Glucuronide conjugation reduces the cytotoxicity but not the mutagenicity of benzo(a)pyrene in the CHO/HGPRT assay

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Abstract
Benzo(a)pyrene (B[a]P) is biotransformed by the mixed‐function oxidase (MFO) system to numerous metabolites some of which are cytotoxic and/or mutagenic to mammalian cells. However, conjugation of B(a)P metabolites with glucuronic acid in vivo is a major pathway of detoxication and elimination. The effects of glucuronide conjugation on B(a)P‐induced cytotoxicity and mutagenicity were studied using the CHO/HGPRT assay with a rat liver homogenate preparation containing MFO system cofactors (S9 mix) and uridine diphosphate α‐D‐glucuronic acid (UDPGA). B(a)P metabolites proximate to the biologically active B(a)P quinones (B[a]P 6‐OH) and to the B(a)P 7,8‐diol‐9,10 epoxide isomers (B[a]P 7,8‐diol), were also assayed with S9 mix in the absence and presence of UDPGA. The addition of UDPGA to S9 mix reduced B(a)P‐induced cytotoxicity but did not affect mutagenicity. B(a)P 6‐OH‐mediated cytotoxicity was also reduced in the presence of UDPGA. UDPGA had no effect on B(a)P 7,8‐diol‐induced cytotoxicity or mutagenicity. B(a)P phenols have been shown to be the preferred B(a)P‐ metabolite substrates for UDP‐glucuronyltransferase enzymes. Thus, the reduction of B(a)P and B(a)P 6‐OH‐induced cytotoxicity by glucuronide conjugation is likely due to the elimination of cytotoxic phenols and quinones. Since B(a)P 7,8‐ diol is a poor substrate for UDP‐glucuronyltransferase enzymes, no effects on B(a)P‐induced mutagenicity or B(a)P 7,8‐diol‐induced cytotoxicity and mutagenicity were observed.

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