The effect of antioxidant polyphenols and related phenolic compounds from plants on the reduction of metmyoglobin (MetMb) was investigated. Potent activity in the reduction of MetMb to oxymyoglobin (MbO2), a bright red protein in meat, was observed for three flavonols, kaempferol, myricetin, and quercetin, at 300 μmol/L against 60 μmol/L MetMb. Sinapic acid, catechin, nordihydroguaiaretic acid, taxifolin, morin, and ferulic acid promoted reduction at 600 μmol/L. A mechanism for the reduction by one of the active flavonols, quercetin, was proposed on the basis of analytical results for redox reaction products derived from quercetin. This suggested the importance of a high propensity toward reduction of the flavonol structure and rapid convertibility of the quinone form to the phenol form for the MbO2 reduction and the maintenance of the level of MbO2 produced.
Inhibitory activity of Fe-ion-catalyzed radical oxidation products from 22 types of phenolic compounds toward xanthine oxidase (XO) was investigated. Phenols are readily oxidizable compounds in nature and, thus, showed potent antioxidant activities. Among the phenols screened in this study, noticeable activity was observed in the oxidation product of caffeic acid, whereas almost no XO-inhibitory activity of caffeic acid was observed. Assay-guided purification of the oxidation product of caffeic acid afforded a highly potent XO inhibitor, with an IC50 value that was calculated to be 60 nmol L(-1), which indicated XO-inhibitory activity much stronger than that of allopurinol (IC50 = 1 μmol L(-1)), a potent XO inhibitor and excellent medicine for the treatment of gout. The chemical structure of this new XO inhibitor was investigated by one- and two-dimensional NMR and HR-ESI-MS analyses, and the unique tetracyclic structure was confirmed by synthesis starting from commercially available 1,2,4-trimethoxybenzene and 3,4-dimethoxylbenzoyl chloride.
Effects of various plant phenolics, including polyphenols, on the oxidation of oxymyoglobin were investigated. Most phenolics promoted the oxidation of oxymyoglobin at both pH 5.4 and 7.4. Potent oxidation-promoting activity was observed by several efficient antioxidant polyphenols with a catechol moiety. Therefore, effects of the catechol structure were investigated using dihydrocaffeic acid analogues. The results clarified that ortho- or para-substituted diphenol structures were important for promoting the oxidation of oxymyoglobin. Inhibition of such prooxidant activity for oxymyoglobin by dihydrocaffeic acid was also investigated. Although the required concentration was relatively higher than that of dihydrocaffeic acid, several amino acids inhibited the oxidation. Among these, cysteine was the most potent. Although cysteine alone completely inhibited oxidation at a concentration above 1 mmol/L, 0.1 mmol/L cysteine showed oxidation-promoting activity. In the presence of 0.1 mmol/L dihydrocaffeic acid, in the range of 0.01 mmol/L to 1 mmol/L cysteine, 0.1 mmol/L cysteine showed the most efficient inhibition. These results suggest the possibility of the formation of some equimolar complexes of dihydrocaffeic acid and cysteine such as 5'-cysteinyl dihydrocaffeic acid, which may be produced during the prooxidation of dihydrocaffeic acid, contributing to the inhibition of the oxidation of oxymyoglobin.
This study investigated antioxidant activities of cysteine derivatives of amino and carboxylic acid moieties against lipid oxidation in anhydrous acetonitrile. Only cysteine derivatives bearing free amino or carboxylate ion were found to exert potent antioxidant activities. Sequential proton loss and electron transfer-like proton shift and subsequent electron transfer (PS-ET) mechanism may facilitate the antioxidant activities of cysteine derivatives against lipid oxidation in anhydrous media.
Xanthine oxidase (XO) inhibitory activity has been found in boiling water extracts from roasted coffee beans. Therefore, assay-guided purification of the extracts was performed using size-exclusion column chromatography, and subsequently with reversed phase HPLC to afford lactone derivatives of chlorogenic acids. Among the tested lactones, crypto-and neochlorogenic lactones showed potent XO inhibitory activities compared with three major chlorogenic acids found in coffee beans. These XO inhibitory lactones may ameliorate gout and hyperuricemia in humans who drink coffee.
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