The function of a peroxidase/phenolics/ascorbic acid system in plant vacuoles has not yet been well elucidated. We wished to study the redox reactions among hydrogen peroxide, phenolics and ascorbic acid (AA) in the presence of horseradish peroxidase. Horseradish peroxidase oxidized rutin and chlorogenic acid (CGA), compounds present in many kinds of plant. The oxidation was inhibited by AA. As a result of the inhibition. AA was oxidized and when almost all of it had been oxidized, oxidation of the phenolics commenced. Monodehydroascorbic acid (MDA) radical was detected during the oxidation of AA, suggesting that the inhibition of oxidation of rutin and CGA was due to reduction of phenoxyl radicals by AA. By comparison of time courses of changes in levels of AA and MDA radicals, and by kinetic calculation, it is suggested that in addition to AA, MDA radicals may also reduce phenoxyl radicals. It is proposed that the peroxidase/phenolics/AA system can function as a hydrogen peroxide scavenging system.

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.

Phenolics as SubstratesSignificance of the Flavonoids and other phenolics are used for various reactions in plants. When used for lignification and browning on aging, the oxidation products are accumulated. When used as substrates of peroxidase (POX) in the presence of ascorbic acid (AA), phenolics are initially oxidized by POX and the phenoxyl radicals formed are reduced by AA producing monodehydroascorbic acid and dehydroascorbic acid. Since the oxidation forms of AA are reduced to AA in plant cells, the phenolics/AA/POX systems can scavenge H202 without accumulating oxidation products of phenolics. Scavenging of H202 by the systems can proceed in vacuoles and the apoplast, because phenolics, AA and POX are normal components of the compartments. AA seems to control lignification because it reduces radicals of lignin monomers which are formed by POX-dependent reactions. On lignification, oxidation of sinapyl alcohol is enhanced by radicals of coniferyl alcohol and hydroxycinnamic acid esters when apoplastic POX rapidly oxidizes coniferyl alcohol and the esters but slowly oxidizes sinapyl alcohol. POX seems to participate in the browning of tobacco leaves and onion scales on aging. H202, which is required for the POX-dependent reactions, can be formed by autooxidation of the phenolics that are transformed to brown components. It is discussed that browning involves the formation of antimicrobial substances.

No abstract

Human saliva contained 4-hydroxyphenylacetic acid (HPA) (2^10 W WM) and nitrite (60^300 W WM). HPA was nitrated to 4-hydroxy-3-nitrophenylacetic acid (NO 2 HPA) when HPA and sodium nitrite were mixed at pH 1.0. NO 2 HPA was also formed when saliva was incubated under acidic conditions. These results suggest that salivary HPA is nitrated to NO 2 HPA when saliva is swallowed into the stomach. ß

No abstract

A salivary component, nitrate, is reduced to nitrite in the oral cavity. Polyphenols in foods are mixed with nitrite in the saliva to be swallowed into the stomach. An objective of the present study is to elucidate reactions between a polyphenol quercetin and a nitrite under acidic conditions. Nitric oxide, which is formed by the reactions between nitrous acid and quercetin or ascorbic acid (AA), can be measured using an oxygen electrode in the saliva as well as a buffer solution. The initial oxidation of quercetin was inhibited by AA, and quercetin enhanced the oxidation of AA, suggesting AA-dependent reduction of quercetin radicals, which might be formed during the oxidation of quercetin by nitrous acid. On the basis of the above results, the usefulness of an oxygen electrode for the measurement of nitrite-dependent nitric oxide formation under acidic conditions is proposed and the possible mechanism of reduction of nitrous acid by quercetin and the interaction between quercetin and AA, which is a normal component in the gastric juice, for the reduction of nitrous acid is discussed.

The objective of the present study was to elucidate how chlorogenic acid in coffee was transformed under acidic conditions simulating the mixture of saliva and gastric juice. When coffee was incubated in acidified saliva that contained nitrite and SCN-, in addition to nitric oxide (NO), four major components were detected. Two of the four components (components 3 and 4) were generated when chlorogenic acid was incubated in acidified saliva and when incubated in an acidic buffer solution in the presence of both nitrite and SCN-. By the incubation of chlorogenic acid in acidic nitrite in the absence of SCN-, components 3 and 4 were not formed but the quinone of chlorogenic acid and nitrated chlorogenic acid were formed. The result indicates that SCN- was indispensable for nitrous acid induced formation of components 3 and 4. Component 4 was isolated and its structure was determined to be (E)-5'-(3-(7-hydroxy-2-oxobenzo[d] [1,3]oxathiol-4-yl)acryloyloxy)quinic acid. Component 3, which was suggested to be 2-thiocyanatochlorogenic acid, seemed to be formed by the reaction between SCN- and the quinone of chlorogenic acid. As it has been reported that the quinone of chlorogenic acid can react with thiols and can decompose producing H2O2, the formation of component 4 can reduce the toxic effects of the quinone of chlorogenic acid.