Phenolic Compounds from the Leaves of Homonoia riparia and their Inhibitory Effects on Advanced Glycation End Product Formation

In a search for novel treatments for diabetic complications from natural resources, we found that the ethyl acetate-soluble fraction from the 80% ethanol extract of the leaves of Homonoia riparia has a considerable inhibitory effect on advanced glycation end product (AGE) formation. Bioassay-guided isolation of this fraction resulted in identification of 15 phenolic compounds (1 – 15). These compounds were evaluated in vitro for inhibitory activity against the formation of AGE. The majority of tested compounds, excluding ethyl gallate (15), markedly inhibited AGE formation, with IC₅₀ values of 2.2 – 89.9 µM, compared with that of the positive control, aminoguanidine (IC₅₀ = 962.3 µM). In addition, the effects of active isolates on the dilation of hyaloid-retinal vessels induced by high glucose (HG) in larval zebrafish was investigated; (–)-epigallocatechin-3-O-gallate (6), corilagin (7), and desmanthine-2 (11) significantly decreased HG-induced dilation of hyaloid–retinal vessels compared with the HG-treated control group.

Microbial Transformation of Two Prenylated Naringenins

Microbial transformation of (±)-6-(1,1-dimethylallyl)naringenin (6-DMAN, 1) and (±)-5-(O-prenyl) naringenin-4′,7-diacetate (5-O-PN, 2) was performed by using fungi. Scale-up fermentation studies with Mucor hiemalis, Cunninghamella elegans var. elegans, and Penicillium chrysogenum led to the isolation of five microbial metabolites. Chemical structures of the metabolites were determined by spectral analyses as (±)-8-prenylnaringenin (3), (2S)-5,4′-dihydroxy-7,8-[(R)-2-(1-hydroxy-1-methylethyl)-2,3-dihydrofurano]flavanone (4), (±)-5-(O-prenyl)naringenin-4′-acetate (5), (±)-naringenin-4′-acetate (6), and (±)-naringenin (7), of which 5 was identified as a new compound.

Phenolic Constituents from the Flowers of Hamamelis japonica Sieb. et Zucc

Hamamelis japonica (Hamamelidaceae), widely known as Japanese witch hazel, is a deciduous flowering shrub that produces compact clumps of yellow or orange-red flowers with long and thin petals. As a part of our ongoing search for phenolic constituents from this plant, eleven phenolic constituents including six flavonol glycosides, a chalcone glycoside, two coumaroyl flavonol glycosides and two galloylated compounds were isolated from the flowers. Their structures were elucidated as methyl gallate (1), myricitrin (2), hyperoside (3), isoquercitrin (4), quercitrin (5), spiraeoside (6), kaempferol 4'-O-beta-glucopyranoside (7), chalcononaringenin 2'-O-beta-glucopyranoside (8), trans-tiliroside (9), cis-tiliroside (10), and pentagalloyl-O-beta-D-glucose (11), respectively. These structures of the compounds were identified on the basis of spectroscopic studies including the on-line LCNMR- MS and conventional NMR techniques. Particularly, directly coupled LC-NMR-MS afforded sufficient structural information rapidly to identify three flavonol glycosides (2 - 4) with the same molecular weight in an extract of Hamamelis japonica flowers without laborious fractionation and purification step. Cytotoxic effects of all the isolated phenolic compounds were evaluated on HCT116 human colon cancer cells, and pentagalloyl-O-beta-D-glucose (11) was found to be significantly potent in inhibiting cancer cell growth.