Inhibitory Effects of Flavonoids from Lespedeza cuneata on Aldose Reductase

Inhibition of aldose reductase (AR) has been shown to prevent the onset and progression of many diabetic complications wherein several AR inhibitors were isolated from plants abundant in polyphenolic compounds. Lespedeza cuneata (Fabaceae), a perennial plant indigenous in East Asian countries, is shown to be abundant in these polyphenolic substances such as flavonoids and tannins. However, there are no studies to date regarding its effects on AR. In this study, the inhibitory activity of the methanol extract and stepwise polarity fractions of Lespedeza cuneata on AR was investigated. The bioactive compounds purified from L. cuneata by repeated column chromatography were also tested for AR inhibition. Results show that the ethyl acetate and n-butanol fractions of L. cuneata exhibited potent inhibition against AR with $IC_{50}$ values of 0.57 and $0.49{\mu}g/mL$, respectively. Further analysis led to the isolation of acacetin (1), afzelin (2), astragalin (3), kaempferol (4), and scutellarein 7-O-glucoside (5). The AR inhibitory effects these five compounds were also determined in which compounds 2, 3, and 5 showed potent AR inhibitory effects with $IC_{50}$ values of 2.20, 1.91, and $12.87{\mu}M$, respectively. Quantitative analysis of astragalin (3) by HPLC-UV was also performed in the leaves and roots of L. cuneata (0.626 and 0.671 mg/g, respectively). This study reports that the flavonoids isolated from L. cuneata show promising AR inhibitory activities which can be further developed as natural therapies for treating and managing diabetic complications.