URINARY THIODIACETIC ACID - A SELECTIVE BIOMARKER FOR THE CYTOCHROME P450-CATALYZED OXIDATION OF 1,2-DIBROMOETHANE IN THE RAT

1,2-Dibromoethane (1,2-DBE) is a carcinogenic compound that is metabol ized both by cytochrome P450 (P450) and glutathione S-transferase (GST ) enzymes, and that has been used by us as a model compound to study i nterindividual variability in biotransformation reactions. In this stu dy, the excretion of thiodiacetic acid (TDA) and S-(2-hydroxyethyl)-N- acetyl-l-cysteine (2-HEMA) were measured in the urine of rats dosed wi th 1,2-DBE, and experiments were performed to investigate to what exte nt P450 and GST enzymes contribute to the formation of TDA. To this en d, CYP2E1, the main P450 isoenzyme catalyzing the oxidation of 1,2-DBE , was inhibited using disulfiram and diallylsulfide. Significant inhib ition of CYP2E1, as confirmed by inhibition of the hydroxylation of ch lorzoxazone, as well as inhibition of the formation of TDA from 1,2-DB E, was observed upon pretreatment of rats with these inhibitors, indic ating that the P450-catalyzed oxidation of 1,2-DBE plays the major rol e in the TDA formation. No significant excretion of TDA was observed a fter administration of intermediate products of the GST pathway [i.e. S-(2-hydroxyethyl)glutathione and 2-HEMA], indicating that the GST-cat alyzed metabolism of 1,2-DBE does not contribute to a significant exte nt to the formation of TDA. The results of this study show that TDA is specifically formed by P450 metabolites of 1,2-DBE, whereas the conju gation of 1,2-DBE to glutathione by GST enzymes does not contribute to the formation of TDA. TDA, excreted in urine, may thus be used as a b iomarker of exposure to 1,2-DBE selectively reflecting the P450-cataly zed oxidation. In addition to 2-HEMA and S-[2-(N-7-guanyl)ethyl]-N-ace tyl-l-cysteine, TDA may be a valuable tool for biomonitoring and mecha nistic studies into the metabolism and toxicity of 1,2-DBE.