Novel isoflavone diglycoside in groundnut (Apios americana Medik)

Abstract In the present HPLC-based analysis of the isoflavone profile of groundnut ( Apios americana Medik) tubers, we identified a major peak that did not correspond to any known isoflavones. A water extract of groundnut tubers prepared at 4 °C showed the major and genistin peaks, whereas that prepared at 45 °C did not yield these peaks but showed a peak for genistein on HPLC analysis. When the extract was treated with β-glucosidase after the inactivation of endogenous enzymes, the major peak decreased over time and the genistin peak also decreased after a transient increase, changes which were accompanied by the appearance of a genistein peak. The HPLC elution pattern of the β-glucosidase-treated extract was markedly similar to that observed for the extract prepared at 45 °C. The major peak was then isolated and purified through HP-20 and ODS columns, and the chemical structure of the resultant component was analysed and identified as genistein-7-O-genitiobioside. The component was degraded by β-glucosidase to produce genistein, suggesting that the groundnut isoflavone is converted to genistein, similarly to soybean isoflavones, by enterobacterial β-glucosidase, a deglycosylation process which has a significant impact on the absorption efficiency and bioactivity of isoflavones. The genistein-7-O-genitiobioside content of groundnut tubers ranged from 106.2 to 352.5 mg/100 g fresh weight, depending on their cultivation area. Additionally, genistein-7-O-genitiobioside showed 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity in a concentration-dependent manner, implying its potential as an antioxidative component. Soybeans have been recognised as the major or only food source of isoflavones; however, the present study suggests that groundnut tubers can serve as potential functional foods that provide a sufficient amount of isoflavones and significant health benefits.