Mechanisms of hydroxylation by cytochrome P-450: metabolism of monohalobenzenes by phenobarbital-induced microsomes.

The monohydroxylation of halobenzenes by phenobarbital-induced rat liver microsomes was studied. The p-halophenol was found to be the major metabolite from all four halobenzenes; o-halophenol formation decreased as the halogen atom size increased. Vmax for total hydroxylation (ortho and para products) correlated well with the sigma + Hammett constant with a negative rho value. This implies a positively charged intermediate in the rate-determining step. Vmax for either ortho or para hydroxylation alone did not correlate with a Hammett constant, implying that the product-determining step occurs after the rate-determining step. Rate-determining formation of a radical cation intermediate is postulated to explain this data.