Oxidation of Quercetin by Salivary Components. I. Salivary Peroxidase-Dependent Oxidation of Quercetin and Characterization of the Oxidation Products.

Abstract

Quercetin glucosides ingested as components of a foodstuff can be transformed to quercetin, and the quercetin is oxidized by peroxidase in the oral cavity. One objective of this study was the characterization of the oxidation products. Although both salivary peroxidase and myeloperoxidase are contained in whole saliva, salivary peroxidase is mainly involved in the oxidation in the saliva. As the oxidation products, 2,3,5,7,3¢ ¢ ¢ ¢,4¢ ¢ ¢ ¢-hexahydroxyflavanone-like compound and two isomers of a compound, which was transformed to quercetin and the above flavanone by a HCl/heattreatment, were isolated. 3,4-Dihydroxybenzoic acid and 2,4,6-trihydroxyphenylglyoxylic acid were also detected in the oxidation products. When dialyzed saliva was used, the salivary peroxidase-dependent oxidation of quercetin and the formation of the oxidation products were partly inhibited by 1 mM thiocyanate which was a substrate for the peroxidase in the oral cavity. The mechanism and the significance of salivary peroxidase-dependent oxidation of quercetin in the oral cavity are discussed.

Oxydation of Quercetin by Salivary Components II. Effects of Quercetin on Reactive Oxygen Metabolism by Salivary Polymorphonuclear Leukocytes.

“…The main products were essentially the same as those formed by oxidation of quercetin by salivary peroxidase (Takahama et al, 2002). The result is summarized in Table 1.…”

“…Two isomers of 2,3-epoxy-2- (3,4-dihydroxyphenyl)-3-[4O-[2-(3-hydroxyphenyl)-3,5,7-]trihydroxy-4H-1-benzopyran-4-onyl]-5,7-dihydroxy-4H-1-benzopyran-4-one are horseradish peroxidase-dependent oxidation products of quercetin (Schreier & Miller, 1985). Compounds, the characteristics of which are similar to the isomers, have been reported as the salivary peroxidase-dependent oxidation products of quercetin (Takahama et al, 2002 (Sorata et al, 1984;Terao & Piskula, 1997), the flavonol can also inhibit the peroxidation of lowdensity lipoprotein which is induced by macrophages in inflamed tissues (Leake, 1997). In addition to myeloperoxidase, peroxidase in saliva may also participate in the scavenging of H 2 O 2 if this oxidant is formed in or diffused into saliva.…”

“…2,3,5,7,3¢,4¢-Hexahydroxyflavanone is an oxidation product of quercetin that still has a flavonoid structure (Barz et al, 1985;Schreier & Miller, 1985). A compound similar to the flavanone has been detected in the salivary peroxidase-dependent oxidation product of quercetin (Takahama et al, 2002). Two isomers of 2,3-epoxy-2- (3,4-dihydroxyphenyl)-3-[4O-[2-(3-hydroxyphenyl)-3,5,7-]trihydroxy-4H-1-benzopyran-4-onyl]-5,7-dihydroxy-4H-1-benzopyran-4-one are horseradish peroxidase-dependent oxidation products of quercetin (Schreier & Miller, 1985).…”

“…The result is summarized in Table 1. 3,4-Dihydroxybenzoic acid has repeatedly been reported as one of the oxidase-and peroxidase-dependent degradation products of quercetin (Barz et al, 1985;Schreier & Miller, 1985;Takahama et al, 2002). 2,3,5,7,3¢,4¢-Hexahydroxyflavanone is an oxidation product of quercetin that still has a flavonoid structure (Barz et al, 1985;Schreier & Miller, 1985).…”

Investigation on biocatalytic properties of a peroxidase-active homogenate from onion solid wastes: An insight into quercetin oxidation mechanism

“…HRP has also been shown to act on quercetin [35], and upon reaction at pH 5.5 the same oxidation products were identified, along with 2,4,6-trihydroxyphenylglyoxylic (PGXA) acid and several quercetin dimers and trimers [36]. Likewise, various hemeproteins including cytochrome c, myoglobin, and hemoglobin [37] and human salivary peroxidase [38] were demonstrated to act on quercetin, yielding dimers of similar structure. In the second instance, PA and PGXA were also generated.…”