title: Bioactivation of 4-methylphenol (p-cresol) via cytochrome P450-mediated aromatic oxidation in human liver microsomes.
abstract: 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.
authors.affiliation: Division of Drug Discovery, R2013, Johnson & Johnson Pharmaceutical Research & Development, LLC, Welsh & McKean Roads, Spring House, Pennsylvania 19477-0779, USA. zyan@prdus.jnj.com
authors.forename: Zhengyin
authors.initials: Z
authors.lastname: Yan
authors.forename: H Marlon
authors.initials: HM
authors.lastname: Zhong
authors.forename: Noureddine
authors.initials: N
authors.lastname: Maher
authors.forename: Rhoda
authors.initials: R
authors.lastname: Torres
authors.forename: Gregory C
authors.initials: GC
authors.lastname: Leo
authors.forename: Gary W
authors.initials: GW
authors.lastname: Caldwell
authors.forename: Norman
authors.initials: N
authors.lastname: Huebert
mesh_terms: D003408:Cresols / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D008862:Microsomes, Liver / Q000378:metabolism*; D010084:Oxidation-Reduction
chemical_list: D003408:Cresols; C032538:4-cresol; D003577:Cytochrome P-450 Enzyme System; D005978:Glutathione