Oxidation of Quercetin by Salivary Components. Quercetin-Dependent Reduction of Salivary Nitrite under Acidic Conditions Producing Nitric Oxide

Abstract

Under acidic conditions, nitrite is protonated to nitrous acid (pK(a) = 3.2-3.4) that can be transformed into nitric oxide by self-decomposition and reduction. When sodium nitrite was mixed with quercetin at pH 1-2, quercetin was oxidized producing nitric oxide. In addition to quercetin, kaempferol and quercetin 4'-glucoside were also oxidized by nitrous acid, but oxidation of apigenin, luteolin, and rutin was slow compared to oxidation of the above flavonols. These results suggested that flavonols, which have a free hydroxyl group at carbon position 3, can readily reduce nitrous acid to nitric oxide. When the pH of saliva was decreased to 1-2, formation of nitric oxide was observed. The nitric oxide formation was enhanced by quercetin, and during this process quercetin was oxidized. These results indicate that there is a possibility of reactions between phenolics and nitrous acid derived from salivary nitrite in the stomach.

Red wine-dependent reduction of nitrite to nitric oxide in the stomach

“…The resulting spectrum is shown in line 3. The formation of the o-semiquinone radical, corresponding to the one-electron oxidation of the parent phenol is in accordance with results by others, showing the formation of chlorogenic and quercetin radicals upon reaction with acidic nitrite [18,19].…”

“…Therefore, these compounds are likely candidates to undergo a redox reaction with nitrite during digestion, yielding U NO. Accordingly, chlorogenic acid and quercetin (phenolics exhibiting a catechol moiety) were shown to enhance the production of U NO from nitrite at acidic pH [18,19]. On the other hand, ascorbic acid, a compound that exhibits a low redox potential, and actively secreted in the stomach, has also been shown to be an efficient one-electron reductant of nitrite [42,43].…”

“…Yet, dietary polyphenols that, depending on its structure, have been shown to redox interact with oxidant radicals, thus preventing oxidative reactions, may be present in large amounts in the stomach in a diet-dependent way and likely promote U NO formation at high fluxes. Accordingly, it has been demonstrated that chlorogenic acid and quercetin enhance the production of U NO from nitrite at acidic pH [18,19].…”

Starch Can Inhibit Nitrite-Dependent Oxidation of Crocin in Gastric Lumen Increasing Bioavailability of Carotenoids

“…This suggests that crocin in foods can reduce salivary nitrite to NO in the stomach. It has been reported that some kinds of polyphenols in foods can reduce salivary nitrite to NO in the stomach (Takahama et al, 2002(Takahama et al, , 2010aPeri et al, 2005), and the function of NO generated in the stomach has been discussed from the viewpoint of increases in various activities of the length range from 400 to 600 nm (Fig. 2).…”

Possible Reactions of Dietary Phenolic Compounds with Salivary Nitrite and Thiocyanate in the Stomach

“…In the stomach, nitrite is protonated producing nitrous acid (pKa = 3.3), an oxidizing and nitrosating agent, to react with food components. It is well known that nitrous acid can transform amines into carcinogenic compounds nitrosoamines [10], and it has been reported that phenolic compounds can reduce nitrous acid to a functional compound nitric oxide ( • NO) [11,12]. The functions of • NO produced in the gastric lumen include the increase in gastric mucosal blood flow, the increase in gastric mucus thickness, and the relaxation of gastric smooth muscle [13][14][15][16].…”