Antioxidative Effects of 6-Methoxysorigenin and Its Derivatives from Rhamnus nakaharai

Abstract

Oxidative stress is well known as a major cause of various chronic diseases, namely cardiovascular diseases, rheumatism, diabetes mellitus, and cancer.1,2) Reactive oxygen species (ROS) are generated in vivo during shock, inflammation, and ischemia/reperfusion injury.3) Antioxidant therapy has been clinically demonstrated to be valuable in the prevention of certain chronic diseases. 3,4) Antioxidants also play an important role in the food industry, because excessive formation of free radicals can accelerate oxidation of lipids in foods and thereby decreases food quality.Rhamnus nakaharai HAYATA (Rhamnaceae) is traditionally used as a folk medicine in Taiwan for treating constipation, inflammation, malignant tumors, and asthma. 5) 3-O-Methyquercetin, the main constituent of the plant, has been reported to inhibit cAMP-and cGMP-phosphodiesterase activity of guinea pig trachealis.6) It was also demonstrated a selective and competitive inhibitor of phosphosdiesterase subtype 3/subtype 4. 5) Isotorachrysone (1), an acetonaphthone isolated from R. nakaharai, showed antioxidant 7) and platelet aggregation inhibitory 8) activities. 6-Methoxysorigenin (2), isolated from the wood of R. nakaharai, possesses a structure closely analogous to 1 (Fig. 1) and is rich in glycosides. 9)To evaluate the antioxidant effect of 2 and its derivatives, glycosides from the wood of R. nakaharai were isolated and characterized. They were 6-methoxysorigenin-8-O-glucoside (3), a-sorinin (4), 6-methoxysorigenin-8-O-rutinoside (5). In addition, peracylates 2a and 2b, and 2 were prepared and evaluated for antioxidant activities using 2,2-diphenyl-1-picrylhydrazyl (DPPH), metal chelating, and electron spin resonance (ESR) assays as well as FeCl 2 -ascorbic acid-induced lipid peroxidation assay in rat liver homogenates in vitro. Their antioxidant activities were compared with those of trolox and with EDTA for results of metal chelating assay. Results and DiscussionDPPH · is a stable free radical that accepts an electron or hydrogen radical to become a stable diamagnetic molecule. 10)In this assay, all test compounds effectively reduced the stable radical DPPH to the yellow-colored diphenylpicrylhydrazine and their scavenging effect was dose-dependent. It was found that compound 2 possesses the most potent DPPH radical-scavenging activity, with an IC 50 value of 3.48 mg/ml, which is close to that of trolox (IC 50 : 3.18 mg/ml) ( Table 1).Chelating agents, which form s-bonds with a metal, are effective as secondary antioxidants because they reduce the redox potential thereby stabilizing the oxidized form of the metal ion.11) Chelation may inhibit lipid oxidation by stabilizing transition metals. In this study, all test compounds including trolox and EDTA interfered with the formation of ferrous and ferrozine complexes, suggesting that they have chelating activity and are able to capture ferrous ion before ferrozine. Compound 2 (IC 50 : 615.90 mg/ml) showed a stronger iron-binding capacity than trolox (IC 50 : 1848.83 mg/ In the search for potentia...

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