Oligomeric proanthocyanidins are the active compounds in Abelmoschus esculentus Moench for its α-amylase and α-glucosidase inhibition activity

We found that Abelmoschus esculentus Moench (okra, or lady's finger) exhibits significant α-amylase and α-glucosidase inhibitory activity. Assay-guided fractionation of the okra extracts resulted in identification of oligomeric proanthocyanidins in unripe okra seeds as the compounds responsible to the activity. High-performance liquid chromatography (HPLC) analysis using a diol column revealed oligomers are a dominant composition with higher degree (>20) of polymerisation appearing as one large peak in the chromatogram. Thiolysis treatment of the proanthocyanidins using methyl thioglycolate produced thioether derivatives of (epi)gallocatechins and (epi)catechins as major products. Based on the ratio of terminal and extension units, the mean degree of polymerisation was estimated to be 21.97. B-type gallocatechin oligomers up to pentadecamer could be detected using matrix-assisted laser desorption/ionisation time of flight (MALDI-TOF) mass spectra.