSION OF A NOVEL ALDO-KETO REDUCTASE WHICH HAS CATALYTIC ACTIVITY TOWARDS A CYTOTOXIC ALDEHYDE-CONTAINING METABOLITE OF THE TOXIN

Fischer 344 rats readily develop liver cancer when exposed to aflatoxi n B1 (AFB1) but dietary administration of the antioxidant ethoxyquin ( EQ) provides protection against hepatocarcinogenesis. Chemoprotection by EQ is accompanied by the overexpression of enzymes which detoxify a ctivated AFB1. Aflatoxin-protein adduct formation takes place followin g metabolism of AFB1 to the dialdehydic form of AFB1-dihydrodiol. The dialdehyde can be detoxified by reduction to a dialcohol through the c atalytic actions of an enzyme present in the hepatic cytosol from rats fed EQ-containing diets; this metabolite is essentially undetectable in reaction mixtures that use hepatic cytosol from rats fed control di ets. The enzyme responsible for catalyzing the formation of dihydroxy- aflatoxin B1 has been purified from the livers of rats fed on diets su pplemented with EQ. It is a soluble monomeric protein with an approxim ate M(r) of 36,600. Besides its activity toward AFB1 this enzyme also catalyzes the reduction of the model substrate 4-nitrobenzaldehyde. Am ino acid sequencing of cyanogen bromide-derived peptides obtained from this reductase indicated that it has not been characterized hitherto, at least not at a molecular level. Therefore, this inducible enzyme h as been designated aflatoxin B1-aldehyde reductase (AFB1-AR). The live rs of adult rats administered dietary EQ contain at least 15-fold grea ter levels of AFB1-AR than the livers from rats fed control diets. Afl atoxin B1-AR was also found to be present in increased amounts in live rs bearing preneoplastic nodules and in rat hepatoma, both of which ar e known to express increased resistance to AFB1. Kidney contains high constitutive levels of AFB1-AR and the administration of EQ increases its concentration in renal cytosol about 3-fold. Although AFB1-AR is p resent in trace amounts in rat lung it was not detected in brain and i n neither tissue was it found to be induced by EQ. Evidence suggests t hat AFB1-AR is a previously unrecognized enzyme that could provide pro tection against the cytotoxic effects of aflatoxin B1 resulting from t he formation of protein adducts. The relative importance of AFB1-AR an d the glutathione-S-transferase Yc2 subunit in conferring resistance t o aflatoxin B1 is discussed.