Strong antihyperglycemic effects of water-soluble fraction of Brazilian propolis and its bioactive constituent, 3,4,5-tri-O-caffeoylquinic acid.

To clarify the suppression of postprandial blood glucose rise via alpha-glucosidase (AGH) inhibitory action by natural compounds, propolis was examined in this study. A single oral administration of propolis extract (50% methanol fraction on XAD-2 column chromatography) in Sprague-Dawley rats demonstrated a potent antihyperglycemic effect with the significant AUC(0-120 min) reduction of 38% at a dose of 20 mg/kg compared to that of controls. Among the active compounds isolated from the fraction, 3,4,5-tri-caffeoylquinic acid was found to be a prominent candidate that exerts the effect and shows a strong maltase-specific inhibition with an IC(50) value of 24 microM. In addition, the noncompetitive inhibition power apparently increased with the number of caffeoyl groups bound to quinic acid.