Beta carotene and its oxidation products have different effects on microsome mediated binding of benzo[a]pyrene to DNA

The effects of β-carotene (βC) and its oxidation products on the binding of benzo[a]pyrene (BaP) metabolites to calf thymus DNA was investigated in the presence of rat liver microsomes. Mixtures of βC oxidation products (βCOP) as well as separated, individual βC oxidation products were studied. One set of experiments, for example, involved the use of the mixture of βCOP obtained after a 2-h radical-initiated oxidation. For this data set, the incorporation of unoxidized βC into microsomal membranes caused the level of binding of BaP metabolites to DNA to decrease by 29% over that observed in the absence of βC; however, the incorporation of the mixture of βCOP caused the binding of BaP metabolites to DNA to increase 1.7-fold relative to controls without βC. Two variations of this experiment were studied: (1) When no NADPH was added, βC decreased the binding of BaP metabolites to DNA by 19%, but the mixture of βCOP increased binding by 3.3-fold relative to that observed in the absence of βC. (2) When NADPH was added under near-anaerobic conditions, βC caused an almost total (94%) decrease in binding whereas βCOP had no effect on the amount of binding relative to that observed in the absence of βC. Both βCOP and cumene hydroperoxide caused BaP metabolites to bind to DNA even when NADPH was omitted from the incubation mixture. Separation of the mixture of βC oxidation products into fractions by HPLC allowed preliminary testing of individual βC oxidation products separately; of the various fractions tested, the products tentatively identified as 11,15′-cyclo-12,15-epoxy-11,12,15,15′-tetrahydro-β-carotene and β-carotene-5,6-epoxide appeared to cause the largest increase in BaP-DNA binding. Microsomes from rats induced with 3-methylcholanthrene (3MC) or Aroclor 1254 produced different levels of binding in some experimental conditions. We hypothesize that, under some conditions, the incorporation of βC into microsomal membranes can be protective against P450-catalyzed BaP binding to DNA; however, the incorporation of βCOP facilitates the formation of BaP metabolites that bind DNA, although only certain P450 isoforms catalyze the binding process.