Both reduced and oxidized ascorbate (AA and DHA) are present in the aqueous phase of the extracellular space, the apoplast, of spinach (Spinacia oleracea 1.) leaves. Fumigation with 0.3 pL 1-' of ozone resulted in ozone uptake by the leaves close to 0.9 pmol cm-' of leaf surface area s-'. Apoplastic AA was slowly oxidized by ozone. The initial decrease of apoplastic AA was ~0 . 1 pmol cm-2 -1 s . The apoplastic ratio of AA to (AA + DHA) decreased within 6 h of fumigation from 0.9 to 0.1. Initially, the concentration of (AA + DHA) did not change in the apoplast, but when fumigation was continued, DHA increased and AA remained at a very low constant level. After fumigation was discontinued, DHA decreased very slowly in the apoplast, reaching control level after 70 h. The data show that insufficient AA reached the apoplast from the cytosol to detoxify ozone in the apoplast when the ozone flux into the leaves was 0.9 pmol cm-'s-'. The transport of DHA back into the cytosol was slower than AA transport into the apoplast. No dehydroascorbate reductase activity could be detected in the apoplast of spinach leaves. In contrast to its extracellular redox state, the intracellular redox state of AA did not change appreciably during a 24-h fumigation period. However, intracellular glutathione became slowly oxidized. At the beginning of fumigation, 90% of the total glutathione was reduced. Only 10% was reduced after 24-h exposure of the leaves to 0.3 pL 1-' of ozone. Necrotic leaf damage started to become visible when fumigation was extended beyond a 24-h period. A close correlation between the extent of damage, on the one hand, and the AA content and the ascorbate redox state of whole leaves, on the other, was observed after 48 h of fumigation. Only the youngest leaves that contained high ascorbate concentrations did not exhibit necrotic leaf damage after 48 h.
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.
Two major flavonol glycosides [quercetin 3,4‘-diglucoside (F1) and quercetin 4‘-monoglucoside (F2)]
and a flavonol aglycon quercetin were mainly localized in the abaxial epidermis of scales. Their
contents increased on aging. Peroxidase in scales oxidized flavonols in the order quercetin ≫ F2 >
isoquercetin ≫ F1, and the activity was higher in the outer than in the inner scales. These results
suggest that the enzyme can participate in the formation of defense substances against infection
and brown compounds in the dry skin from quercetin. Contents of F1 and F2 in scales were decreased
by cooking by boiling. This decrease was due to the release of F1 and F2 into cooking water and
their oxidation. F2 was oxidized more rapidly than F1 during cooking. The difference in the stability
between F1 and F2 was due to the presence or absence of a hydroxyl group at the C-3 position of
the glucosides.
Keywords: Flavonols; peroxidase; distribution in tissues; thermostability; onion (Allium cepa)
Outer scales of yellow onion bulbs turn brown during maturing. The brown outer scales contain an antifungal component, 3,4-dihydroxybenzoic acid. An aim of the present study is to elucidate the mechanism of formation of the benzoic acid. In a browning scale, the scale was divided into three areas; fleshy, drying and dried brown areas. Levels of quercetin glucosides in dried brown areas were less than 10% of the glucosides in fleshy and drying areas, whereas levels of quercetin were high in dried brown areas. This result suggests that quercetin was formed by deglucosidation of quercetin glucosides on the border between drying and dried brown areas. Peroxidase (POX) activity of dried brown areas was about 10% of those of fleshy and drying areas. Quercetin was oxidized by autooxidation, and cell-free extracts of drying areas and POX isolated from onion scales enhanced the oxidation even in the absence of externally added hydrogen peroxide. The enhancement of quercetin oxidation was suppressed by catalase. No tyrosinase-like activity was detected in the cell-free extracts and the POX preparation. These results suggest that, during the enhanced oxidation of quercetin, hydrogen peroxide is formed. 3,4-Dihydroxybenzoic acid and 2,4,6-trihydroxyphenylglyoxylic acid, which were the oxidation products of quercetin, were found in dried brown area. These results suggest that an antifungal agent 3,4-dihydroxybenzoic acid is formed by POX-dependent oxidation of quercetin on browning of onion scales.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.