p-Nitrophenol hydroxylation. A microsomal oxidation which is highly inducible by ethanol

p-Nitrophenol is widely employed as a substrate for the study of the glucuronide and sulfate conjugation pathways. However, in perfused livers from ethanol-treated rats, p-nitrophenol was rapidly metabolized to 4-nitrocatechol. The 4-nitrocatechol formed competed with p-nitrophenol for conjugation with glucuronic acid and sulfate, and interfered with the direct spectral measurement of p-nitrophenol. In isolated microsomes, rates of p-nitrophenol hydroxylation were increased 6-fold after chronic ethanol treatment. Much smaller induction was observed after pretreatment of rats with phenobarbital (70% increase) or beta-naphthoflavone (30% increase). In addition, the 6-fold increase in rates of p-nitrophenol hydroxylation after chronic ethanol treatment was greater than increases in activity observed for the microsomal metabolism of aniline, 7-ethoxycoumarin, benzo(a)pyrene, ethanol, or aminopyrine. These data demonstrate that p-nitrophenol may be an extremely useful substrate for the study of changes in drug-metabolizing activity induced by ethanol treatment.