Plant tannins, including hydrolysable and condensed varieties, are well known antioxidants in medicinal plants, foods, and edible fruits. Their diverse biological properties and potential for disease prevention have been demonstrated by various in vitro and in vivo assays. A number of ellagitannins, the largest group of hydrolysable tannins, have been isolated from dicotyledoneous angiosperms and characterized. This diverse class of tannins is sub-grouped into simple ellagitannins, C-glycosidic ellagitannins, complex tannins (condensates of C-glycosidic tannins with flavan-3-ol), and oligomers up to pentamers. This review outlines and describes the chemotaxonomic significance of structural features in various types of ellagitannins found in plants belonging to the Myrtaceae, Onagraceae, and Melastomataceae families, which are all included in the order Myrtales. Any biological activities that have been reported, including antitumor and antibacterial effects as well as enzyme inhibition, are also reviewed.

In order to establish the marker constituents of the natural antioxidant food-additive Eucalyptus leaf extract, the UV-absorbing constituents of two eucalyptus leaf extracts registered as food additives (eucalyptus A and B) were investigated. Several major peaks on the reversed-phase HPLC chromatogram of eucalyptus A were characterized as gallic acid, ellagic acid, 3-O-beta-D-glucuronides of quercetin and kaempferol, and a hydrolyzable tannin dimer, oenothein B, by direct comparison with authentic specimens isolated from Eucalyptus globulus leaves. A new gallotannin was found in the E. globulus leaf extract, and its structure was found to be 1,2,3,6-tetra-O-galloyl-beta-D-galactose. Two major peaks on the HPLC chromatogram of eucalyptus B were identified as gallic acid and ellagic acid, indicative of degradation products from hydrolyzable tannins in the leaves. Considering the evaluation of antioxidant activity by radical scavenging ability, a standardization of eucalyptus leaf extract, including the antioxidative polyphenol, oenothein B, is proposed.

We previously reported that the citrus flavonoid 3,5,6,7,8,3′,4′-heptamethoxyflavone (HMF) increased the expression of brain-derived neurotrophic factor (BDNF) in the hippocampus of a transient global ischemia mouse model. Since the BDNF hypothesis of depression postulates that a reduction in BDNF is directly involved in the pathophysiology of depression, we evaluated the anti-depressive effects of HMF in mice with subcutaneously administered corticosterone at a dose of 20 mg/kg/day for 25 days. We demonstrated that the HMF treatment ameliorated (1) corticosterone-induced body weight loss, (2) corticosterone-induced depression-like behavior, and (3) corticosterone-induced reductions in BDNF production in the hippocampus. We also showed that the HMF treatment restored (4) corticosterone-induced reductions in neurogenesis in the dentate gyrus subgranular zone and (5) corticosterone-induced reductions in the expression levels of phosphorylated calcium-calmodulin-dependent protein kinase II and extracellular signal-regulated kinase1/2. These results suggest that HMF exerts its effects as an anti-depressant drug by inducing the expression of BDNF.

Nine known compounds: trans-cinnamic acid, catechin, syringin, epicatechin, symplocoside, kaempferol 3-O-rhamnoside 7-O-glucoside, isovitexin 2-O-rhamnoside, herbacetin 7-O-glucoside, and pollenitin B and a new flavonoid glycoside, characterized as herbacetin 7-O-neohesperidoside (1) on the basis of spectroscopic analysis and chemical evidence, were isolated from a traditional crude drug, “Ephedra herb extract”. Compound 1 had no effects on HGF-induced motility, whereas herbacetin, which is an aglycone of 1, significantly inhibited it.