Chemical Constituents of Malaysian U. cordata var. ferruginea and Their in Vitro α-Glucosidase Inhibitory Activities

Continuing our interest in the Uncaria genus, the phytochemistry and the in-vitro alpha-glucosidase inhibitory activities of Malaysian Uncaria cordata var. ferruginea were investigated. The phytochemical study of this plant, which employed various chromatographic techniques including recycling preparative HPLC, led to the isolation of ten compounds with diverse structures comprising three phenolic acids, two coumarins, three flavonoids, a terpene and an iridoid glycoside. These constituents were identified as 2-hydroxybenzoic acid or salicylic acid (1), 2,4-dihydroxybenzoic acid (2), 3,4-dihydroxybenzoic acid (3), scopoletin or 7-hydroxy-6-methoxy-coumarin (4), 3,4-dihydroxy-7-methoxycoumarin (5), quercetin (6), kaempferol (7), taxifolin (8), loganin (9) and beta-sitosterol (10). Structure elucidation of the compounds was accomplished with the aid of 1D and 2D Nuclear Magnetic Resonance (NMR) spectral data and Ultraviolet-Visible (UV-Vis), Fourier Transform Infrared (FTIR) spectroscopy and mass spectrometry (MS). In the alpha-glucosidase inhibitory assay, the crude methanolic extract of the stems of the plant and its acetone fraction exhibited strong alpha-glucosidase inhibition activity of 87.7% and 89.2%, respectively, while its DCM fraction exhibited only moderate inhibition (75.3%) at a concentration of 1 mg/mL. The IC50 values of both fractions were found to be significantly lower than the standard acarbose suggesting the presence of potential alpha-glucosidase inhibitors. Selected compounds isolated from the active fractions were then subjected to alpha-glucosidase assay in which 2,4-dihydroxybenzoic acid and quercetin showed strong inhibitory effects against the enzyme with IC50 values of 549 and 556 mu g/mL compared to acarbose (IC50 580 mu g/mL) while loganin and scopoletin only showed weak alpha-glucosidase inhibition of 44.9% and 34.5%, respectively. This is the first report of the isolation of 2-hydroxybenzoic acid, 2,4-dihydroxybenzoic acid and loganin from the genus and the first report of the alpha-glucosidase inhibitory potential of 2,4-dihydroxybenzoic acid.