Effect of Different Membrane Separation Parts on the Activity of Dipeptidyl Peptidase-IV in the Aqueous Extract of Asparagus officinalis and Its Compositional Analysis

To identify the main functional components of asparagus aqueous extract that inhibit dipeptidyl peptidase-IV (DPP-4) activity, five asparagus aqueous extract parts with different molecular weight cut-offs, including >30, 10~30, 6~10, 3~6, and <3 kDa, were successfully prepared by membrane separation technology in this study. The DPP-4 inhibition effect of each part was compared, and its total saponin, total flavonoid, total polysaccharide, total polyphenol, and total nitrogen contents were analysed. The results showed that the total nitrogen content showed the highest positive correlation with the inhibition rate (P<0.01), and the molecular weight cut-off (MWCO) greater than 30 kDa (UAJ1) was the site with the highest total nitrogen content and showed the strongest inhibition, with an IC50 value of 8.19 mg/mL, which was of the mixed inhibition type. Total polysaccharide content had the second highest positive correlation with the inhibition rate (P<0.01), total polyphenol content also showed a significant positive correlation with the inhibition rate (P<0.05), while total flavonoid content showed a very significant negative correlation with the inhibition rate (P<0.01). There was no significant correlation between total saponins and inhibition rate. In summary, the components in the aqueous extract of asparagus that exerted major inhibitory effects on DPP-4 were protein fractions with molecular weights greater than 30 kDa. This results would provide a reference for natural DPP-4 inhibitors in hypoglycaemic health food and drug development.