32 P-Postlabeling analysis of DNA adduction in mice by synthetic metabolites of the environmental carcinogen, 7 H -dibenzo[c,g]carbazole: chromatographic evidence for 3-hydroxy-7 H -dibenzo[c,g]carbazole being a proximate genotoxicant in liver but not skin

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Abstract
The DNA adduction by the environmental carcinogen 7 H -dibenzo[c,g]carbazole (DBC) and chemically synthesized 2-OH, 3-OH, and 4-OH metabolites of DBC was investigated in liver and skin of female CD-1 mice. After topical application to the skin of 37 μmol/kg of DBC or the phenolic metabolites, DNA adducts were measured by a 32 P-post-labeling assay employing carrier-free [γ- 32 P]ATP and ATP-deficient conditions. In liver, DBC produced four major and several minor chromatographically distinct adducts of as yet undetermined chemical structure. The adduct pattern elicited by 3-OH-DBC was qualitatively similar to the DBC adduct pattern, while this was not the case for 2-OH-DBC and 4-OH-DBC. On the basis of co-chromatography experiments under various conditions, the DBC and 3-OH-DBC adducts appeared identical, and the total level of adduction elicited by these compounds in liver was substantial. Similar results were observed when DBC or 3-OH-DBC were administered i.p. As a major difference between the two compounds, one 3-OH-DBC adduct (no. 3) was 4.4- and 7.0-fold lower than the corresponding DBC adduct after i.p. and topical dosing, respectively. In skin, DBC produced two major adduct fractions after topical application, one of which could be chromatographically resolved into three subcomponents. Prominent adducts produced in skin DNA by each of the three metabolites were different from those elicited by DBC, and the level of adduction by the metabolites was significantly lower than that by DBC. Comparison of the skin and liver DBC-DNA adduct patterns after topical application of DBC showed that only one of the four major chromatographically resolved skin adducts corresponded to a major liver adduct (no. 3), and that total adduction in liver was 13.5-fold higher than in skin. These results suggested that (i) activation of DBC to DNA-binding compounds in liver occurs through at least two pathways with 3-OH-DBC being a proximate carcinogen involved in the formation of most of the adducts; (ii) 3-OH-DBC and the other two phenolic metabolites investigated play a minor role, if any, in the formation of DBC-DNA adducts in skin; (iii) metabolic activation of DBC to DNA-binding compounds in liver and skin appears to follow pathways that are different in terms of both the chemical nature and the amount of the adducts formed; and iv) DBC and 3-OH-DBC exhibit a strong preference for liver versus skin DNA.