URINARY MERCAPTURIC ACID DIASTEREOISOMERS IN RATS SUBCHRONICALLY EXPOSED TO STYRENE AND ETHANOL

Styrene is stereoselectively oxidized by cytochrome P450 to its reacti ve metabolite, styrene oxide. The (R)- and (S)-enantiomers of styrene oxide can be conjugated with glutathione (GSH) to both (R)- and (S)-di astereoisomers of the specific mercapturic acids, N-acetyl-S-(1-phenyl -2-hydroxyethyl)-L-cysteine (M1) and N-acetyl-S-(2-phenyl-2-hydroxyeth yl)-L-cysteine (M2). Several investigations have indicated different t oxic potential of the (R)- and (S)-configurations of styrene oxide and its GSH- and N-acetyl-conjugates. In this study the mercapturic acid diastereoisomers were measured in the urine of rats exposed to styrene in combination with ethanol, a good inducer of styrene metabolism. Ma le Sprague-Dawley rats were given an isocaloric liquid diet containing ethanol (5% w/v) for 3 weeks. Starting from the 2nd week, the animals were also exposed to styrene vapours (300 ppm, 6 h/day, 5 days/week) in a dynamic exposure chamber. Both the (R)- and (S)-diastereoisomers of the M1 and M2 as well as the conventional biomarkers, mandelic acid (MA) and phenylglyoxylic acid (PGA) were measured in urinary samples. Approximately 30 and 25% reduction of the levels or brain non-protein sulfhydryls (NPS) was observed in the animals given styrene aad ethan ol, respectively, while the combined ethanol and styrene treatment res ulted in a 60% decrease. Ethanol consumption also resulted in higher u rinary levels of the M1-R, M1-S and M2 metabolites associated with inc reased M1-R/S ratio and higher urinary MA excretion compared to animal s treated with styrene. These results suggest that the urinary mercapt uric acid diastereoisomers may be used as a noninvasive tool to examin e stereoselective patterns of styrene metabolism in vivo, as well as t heir alterations caused by ethanol. These compound-specific mercapturi c acids may also be valuable indicators of styrene-induced disorders o f GSH homeostasis in nonaccessible organs.