Phenolic compounds, antioxidant and α-glucosidase inhibitory activities from moringa oleifera seed by-products

Moringa oleifera seed husks, the traditional non-medicinal parts of M. oleifera seed, are usually discarded or burned, which pollute the environment and waste resources. In an unprecedented study, the antioxidant and α-glucosidase inhibitory activities of crude extracts from the medicinal and by-product parts of M. oleifera seed were compared and evaluated. The results showed that the scavenging ability of M. oleifera seed husks on 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radicals, and the α-glucosidase inhibitory activities were significantly stronger than that of kernels. 24 phenolic compounds, including 1 new compound, 2 new natural products and 21 known phenolic compounds, were isolated from M. oleifera seed husks. Among of them, erythro-guaiacylg-lycerol (23) and threo-guaia-cylglycerol (24) exhibited significant antioxidant activity, and their half-maximal inhibitory concentration (IC₅₀) values for scavenging DPPH and ABTS radicals were 32.39, 7.16 µg/mL and 24.24, 7.35 µg/mL, respectively. 2-(4-hydroxy-3-(4-hydroxybenzyl) phenyl) acetamide (1), p-hydroxybenzoic acid (11), 4,4-dihydroxydiphenylmethane (16) and 4-hydroxy-3-[(4-hydroxyphenyl) methyl] benzeneacetonitrile (17) showed strong α-glucosidase inhibitory activity, with IC₅₀ values of 16.55, 17.45, 25.12, and 14.10 µg/mL, respectively. The structure-activity relationship (SAR) study revealed the importance of the presence of 1,2,4-trisubstituted phenol structural fragment in enhancing the antioxidant and α-glucosidase inhibitory activities of the compounds. Compounds with such structural fragment may be developed into potential hypoglycemic drugs. According to the results, M. oleifera seed husks could be an alternative to develop novel drugs, functional foods and nutritional health products with hypoglycemic effects. The findings also indicate that M. oleifera seed by-products have the potential to be converted into value-added products, thereby contributing to sustainable waste management and environmental protection.