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-β-methylstyrene, 7,8-oxide, trans-β-ethylstyrene, 7,8-oxide, trans-β-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 1/10⁶ of the activity of the positive control, benzoa]pyrene 4,5-oxide, depending on the compound and the bacterial strain. Trans-β-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.