Quercetin-Dependent Inhibition of Nitration Induced by Peroxidase/H₂O₂/Nitrite Systems in Human Saliva and Characterization of an Oxidation Product of Quercetin Formed during the Inhibition

Abstract

Local pH in the oral cavity can decrease to below 7 at the site where acid-producing bacteria are proliferating. Effects of pH on nitration of 4-hydroxyphenylacetic acid were studied using dialyzed human saliva. Dialyzed saliva nitrated 4-hydroxyphenylacetic acid to 4-hydroxy-3-nitrophenylacetic acid in the presence of nitrite and H(2)O(2). The rate of the nitration was dependent on pH, and the maximal rate was observed between pH 5.5 and 7.2. The optimum pH seemed to reflect rates of formation of nitrogen dioxide and 4-hydroxyphenylacetic acid radicals. Quercetin inhibited the nitration. The quercetin-dependent inhibition might be due to scavenging of nitrogen dioxide and 4-hydroxyphenylacetic acid radicals, which were formed by salivary peroxidase-dependent oxidation of nitrite and 4-hydroxyphenylacetic acid, respectively, and competition with nitrite and 4-hydroxyphenylacetic acid for peroxidase in saliva. An oxidation product of quercetin was formed during inhibition of the nitration by quercetin. The oxidation product was identified as 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. This component could also be oxidized by salivary peroxidase and nitrogen dioxide radicals. The oxidation products were 2,4,6-trihydroxyphenylglyoxylic and 3,4-dihydroxybenzoic acids. On the basis of the results, the significance of quercetin for inhibition of nitrogen dioxide formation and for scavenging of nitrogen dioxide radicals in the oral cavity is discussed.

Biodegradation of 3-Nitrotyrosine by Burkholderia sp. Strain JS165 and Variovorax paradoxus JS171

“…HNPA is excreted into the environment at concentrations that are an order of magnitude greater than the 3NTyr concentration (15,26) and so might be more available as a growth substrate than 3NTyr. Other studies have reported that mammalian peroxidases, including enzymes in human saliva, catalyze the nitration of 4-hydroxyphenylacetate to HNPA (9,29). On the other hand, the environmental flux of 3NTyr may be much higher than previously thought.…”

Reactions of polyphenols in masticated apple fruit with nitrite under stomach simulating conditions: Formation of nitroso compounds and thiocyanate conjugates

“…5). Quercetin radical can be transformed into 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone (A) via the quinone (Hirota, Takahama, Ly, & Yamauchi, 2005). Quercetin 7-glucoside was also oxidized to A glucoside by nitrous acid (Morina, Takahama, Yamauchi, Hirota, & Veljovic-Jovanovic, 2015).…”

Protective action of a hexane crude extract of Pterodon emarginatus fruits against oxidative and nitrosative stress induced by acute exercise in rats

“…However, Gunther et al [13] described a mechanism for NO-dependent tyrosine nitration that does not require the formation of highly reactive nitrogen oxide intermediates such as peroxynitrite or nitrogen dioxide. Peroxidases can also oxidize nitrite to nitrogen dioxide radical, which can cause nitration of tyrosine and tyrosine residues in proteins [14]. …”