PT - JOURNAL ARTICLE AU - G. B. CORCORAN AU - J. R. MITCHELL AU - Y. N. VAISHNAV AU - E. C. HORNING TI - Evidence That Acetaminophen and <em>N</em>-Hydroxyacetaminophen Form a Common Arylating Intermediate, <em>N</em>-Acetyl-<em>p</em>-Benzoquinoneimine DP - 1980 Nov 01 TA - Molecular Pharmacology PG - 536--542 VI - 18 IP - 3 4099 - http://molpharm.aspetjournals.org/content/18/3/536.short 4100 - http://molpharm.aspetjournals.org/content/18/3/536.full SO - Mol Pharmacol1980 Nov 01; 18 AB - Adding ascorbic acid to microsomal incubations containing acetaminophen inhibited covalent binding of the reactive metabolite. Adding ascorbic acid to incubations containing acetaminophen and cysteine markedly decreased acetaminophen-cysteine adduct formation. Ascorbic acid addition to aqueous incubations containing N-hydroxyacetaminophen and cysteine similarly inhibited the nonenzymatic formation of an acetaminophen-cysteine adduct. Therefore, the chemical reactions responsible for the nonenzymatic decomposition of N-hydroxyacetaminophen to yield acetaminophen-cysteine adducts were examined. In aqueous solutions above pH 7, N-hydroxyacetaminophen rapidly dehydrated to N-acetyl-p-benzoquinoneimine. In the absence of reducing compounds N-acetyl-p-benzoquinoneimine reacted with another molecule of N-hydroxyacetaminophen to give equal amounts of nitrosophenol and acetaminophen. The addition of cysteine or ascorbic acid slowed the decomposition of N-hydroxyacetaminophen and inhibited the formation of nitrosophenol. Cysteine effected these changes through decreasing the concentration of N-acetyl-p-benzoquinoneimine, primarily by reducing it to acetaminophen at low pH (5.5-7.0) or by conjugating with it to yield an acetaminophen-cysteine adduct at high pH (7.5-11.0). Ascorbic acid produced its effects only through reduction of the N-acetyl-p-benzoquinoneimine intermediate; thus acetaminophen was the only product. These observations provide strong evidence that the reactive intermediate formed in microsomes from acetarninophen and in solution from N-hydroxyacetaminophen is N-acetyl-p-benzoquinoneimine.