In vitro metabolic conversion of aflatoxins and benzo(alpha)pyrene to nucleic acid-binding metabolites

Abstract

An aflatoxin B1 metabolite was found to become covalently bound to rat liver RNA and calf thymus DNA in vitro, and it formed complexes with increased spectral absorbance in the 360 nm region. The formation of such complexes was reduced nicotinamide adenine dinucleotide phosphate and microsome dependent, was inhibited by theta-diethylaminoethyl diphenylpropylacetate-HC1, and by CO and N2, when the latter were used to replace the gas phase of the incubations. The formation of the complexes was enhanced about 2-fold with cicrosomes from phenobarbital-treated rats but not from 3-methylcholanthrene-treated rats. More binding was observed with DNA than RNA. Dentured DNA was about 70% as effective as native DNA. Nucleic acids from various sources showed the following order of binding potency: DNA from Micrococcus luteus greater than DNA from calf thymus equal to DNA from rat liver greater than RNA from rat liver greater than transfer RNA from rat liver. In the presence of reduced nicotinamide adenine dinucleotide phosphate and microsomes from phenobarbital-treated rats, aflatoxin G1 was also converted into metabolite(s) that became covalently bound to nucleic acids and formed complexes with increased spectral absorbances in the 360 nm region: this reaction was also inhibited by theta-diethylaminoethyl diphenylpropylacetate-HC1. Under the same conditions, aflatoxin B2, aflatoxin G2, aflatoxin B2a, and "Compound 11," which lack a C2-C3 double bond, did not show any noticeable binding to either DNA or RNA. These data strongly support the concept that the microsomal mixed-funciton oxygenase-catalyzed oxidation of the C2-C3 double bond of aflatoxins is a prerequisite for the formation of nucleic acid-binding metabolites. Microsomes from untreated, phenobarbital-treated, and 3-methylcholanthrene-treated rats were compared in vitro for their ability to catalyze the formation of DNA-binding metabolites from aflatozin B1 and benzo(a)pyrene. In assays involving benzo(a)pyrene, microsomes from 3-methylcholanthrene-treated rats were 12- and 5-fold more active than microsomes from untreated and phenobar-bital-treated rats, respectively. This is in contrast to the results obtained with aflatoxin B1 and suggests that different enzymes in the hepatic microsomal mixed-function oxygenase complex are involved in the generation of reactive metabolites from various polycyclic hydrocarbons.