Inhibition of Three Diabetes-Related Enzymes by Procyanidins from Lotus ( Nelumbo nucifera Gaertn.) Seedpods

The inhibitory effects of procyanidins from lotus ( Nelumbo nucifera Gaertn.) seedpods on the activities of α -amylase, α -glucosidase and protein tyrosine phosphatase 1B (PTP1B), were studied and compared with those of (+)-catechin, (−)-epicatechin, epigallocatechin gallate (EGCG), procyanidin dimer B2 and trimer C1. The results showed that Lotus procyanidin extract (LPE) significantly inhibited α -amylase, α -glucosidase and PTP1B with IC 50 values of 5.5, 1.0, and 0.33 μ g/mL, respectively. The inhibition increased with the degree of polymerization and the existence of galloyl or gallocatechin units. Kinetic analysis showed that LPE inhibited α -glucosidase activity in a mixed competitive and noncompetitive mode. Fluorescence quenching revealed that α -glucosidase interacted with LPE or EGCG in an apparent static mode, or the model of “sphere of action”. The apparent static ( K ) and bimolecular ( k q ) constants were 4375 M −1 and 4.375 × 10 11 M −1 s −1 , respectively, for LPE and 1195 M −1 and 1.195 × 10 11 M −1 s −1 , respectively, for EGCG. Molecular docking analysis provided further information on the interactions of (+)-catechin, (−)-epicatechin, EGCG, B2 and C1 with α -glucosidase. It is hypothesized that LPE may bind to multiple sites of the enzyme through hydrogen bonding and hydrophobic interactions, leading to conformational changes in the enzyme and thus inhibiting its activity. These findings first elucidate the inhibitory effect of LPE on diabetes-related enzymes and highlight the usefulness of LPE as a dietary supplement for the prophylaxis of diabetes.