Momilactones A and B Are α-Amylase and α-Glucosidase Inhibitors

Momilactones A (MA) and B (MB) are the active phytoalexins and allelochemicals in rice. In this study, MA and MB were purified from rice husk of Oryza sativa cv. Koshihikari by column chromatography, and purification was confirmed by high-performance liquid chromatography, thin-layer chromatography, gas chromatography-mass spectrometry, liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS), and H-1 and C-13 nuclear magnetic resonance analyses. By in vitro assays, both MA and MB exerted potent inhibition on alpha-amylase and alpha-glucosidase activities. The inhibitory effect of MB on these two key enzymes was greater than that of MA. Both MA and MB exerted greater alpha-glucosidase suppression as compared to that of the commercial diabetic inhibitor acarbose. Quantities of MA and MB in rice grain were 2.07 +/- 0.01 and 1.06 +/- 0.01 mu g/dry weight (DW), respectively. This study was the first to confirm the presence of MA and MB in refined rice grain and reported the alpha-amylase and alpha-glucosidase inhibitory activity of the two compounds. The improved protocol of LC-ESI-MS in this research was simple and effective to detect and isolate MA and MB in rice organs.