Methyl Caffeate as an [alpha]-Glucosidase Inhibitor from Solanum torvum Fruits and the Activity of Related Compounds

In screening experiments for rat intestinal α-glucosidase (sucrase and maltase) inhibitors in 325 plants cultivated in Japan's southern island, of Tanegashima, marked inhibition against both sucrase and maltase was found in the extract of the fruit of Solanum torvum. Enzyme-assay guided fractionation of the extract led to the isolation of methyl caffeate (1) as a rat intestinal sucrase and maltase inhibitor. We examined 13 caffeoyl derivatives for sucrase- and maltase-inhibitory activities. The results showed that methyl caffeate (1) had a most favorable structure for both sucrase and maltase inhibition, except for a higher activity of methyl 3,4,5-trihydroxycinnamate (14) against sucrase. Its moderate inhibitory action against α-glucosidase provides a prospect for antidiabetic usage of S. torvum fruit.