Transport of DNA-adducting metabolites in mouse serum following benzo[a]pyrene administration.

Abstract

Metabolic activation of benzo[a]pyrene (BaP) by cellular enzymes is required for DNA adduct formation. In vivo DNA adducts might also arise from BaP metabolites supplied via the systemic circulation, rather than from in situ activation. We determined whether electrophilic metabolites could be detected in mouse serum 4 h after BaP dosing (i.p.) by trapping metabolites with salmon sperm DNA (ssDNA), followed by 32P-postlabeling analysis for DNA adducts. In vitro studies demonstrated that mouse serum sequesters BaP-7,8-diol-9,10-epoxide (BPDE) and protects it from hydrolysis. BPDE was rapidly transferred from serum to ssDNA or splenocytes, with adduct levels in ssDNA 4- to 7-fold greater than in splenocytes. After BaP administration, mouse serum produced two adduct spots when incubated with ssDNA. The major adduct (spot 3) co-chromatographed with a BPDE adduct standard, while the minor adduct (spot 2) was unrelated to BPDE. A BPDE standard curve in control serum was developed to quantitate BPDE levels in dosed serum. These levels ranged from 13.1 to 19.1 nM. Tissue DNA contained three adduct spots: spots 2 and 3 appeared identical to the respective adducts arising from dosed serum. BPDE-DNA adducts in tissues were highest in liver, lung and spleen, with kidney and stomach levels significantly lower. Levels of adduct 2 did not correlate with levels of adduct 3, especially in spleen where the adduct 2/adduct 3 ratio was very low. In vitro studies in which splenocytes were presented with both adducting metabolites suggested that splenocytes preferentially form adduct 3. These results indicate that two of the three BaP electrophilic metabolites responsible for cellular DNA damage are present in mouse serum. The levels of BPDE in serum may be sufficient to account for a substantial portion of the tissue load of BPDE-DNA adducts.