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The radical scavenging activity of marine polysaccharides was enhanced by their high-temperature treatment (roasting reaction model). The product obtained from alginic acid exhibited maximum activity, and a radical scavenger, alginetin, was identified in the product. Its antioxidant activities were examined by chemical methods, which confirmed that it possessed a stoichiometrically greater antioxidant capacity than that of Trolox., (© The Author(s) 2021. Published by Oxford University Press on behalf of Japan Society for Bioscience, Biotechnology, and Agrochemistry.)

The radical scavenging activity of commercially available roasted (deep colored) and unroasted (light colored) egoma (Perilla frutescens var. frutescens) oils was evaluated by the DPPH radical scavenging method. The antiradical activity of roasted oils was higher than that of unroasted oils, and the activity of methanol-water extracts from the roasted egoma oils was significantly higher than that of unroasted oils. The antiradical activity of the methanol-water fractions was strongly correlated to that of whole oils (r=0.72) and the color depth of oils (r=0.93), which was an index of roasting. Fractionation of the methanol-water extract of a roasted egoma oil according to molecular size using ultra membrane filters revealed that the fraction under 3 kDa had the strongest radical scavenging activity. Subsequent preparative HPLC separation using an ODS column also revealed that the second fraction was the most active. Our HPLC analytical method for DPPH radical scavengers in complex mixtures detected four strong radical scavenger peaks in the fraction. Among the detected peaks, two peaks were determined to be derived from rosmarinic acid and luteolin by comparison with the retention times and UV spectra of the authentic samples, and the other two compounds could not be identified because no characteristic UV spectra were observed. These identified polyphenols (rosmarinic acid and luteolin) have been reported to be present in the non-oily part of egoma seeds. They probably migrated to the oily part during the egoma oil roasting process.

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., (Copyright © 2014 Elsevier Inc. All rights reserved.)

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.

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.

Curcumin is a yellow pigment found in the spice turmeric and a main functional constituent of the rhizomes of Curcuma longa. The multi-functionality of curcumin has recently attracted much attention in various fields, including food, medicinal, and clinical industries. Three major curcuminoids, curcumin, demethoxycurcumin, and bis-demethoxycurcumin, are distributed in not only Curcuma, but also the Zingiber species; however, distribution of other curcuminoids is very limited in nature. The radical scavenging ability of curcumin, which is the basis of medicinal functionality, is its most important property. Herein, the structures of several rare curcuminoids that have been identified during the past four decades are summarized. Mechanistic studies of the radical trapping stage of curcumin are also comprehensively reviewed, and the studies on antioxidant effects derived from this radical trapping ability of curcuminoids are discussed. Various chemical reactions of curcumin, such as photo-oxidation, enzymatic oxidation, etc. are also summarized. In particular, antioxidation of curcumin in lipid media is introduced as a reaction unique to curcumin.

The enhancing effect of a cysteinyl thiol N-benzoylcysteine methyl ester on the antioxidant activity of several flavonoids was investigated in a lipid oxidation system. Obvious enhancement was apparent for catechin, myricetin, quercetin, and taxifolin, the activity for myricetin being the most potent among them. An HPLC analysis of the products from the antioxidation reaction of myricetin in the presence of the thiol was carried out and the structures of the products were determined to clarify the enhancing effect chemically. The obtained data indicated that two thiol adducts on the B ring, and probably C-ring adducts, which were produced in the antioxidation process, exerted an enhancing effect on the antioxidant activity of myricetin.