Tissue and Spatial Distribution of Flavonol and Peroxidase in Onion Bulbs and Stability of Flavonol Glucosides during Boiling of the Scales

Abstract

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)

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

“…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 .…”

Stability and transformation of major flavonols in onion (Allium cepa) solid wastes

“…The reaction was initiated by adding 0.8 mL H 2 O 2 (1 mM in phosphate buffer, pH 7). The decrease in absorbance at 370 nm (λ max of quercetin) was recorded using a HP8452A diode array spectrophotometer (Hirota et al 1998). One peroxidase unit was defined as ΔA 370 min −1 and the specific activity (υ s ) as U mg −1 of protein.…”

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

“…(Osaka, Japan). Quercetin 4¢-monoglucoside and quercetin 3,4¢-diglucoside from onion bulbs were prepared by thin-layer chromatography (silica gel) and the concentrations were estimated from the absorbance at 360 nm using isoquercitrin as a standard as reported previously (Hirota et al, 1998).…”