Use of genetically manipulated Salmonella typhimurium strains to evaluate the role of sulfotransferases and acetyltransferases in nitrofen mutagenicity

Nitrofen had been used as a herbicide, until its carcinogenic and teratogenic activity in rodents was detected. A food contamination occurring in 2002 in Germany led to the initiation of new studies in order to better understand the potential risk for humans. Nitrofen is a nitroarene and as such might be activated to a mutagen via reduction to the corresponding hydroxylamine and subsequent formation of a reactive acetic or sulfuric acid ester. Therefore, we have investigated the mutagenicity of nitrofen in Salmonella typhimurium strains engineered for the expression of all human xenobiotic-metabolizing sulfotransferases (SULTs) and acetyltransferases (NATs) identified. Nitrofen was inactive in the parental strains TA1538, TA98 and TA100, but was mutagenic even at low doses when human sulfotransferase SULT1A1 (the major broad-spectrum phenol SULT) was expressed in these strains, but not when it was expressed in a TA1538-derived strain deficient in an endogenous nitroreductase. Several other human SULTs (in particular 1A3 and 1C1) as well as human NAT2 (unlike NAT1) also activated nitrofen, but were markedly less efficient than SULT1A1. Likewise, expression of rat and mouse SULT1A1 led to weaker mutagenic activity of nitrofen than expression of the corresponding human enzyme. An endogenous acetyltransferase only activated nitrofen to a mutagen when it was strongly over-expressed in the TA98-derived strain YG1024. Thus, humans might be more susceptible to the carcinogenic effects of nitrofen than mice and rats, which have been used in long-term studies. The fact that several SULTs show particular high expression in fetal tissues suggests that this activation pathway may also play a role in the teratogenic effects observed.