Antioxidative and α-glucosidase inhibitory constituents of Polyscias guilfoylei: experimental and computational assessments

Abstract

Searching for bioactive agents from medicinal plants, eleven constituents were isolated from Polyscias guilfoylei stem for the first time, including a nucleoside uracil (1), two sterols β-sitosterol (2) and daucosterol (3), a saponin androseptoside A (4), two lignans (+)-pinoresinol (5) and (+)-syringaresinol (6), four phenolic acids protocatechuic acid (7), methyl protocatechuate (8), caffeic acid (9), and 5-O-caffeoylquinic acid (10), and a flavonoid quercitrin (11). Metabolites 1, 4, and 6-11 have never been observed in genus Polyscias before. Phenolic compounds 7 and 9 possessed the respective IC₅₀ values of 21.33 and 13.88 µg/mL in DPPH (2,2-diphenyl-1-picrylhydrazyl) antioxidative assay, as compared with that of the positive control resveratrol (IC₅₀ = 13.21 µg/mL). From density functional theory (DFT) calculated approach, the DPPH free radical scavenging capacity of two compounds 7 and 9 can be explained by the role of OH groups at carbons C-3 and C-4. Antioxidative actions of these two potential agents are followed HAT (H atom transfer) mechanism by OH bond disruption in gas, but SPLET (sequential proton loss electron transfer) mechanism in solvents water and methanol. Compared to 4-OH group, 3-OH group showed better bond disruption enthalpies and better kinetic energies since it reacted with HOO^• and DPPH radicals. Sterols 2-3 and flavonoid 11 induced the IC₅₀ values of < 2.0 µg/mL better than the positive control acarbose (IC₅₀ = 184.0 µg/mL) in α-glucosidase inhibitory assay. Their interactions with human intestinal C- and N-terminal domains of α-glucosidase were explored using molecular docking study. The obtained results proved that compounds 2, 3, and 11 bind relatively stronger with the C-terminal domain than to the N-terminal domain through pivotal residues in the binding site and could be hypothesized as mixed inhibitors.

Keywords: Antioxidant; Density functional theory; Molecular docking; Polyscias guilfoylei; α-Glucosidase inhibition.