[Mercapturates and biologic monitoring: styrene]

The biological monitoring of styrene exposure is currently performed by the measurement of the urinary excretion of mandelic acid (MA) and phenylglyoxylic acid (PGA), which originate from the conversion of styrene 7,8-epoxide (SO), an electrophilic compound considered responsible for most toxic effects of styrene. In rats, an alternative pathway in the detoxification processes of SO has been shown, which involves its conjugation with glutathione (GSH), leading to the excretion of N-acetyl-S-(1-phenyl-2-hydroxyethyl)-cysteine (M1) and N-acetyl-S-(2-phenyl-2-hydroxyethyl)-cysteine (M2). Giving the chiral nature of SO, which is present in two enantiomeric forms (R and S), both M1 and M2 consist of two diastereoisomers: thus, four specific mercapturic acids are excreted in styrene-exposed rats, namely M1-R, M1-S, M2-R and M2-S. Until now, the excretion of these compounds in man has not been confirmed directly, giving the analytical difficulties in measuring the low levels of urinary mercapturic acids which are expected following styrene exposure in the working environment. In the present study we applied an analytical method, based on HPLC with fluorometric detection, to measure the excretion of M1-R, M1-S, and M2 in post-shift urines from 22 workers exposed to styrene and in 10 unexposed subjects. The results clearly demonstrated that the GSH pathway is involved in the detoxification processes of styrene, even if to a low extent (the biotransformation rates of styrene to mercapturic acids varied from 0.021 to 0.325%) and that M1 and M2 are specific for styrene exposure, in fact unexposed subjects showed no detectable amounts of these metabolites. In spite of the marked interindividual variability, significative correlations were found between mercapturic acids excretion and environmental styrene concentration or urinary levels of MA and PGA. The urinary levels of M1-S and M1-R were significantly different, thus indicating a stereoselectivity of the enzymes involved in the biotransformation of styrene to mercapturic acids.