Inhibitory evaluation of oligonol on α-glucosidase, protein tyrosine phosphatase 1B, cholinesterase, and β-secretase 1 related to diabetes and Alzheimer's disease

Oligonol is a low-molecular-weight form of polyphenol that is derived from lychee fruit extract and contains catechin-type monomers and oligomers of proanthocyanidins. This study investigates the anti-diabetic activities of oligonol via α-glucosidase and human recombinant protein tyrosine phosphatase 1B (PTP1B) assays, as well as its anti-Alzheimer activities by evaluating the ability of this compound to inhibit acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and β-site amyloid precursor protein cleaving enzyme 1 (BACE1). Oligonol exhibited potent concentration-dependent anti-diabetic activities by inhibiting α-glucosidase and PTP1B with IC₅₀ values of 23.14 μg/mL and 1.02 μg/mL, respectively. Moreover, a kinetics study revealed that oligonol inhibited α-glucosidase (K _i = 22.36) and PTP1B (K _i = 8.51) with characteristics typical of a mixed inhibitor. Oligonol also displayed potent concentration-dependent inhibitory activity against AChE and BChE with IC₅₀ values of 4.34 μg/mL and 2.07 μg/mL, respectively. However, oligonol exhibited only marginal concentration-dependent BACE1 inhibitory activity with an IC₅₀ value of 130.45 μg/mL. A kinetics study revealed mixed-type inhibition against AChE (K _i = 4.65) and BACE1 (K _i = 58.80), and noncompetitive-type inhibition against BChE (K _i = 9.80). Furthermore, oligonol exhibited dose-dependent inhibitory activity against peroxynitrite (ONOO^-)-mediated protein tyrosine nitration. These results indicate that oligonol has strong preventative potential in diabetes mellitus and in Alzheimer's disease.