Radioactively labelled epoxides. part V. Tritium labelled K-region oxides and trans-dihydrodiols of pyrene, benzo[a]pyrene and dibenz[a, h]anthracene

Some substrates and inhibitors of cytosolic epoxide hydrolase induce sister-chromatid exchanges in mammalian cells, but do not induce gene mutations in Salmonella typhimurium and V79 cells

Trans-stilbene oxide, trans-beta-methylstyrene 7,8-oxide, trans-beta-ethylstyrene 7,8-oxide, trans-beta-propylstyrene 7,8-oxide and 4-fluorochalcone oxide were investigated for genotoxic activity in bacterial and mammalian cells, in the absence of external xenobiotic-metabolising systems. All compounds strongly enhanced the frequency of sister-chromatid exchanges (SCE) in cultured human lymphocytes. None of them was mutagenic in Salmonella typhimurium (reversion of the his- strains TA98, TA100 and TA104). The limit of detection was 1/20,000 to 1/10(6) of the activity of the positive control, benzo[a]pyrene 4,5-oxide, depending on the compound and the bacterial strain. Trans-beta-methylstyrene 7,8-oxide and 4-fluorochalcone oxide were additionally tested for induction of SCE and gene mutations in the same target cells, namely Chinese hamster V79 cells. Their influence on the level of SCE was similar to that observed in human lymphocytes, whilst gene mutations (at the hprt locus) were not induced. The four investigated styrene oxide derivatives are known to be excellent substrates for a mammalian enzyme, cytosolic epoxide hydrolase (cEH). 4-Fluorochalcone oxide is a potent selective inhibitor of this enzyme and is structurally similar to the investigated styrene oxide derivatives. These properties of the test compounds however cannot explain the observed discrepancies in the results, since the genetic end point (SCE versus gene mutations) was decisive, and SCE were induced in cEH-proficient human lymphocytes as well as in cEH-deficient V79 cells.

The oxidation of the highly tumorigenic benz[a]anthracene 3,4-dihydrodiol by rat liver dihydrodiol dehydrogenase

Rat liver dihydrodiol dehydrogenase (DDH, EC 1.3.1.20) has been shown to reduce the mutagenicity of benz[a]anthracene (BA) in the bacterial Ames test. BA-3,4-dihydrodiol is a highly mutagenic and tumorigenic metabolite of BA. In order to test the hypothesis that this dihydrodiol may be a substrate of DDH, we established two novel assay systems for the NADP(+)-dependent oxidation of BA-3,4-dihydrodiol by rat liver DDH, an HPLC-based assay procedure and a radiometric assay with specifically labelled [3,4-3H]-BA-3,4-dihydrodiol as substrate. With the HPLC-based assay, the kinetic constants of the enzymatic catalysis were as follows: Km(app) = 21 microM for BA-3,4-dihydrodiol and Vmax = 20.0 nmol/min.mg enzyme. The reaction product was identified by cochromatography, fluorimetry and mass spectroscopy as BA-3,4-catechol, but interconversions between the catechol and the corresponding o-quinone during the analytical procedures were detected. With the radiolabelled substrate, a linear relationship between substrate concentration and reaction velocity was found. The V/K value for labelled substrate was 0.155 ml/min.mg enzyme and a (V/K)H/(V/K)T kinetic isotope effect of 6.7 was observed. The non-labelled substrate acted as a competitive inhibitor of the enzymatic oxidation of tritiated BA-3,4-dihydrodiol with a Ki value of 56.4 microM. The reaction rates determined in this study suggest an important role of DDH activity in the metabolism of BA.