Quercetin Glucosides are Hydrolyzed to Quercetin in Human Oral Cavity to Participate in Peroxidase-Dependent Scavenging of Hydrogen Peroxide.

Abstract

It has been reported that consuming food which contains flavonols is related to the decrease in risks to arteriosclerosis and cancer. One objective of this study was to elucidate the metabolism of quercetin and the glucosides (quercetin 3,4¢-diglucoside and quercetin 4¢-glucoside) present in onion soup, in the oral cavity. Not only quercetin glucosides but also quercetin were found in whole saliva which was collected 15 min after eating onion soup. Quercetin seemed to be formed by deglucosidation of the glucosides. The concentrations of the flavonols were much higher in sediments than in supernatants of the saliva suggesting that the flavonols bound to components of the sediments like detached epithelial cells. Concentrations of quercetin and the glucosides in the oral cavity decreased as a function of time after the intake. The decreases were mainly due to the washing away from this cavity by saliva. In addition, deglucosidation of quercetin glucosides to quercetin was also assumed to contribute to the decrease in the glucosides and oxidation of quercetin by peroxidase (POX) in saliva to the decrease in quercetin. Based on the data that quercetin was oxidized by POX in saliva in the presence and absence of thiocyanate, it is suggested that quercetin as well as thiocyanate can be a substrate for POX in saliva to scavenge H 2 O 2 in oral cavity. Keywords: H 2 O 2-scavenging, metabolism of quercetin and the glucoside, onion (Allium cepa), oral cavity, peroxidase in saliva

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

“…5). The inhibition may be due to competition between thiocyanate and quercetin for compound I of salivary peroxidase, which is formed by direct reaction between H 2 O 2 and peroxidase, as discussed previously (Hirota et al, 2001). The inhibition of quercetin oxidation by thiocyanate also suggests that the oxidation product of thiocyanate, hypothiocyanate, does not rapidly oxidize quercetin at pH 7.6.…”

“…Since polyphenols are normally ingested by mouth as vegetables and fruits, elucidation of their metabolism in the oral cavity is also important. In previous papers, we reported that when onion soup, which contains quercetin glucosides (Hirota et al, 1998(Hirota et al, , 1999, is ingested, the glucosides are hydrolyzed to the aglycone quercetin, and that quercetin is degraded in the oral cavity (Hirota et al, 2001). The degradation is also observed when quercetin is added to collected saliva with and without externally added H 2 O 2 .…”

“…The degradation is also observed when quercetin is added to collected saliva with and without externally added H 2 O 2 . Since peroxidases in the oral cavity can catalyze the oxidative degradation of quercetin, the oxidation reaction may, in part, participate in scavenging H 2 O 2 (Hirota et al, 2001). If quercetin is oxidized by peroxidases in the oral cavity, characterization of the oxidation products is required.…”

Transformation of Nitrite and Nitric Oxide Produced by Oral Bacteria to Reactive Nitrogen Oxide Species in the Oral Cavity

“…In fact, the concentrations of nitrite and HPA tend to be higher in individuals with age of 60-year-old or more. Quercetin (30 µM) suppressed nitrite-induced O 2 NHPA formation more than 90% in bacterial fraction at pH 5 and quercetin can stay in the oral cavity for several hours after ingestion of onion soup (Hirota et al, 2001). Chlorogenic acid, which can also scavenge NO 2 , stays in the oral cavity for several hours after ingestion of coffee (Takahama et al, 2007b).…”

Quercetin and the Glucosides Inhibit Nitration of a Salivary Component 4-Hydroxyphenylacetic Acid Catalyzed by Salivary Polymorphonuclear Leukocytes

“…Among these components, antioxidative polyphenols may be important from the standpoint of health in the oral cavity. It has been reported that quercetin and the glucosides are present in the saliva after ingestion of onion soup, and that the flavonols remained in the oral cavity for several hours after the ingestion (Hirota et al, 2001). The importance of the phenolics as antioxidants in the oral cavity is deduced from the report that polyphenols can scavenge reactive oxygen species generated by SPMN (Hirota et al, 2002).…”