Gen-ichiro Nonaka

Inhibitory effects of polyphenols from water chestnut (Trapa japonica) husk on glycolytic enzymes and postprandial blood glucose elevation in mice

Water chestnut is an annual aquatic plant that grows in Asia and Europe. Although water chestnut has been used as food and herbal medicine, its physiological functions and active ingredients are unknown. Here, we extracted polyphenols from the husk of the Japanese water chestnut (Trapa japonica) and assessed their effects on blood glucose levels. Three hydrolysable polyphenolics (WCPs), eugeniin, 1,2,3,6-tetra-O-galloyl-β-d-glucopyranose, and trapain, were predominant with dry-weight contents of 2.3 ± 0.0, 2.7 ± 0.1, and 1.2 ± 0.1g/100g, respectively. These WCPs exhibited inhibitory activity against α-amylase and α-glucosidase. Whereas (-)-epigallocatechin gallate does not inhibit α-amylase, WCPs exhibited high inhibitory activity (>80% at 0.15 mg/mL). In mice, administration of WCPs (40 mg/kg) significantly reduced blood glucose and serum insulin levels as assessed by the carbohydrate tolerance test.

Triterpene hexahydroxydiphenoyl esters and a quinic acid purpurogallin carbonyl ester from the leaves of Castanopsis fissa.

Triterpene hexahydroxydiphenoyl (HHDP) esters have only been isolated from Castanopsis species, and the distribution of these esters in nature is of chemotaxonomical interest. In this study, the chemical constituents of the leaves of Castanopsis fissa were examined in detail to identify and isolate potential HHDP esters. Together with 53 known compounds, 3,4-di-O-galloyl-1-O-purpurogallin carbonyl quinic acid (1) and 3,24-(S)-HHDP-2α,3β,23,24-tetrahydroxytaraxastan-28,20β-olide (2) were isolated and their structures were elucidated by spectroscopic and chemical methods. The polyphenols of the leaves were mainly composed of galloyl quinic acids, triterpenes HHDP esters, ellagitannins and flavonol glycosides. In particular, the isolation yields of 1,3,4-trigalloyl quinic acid and compound 2 were 1.53% and 0.27%, respectively, from the fresh leaves. The presence of lipid soluble HHDP esters of oleanane-type triterpenes as one of the major metabolites is an important chemotaxonomical discovery. Lipase inhibition activities and ORAC values of the major constituents were compared. The triterpene HHDP ester showed moderate lipase inhibition activity and myricitrin gave the largest ORAC value.

Modification of the Solubility of Tannins: Biological Significance and Synthesis of Lipid-Soluble Polyphenols

Tannins are believed to play a significant role in plant defense systems because they possess a severe astringent taste and decrease nutritional value of the plant by precipitating proteins and inhibiting digestive enzymes.1–3 Usually, tannins exist as a complex mixture of slightly varying but basically very similar compounds, which are derived from several key compounds such as catechin and pentagalloylglucose, by oxidation, substitution, hydrolysis, and so forth.3 The structural variation leads to differences in their physico-chemical properties including water solubility,4 chemical reactivity,5–7 and the ability of complex formation with proteins.8–10 Among these properties, the water solubility is an important factor in showing their biological activities.