BENZENE METABOLISM BY HUMAN LIVER-MICROSOMES IN RELATION TO CYTOCHROME-P450 2E1 ACTIVITY

Low levels of benzene from sources including cigarette smoke and autom obile emissions are ubiquitous in the environment. Since the toxicity of benzene probably results from oxidative metabolites, an understandi ng of the profile of biotransformation of low levels of benzene is cri tical in making a valid risk assessment. To that end, we have investig ated metabolism of a low concentration of [C-14]benzene (3.4 mu M) by microsomes from human, mouse and rat liver, The extent of phase I benz ene metabolism by microsomal preparations from 10 human liver samples and single microsomal preparations from both mice and rats was then re lated to measured activities of cytochrome P450 (CYP) 2E1. Measured CY P 2E1 activities, as determined by hydroxylation of p-nitrophenol, var ied 13-fold (0.253-3.266 nmol/min/mg) for human samples. The fraction of benzene metabolized in 16 min ranged from 10% to 59%. Also at 16 mi n, significant amounts of oxidative metabolites were formed. Phenol wa s the main metabolite formed by all but two human microsomal preparati ons. In those samples, both of which had high CYP 2E1 activity, hydroq uinone was the major metabolite formed. Both hydroquinone and catechol formation showed a direct correlation with CYP 2E1 activity over the range of activities present. A simulation model was developed based on a mechanism of competitive inhibition between benzene and its oxidize d metabolites, and was fit to time-course data for three human liver p reparations. Model. calculations for initial rates of benzene metaboli sm ranging from 0.344 to 4.442 nmol/mg/min are directly proportional t o measured CYP 2E1 activities. The model predicted the dependence of b enzene metabolism on the measured CYP 2E1 activity in human liver samp les, as well as in mouse and rat liver samples. These results suggest that differences in measured hepatic CYP 2E1 activity may be a major f actor contributing to both interindividual and interspecies variations in hepatic metabolism of benzene. Validation of this system in vivo s hould lead to more accurate assessment of the risk of benzene's toxici ty following low-level exposure.