Characterization and bioactivity of proanthocyanidins during Malay cherry (Lepisanthes alata) fruit ripening

Bioactive proanthocyanidins are the main inhibitors of starch hydrolases in unripe and ripe Malay cherry fruits. Proanthocyanidins from both unripe and ripe fruits showed stronger inhibition of alpha-amylase with IC50 values of 3.54 +/- 0.03 and 1.19 +/- 0.02 mu g/mL, respectively, than that of alpha-glucosidase. The structural fingerprints of proanthocyanidins were identified using high-performance liquid chromatography electrospray ionization mass spectrometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analyses. Results showed the subunits of proanthocyanidins were (epi)catechins and (epi)gallocatechins, and they formed tetramers to dodecamers in unripe fruits and tetramers to hexadecamers in ripe fruits. The mean degree of polymerization (mDP) of proanthocyanidins was found to increase from 8 to 18 as fruit ripened. Larger proanthocyanidins (higher mDP values) in ripe fruits exerted significantly stronger inhibition of alpha-amylase and alpha-glucosidase than less polymerized proanthocyanidins in unripe fruits, indicating the dependence of inhibition on mDP. Total phenolics and total proanthocyanidins significantly decreased as fruit ripened.