Total Phenolic, Flavonoid, and Coixol Contents, and In vitro α-amylase and α-glucosidase Inhibitory Activities of Coix lacryma-jobi L. Seed Extracts

Abstract

Job’s tears (Coix lacryma-jobi L., Poaceae) seeds have been traditionally used for their hypoglycemic effects, but the phytochemicals and underlying mechanisms remain unclear. This study investigates the phytochemicals and inhibitory effects against α-amylase and α-glucosidase of the raw and cooked seeds. The seeds were extracted with 95% ethanol, then fractionated with dichloromethane, ethyl acetate, and water to obtain fractions designated RE, RD, RA, RW (raw seeds), and CE, CD, CA, and CW (cooked seeds), respectively. The phytochemical content, coixol content, and effects on α-amylase and α-glucosidase activities were determined using colorimetry, high-performance liquid chromatography, and enzymatic assays, respectively. The extracts yielded phenolic compounds (4.46-18.84 mg gallic acid equivalents/g), flavonoids (6.21-69.60 mg rutin equivalents/g), and coixol (0.0-30.70 µg coixol/g), with the highest levels of those compounds found in CA, RW, and CE, respectively. Since the tested extracts showed stronger inhibitory effects on α-glucosidase (48.94-98.90%, at 0.1 mg/mL) than on α-amylase (0.30-45.67%, at 5.0 mg/mL), the half-inhibitory concentration (IC50) values were determined only for the potent fractions (RE, RD, RA, CD, and CA) against α-glucosidase. The IC50 values of selected fractions (17.92 to 86.34 µg/mL) were significantly lower than that of acarbose (461.10 µg/mL). Among these, the RA fraction showed the highest efficacy and potency. Flavonoid content correlated strongly with α-glucosidase inhibition, while phenolics and coixol correlated moderately. Dichloromethane and ethyl acetate fractions from raw or cooked Job’s tears seeds, rich in phytochemicals, displayed potential for further investigation of their antihyperglycemic activity in cell-based experiments or in an animal model.