The formation and toxicity of catechol metabolites of acetaminophen in mice

Acetaminophen is metabolized to a catechol, 3'-hydroxy-4'-hydroxyacetanilide (3-hydroxyacetaminophen), by mouse liver microsomes, and to both catechol and methylated catechol metabolites by the mouse in vivo. Although 3-hydroxyacetaminophen is less hepatotoxic in mice than acetaminophen itself, 3-methoxyacetaminophen is as hepatotoxic as acetaminophen and is subject to a glutathione threshold effect. However, neither metabolite is formed in sufficient amounts to account for the hepatotoxicity caused by acetaminophen in the mouse. Mouse liver microsomes catalyze the oxidation of acetaminophen to the catechol in an apparent cytochrome P-450-mediated reaction that is induced by phenobarbital and inhibited by piperonyl butoxide, but is surprisingly not altered by cobaltous chloride. Lineweaver-Burk analysis of the oxidation carried out by liver microsomes from control animals gave curvilinear plots that may indicate catalysis by two enzyme sites with apparent Km values of 0.011 and 0.271 mM, and apparent Vmax values of 87 and 162 pmol/mg/min, respectively. Neither an isotope effect nor an NIH shift were measurable in the microsomal metabolism of selectively deuterated analogs of acetaminophen to 3-hydroxyacetaminophen. These results, coupled with results of previous investigations with 18O2 and epoxide hydrolase, indicate that a mechanism different from either direct insertion or epoxidation is involved in the formation of the catechol metabolite of acetaminophen.