Scand J Work Environ Health 1978;4 suppl 2:142-155    pdf

doi:10.5271/sjweh.2769

Metabolism and mutagenicity of styrene.

by Watabe T, Isobe M, Sawahata T, Yoshikawa K, Yamada S, Takabatake E

Distilled styrene showed mutagenic activity towards Salmonella typhimurium TA 100 after activ ation with hepatic 9,000 X g supernatant fractions (S-9). The S-9 fractions ,had been obtained from rats treated with 3-methylcholanthrene or phenobarbital and fortified with an NADPH-generating system in the presence of 3,3,3-trichloropropene oxide (TCPO), an hepatic microsomal epoxide hydratase inhibitor. No mutagenic activity was observed when either the TCPO was omitted or rats were treated with a polychlorinated biphenyl mixture (PCBs). The S-9 fraction from 3-methylcholanthrene-treated rats activated styrene more effectively than the liver homogenate from phenobarbital-treated animals. As has already been pointed out, phenyloxirane, an intermediate in the hepatic microsomal biotransformation of styrene, induced the mutagenesis of TA 100 cells in the absence of 8-9 and TCPO, but the mutagenesis was totally abolished by the addition of S-9. A further addition of TCPO partially restored the mutagenetic activity of phenyloxirane. Gas chromatographic data on phenyloxirane and I-phenyl1,2-ethanediol in the testing system for styrene ind icat ed that phenyloxirane could not account for the mutagenesis induced by the hepatic activation of styrene since sums of phenyloxirane and its hydrolytic product, I-phenyl-l,2-ethanediol (formed from styrene during incubation with 8-9 in the presence and absence of TCPO) were much smaller than the amount which induced mutagenesis; they also showed that PCBs were the most potent inducers for hepatic rnonooxygenase, which catalyzes the conversion of styrene to phenyloxirane. PCBs induced hepatic monooxygenase approximately 1.4 times as effectively as 3-methylcholanthrene, in spite of its inability to increase the hepatic activation system for the mutagenicity of styrene. There is therefore a possibility that at least one more unknown active metabolite with an epoxide structure is present. The ar ene oxide I-vinvlbenzene 3,4-oxide, which was synthesized in our laboratory and was assumed to be a precusor to 4-vinylphenol (a previously isolated urinary metabolite of styrene), had a potent killing effect but no mutagenic activity towards TA 100 cells. A search for the ultimate mutagen is now in progress in our laboratory. l4C-styrene bound covalently to microsomal protein of rat liver when incubated in the presence of an NADPH-generating system. The absence of the cofactor decreased the binding remarkably. The NADPH-dependent protein-binding was increased by TCPO in vitro and by the phenobarbital pretreatment of rats in vivo. Pretreatment of rats with carbon tetrachloride decreased the amount of radioactivity bound to hepatic microsomal protein.