[Mercapturates and biologic monitoring: benzene]

S-Phenylmercapturic acid (PMA), a urinary metabolite which derives from the conjugation of benzene epoxide with glutathione, has been recently included in the list of the biological markers of benzene exposure. We have evaluated the urinary PMA levels in 145 workers exposed to benzene and in 87 subjects not occupationally exposed to the solvent (45 smokers and 42 non-smokers). In non-exposed persons, the background PMA excretion was nearly constant during one day (urine samples were collected in the morning, afternoon, and evening) and in smokers the mean PMA levels were higher than in non-smokers (9.6 vs 1.3 micrograms/g creatinine). This difference was presumably due to the extra-exposure to benzene (from cigarette smoke) in smokers compared with non-smokers, in fact a close relationship was found between PMA excretion and urinary benzene concentration (r = 0.86). Urine samples from workers exposed to benzene (airborne mean concentration = 0.52 ppm) showed higher mean levels of PMA (37.6 micrograms/g creatinine) than samples taken from not occupationally exposed subjects, both smokers and non-smokers. The mean biotransformation rate of benzene to PMA was 0.073%, with a large inter-individual variability (range = 0.002-0.21%). Also, statistical analysis of data revealed a clear bimodal distribution pattern of the conversion rate frequencies in the population examined: this is consistent with the hypothesis of the presence of "slow" and "fast" converters, due to the polymorphism of glutathione-S-transferase isozymes.