title: Consequences of quercetin methylation for its covalent glutathione and DNA adduct formation.
abstract: This study investigates the pro-oxidant activity of 3'- and 4'-O-methylquercetin, two relevant phase II metabolites of quercetin without a functional catechol moiety, which is generally thought to be important for the pro-oxidant activity of quercetin. Oxidation of 3'- and 4'-O-methylquercetin with horseradish peroxidase in the presence of glutathione yielded two major metabolites for each compound, identified as the 6- and 8-glutathionyl conjugates of 3'- and 4'-O-methylquercetin. Thus, catechol-O-methylation of quercetin does not eliminate its pro-oxidant chemistry. Furthermore, the formation of these A-ring glutathione conjugates of 3'- and 4'-O-methylquercetin indicates that quercetin o-quinone may not be an intermediate in the formation of covalent quercetin adducts with glutathione, protein and/or DNA. In additional studies, it was demonstrated that covalent DNA adduct formation by a mixture of [4-(14)C]-3'- and 4'-O-methylquercetin in HepG2 cells amounted to only 42% of the level of covalent adducts formed by a similar amount of [4-(14)C]-quercetin. Altogether, these results reveal the effect of methylation of the catechol moiety of quercetin on its pro-oxidant behavior. Methylation of quercetin does not eliminate but considerably attenuates the cellular implications of the pro-oxidant activity of quercetin, which might add to the mechanisms underlying the apparent lack of in vivo carcinogenicity of this genotoxic compound. The paper also presents a new mechanism for the pro-oxidant chemistry of quercetin, eliminating the requirement for formation of an o-quinone, and explaining why methylation of the catechol moiety does not fully abolish formation of reactive DNA binding metabolites.
authors.affiliation: Division of Toxicology, Wageningen University, Tuinlaan 5, 6703 HE Wageningen, The Netherlands.
authors.forename: Hester
authors.initials: H
authors.lastname: van der Woude
authors.forename: Marelle G
authors.initials: MG
authors.lastname: Boersma
authors.forename: Gerrit M
authors.initials: GM
authors.lastname: Alink
authors.forename: Jacques
authors.initials: J
authors.lastname: Vervoort
authors.forename: Ivonne M C M
authors.initials: IM
authors.lastname: Rietjens
mesh_terms: D000818:Animals; D000975:Antioxidants / Q000737:chemistry / Q000378:metabolism; D018736:DNA Adducts / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism*; D006735:Horseradish Peroxidase / Q000378:metabolism; D008745:Methylation; D010084:Oxidation-Reduction; D011794:Quercetin / Q000378:metabolism*; D011809:Quinones / Q000737:chemistry / Q000378:metabolism*; D051381:Rats; D013329:Structure-Activity Relationship
chemical_list: D000975:Antioxidants; D018736:DNA Adducts; D011809:Quinones; D011794:Quercetin; D006735:Horseradish Peroxidase; D005978:Glutathione