フラボノイド類の抗酸化活性と構造との相関性に関する研究(1)

Lipid peroxidation induces qualitative decay of foods, so various compounds possess antioxidant activity have been studied and used to protect against the alteration. We have established and reported the quantitative simultaneous analytical method using HPLC for three antioxidants of rutin, isoquercitrin and quercetin in foodstuffs so far. These compounds called flavonoids are plant constituents, contained in many foods such as fruits, vegetables, beans and spices. They exhibit antioxidant activities in both hydrophilic and lipophilic systems, and have been attracted much attention because of their various activities such as effects on the plant growth, protection against harmful UV radiation and resistance to microorganisms. Most of flavonoids exist as O-glycosylated or O-methylated derivatives of one or more hydroxyl group in nature, but sometimes may be manufactually conjugated with sugars in order to increase polarities for special uses. In this paper, we determined antioxidant activities and investigated structure-activity relationships of quercetin and its major glycosides and methoxides, using an oxidation system with β-carotene-linoleic acid. We applied this method for the model of aqueous foods like vegetables and fruits, because their major fatty acid was linoleic acid and oxidation progresses in the aqueous emulsion at this system. This method has an advantage of short time measurement in the handy way of visible adsorption spectrophotometry. All tested compounds inhibited the decolorization rate of β-carotene compared with control, and these inhibitions were used as criteria of antioxidant activities. In this study, quercetin showed stronger delay effect than its methoxides and glycosides. It is supported that conjugated forms with methyl group or sugars may be a major cause of decreasing antioxidant activities. In addition, it is also considered that antioxidant activities are affected by the binding position of methyl group on the methoxides or the structures of binding sugars on the glycosides. Antioxidant activities of methoxides were 50 to 65% while quercetin showed about 80% at the concentration of 2μmol/L each. The differences of these activities may be dependent not only on what kind of rings but also on either of 3' or 4'-position on the same B ring for the place of methoxyl substitution. It is suggested that OH groups on quercetin molecular indicate various degree of contribution to antioxidant activities depending on binding position. On the other hand, glycosides showed high antioxidant activities from 80 to 90% at 20μmol/L, whereas low activities less than 40% at 2μmol/L. Since all of tested glycosides are conjugated with sugars at 3-position on the C ring, it is suggested that the 3-OH moiety on the C ring is an important factor for its antioxidant effectiveness. It is also considered that antioxidant activities of glycosides may be increased after hydrolysis. It seems to be necessary to examine further study about relationships between antioxidant activities and structures focused OH substitution patterns of flavonoids. Table. Inhibition of thermal oxidation by quercetin and its derivatives [table]