Investigating phytochemical and antioxidant activity of free and bound phenolics from brown rice bran and their correlation with enzymatic inhibitory and in vitro starch digestibility

Abstract

Inhibiting the activity of digestive enzymes is a highly effective approach to controlling glucose absorption and, consequently, aiding in controlling type-2 diabetes mellitus. Brown rice bran, a by-product of the rice grain polishing process, represents a natural source rich in bioactive compounds that could be explored as a potential solution to reduce postprandial hyperglycemia. The study focused on evaluating the inhibitory effects of free and bound phenolic compounds on α-amylase and α-glucosidase enzymes, which play significant roles in carbohydrate metabolism. The IC50 values were found to be 0.947±0.05 mg/mL for α-amylase inhibition and 2.188±0.69 mg/mL for α-glucosidase inhibition in the free phenolic compound extract. In contrast, the bound phenolic compound extract displayed higher IC50 values of 13.861±0.03 mg/mL and 16.883±0.15 mg/mL for α-amylase and α-glucosidase inhibition, respectively. These differences in inhibitory potential between the free and bound phenolic compounds were statistically significant (p < 0.005). The superior inhibition observed in the free phenolic extract can be attributed to its higher levels of phenolic, flavonoid, and anthocyanin compounds compared to the bound form. Additionally, the antioxidant content analysis using the DPPH and FRAP methods revealed that the bound phenolic compounds exhibited lower antioxidant levels compared to the free form. Furthermore, the study investigated the impact of these bioactive compounds on starch digestibility by adding 10% sample extracts, both in free and bound forms, to high amylose rice starch. The results showed that the rate of starch digestibility was reduced to 54.04% and 58.2%, respectively, compared to the control (60.52%). Phytochemical content exhibited highly positive correlations with antioxidant activity leading to stronger enzymatic inhibition and slower the digestion rate. These findings suggest that the free phenolic extract from brown rice bran holds promise as a potential antidiabetic agent due to its ability to inhibit enzymatic digestion, leading to reduced postprandial glucose levels.