TY - JOUR T1 - BIOACTIVATION OF 4-METHYLPHENOL (<em>p</em>-CRESOL) VIA CYTOCHROME P450-MEDIATED AROMATIC OXIDATION IN HUMAN LIVER MICROSOMES JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1867 LP - 1876 DO - 10.1124/dmd.105.006387 VL - 33 IS - 12 AU - Zhengyin Yan AU - H. Marlon Zhong AU - Noureddine Maher AU - Rhoda Torres AU - Gregory C. Leo AU - Gary W. Caldwell AU - Norman Huebert Y1 - 2005/12/01 UR - http://dmd.aspetjournals.org/content/33/12/1867.abstract N2 - It has previously been proposed that 4-methylphenol (p-cresol) is metabolically activated by oxidation of the methyl group to form a reactive quinone methide. In the present study a new metabolism pathway is elucidated in human liver microsomes. Oxidation of the aromatic ring leads to formation of 4-methyl-ortho-hydroquinone, which is further oxidized to a reactive intermediate, 4-methyl-ortho-benzoquinone. This bioactivation pathway is fully supported by the following observations: 1) one major and two minor glutathione (GSH) adducts were detected in microsomal incubations of p-cresol in the presence of glutathione; 2) a major metabolite of p-cresol was identified as 4-methyl-ortho-hydroquinone in microsomal incubations; 3) the same GSH adducts were detected in microsomal incubations of 4-methyl-ortho-hydroquinone; and 4) the same GSH adducts were chemically synthesized by oxidizing 4-methyl-ortho-hydroquinone followed by the addition of GSH, and the major conjugate was identified by liquid chromatography-tandem mass spectrometry and NMR as 3-(glutathione-S-yl)-5-methyl-ortho-hydroquinone. In addition, it was found that 4-hydroxybenzylalcohol, a major metabolite derived from oxidation of the methyl group in liver microsomes, was further converted to 4-hydroxybenzaldehyde. In vitro studies also revealed that bioactivation of p-cresol was mediated by multiple cytochromes P450, but CYP2D6, 2E1, and 1A2 are the most active enzymes for formation of quinone methide, 4-methyl-ortho-benzoquinone, and 4-hydroxybenzaldehyde, respectively. Implications of the newly identified reactive metabolite in p-cresol-induced toxicity remain to be investigated in the future. The American Society for Pharmacology and Experimental Therapeutics ER -