Effects of Ultrasonication on the Antioxidant and Anti-diabetic Properties of Hydrolyzed Oat Proteins

Abstract

The method of protein extraction from foods can affect yields, compositions, and functionalities of the isolated proteins. The first objective of this work was to determine the effects of ultrasonic treatments on protein yields and polypeptide compositions. Proteins were consequently extracted from oat brans under normal (CTL) conditions and after pre-treatments of brans with ultrasonic bath (UB), and high power (HP) sonication. There was no significant difference in extraction yields (2.06 -3.79%), meanwhile gel electrophoresis and tandem mass spectrometry (LC-MS/MS) analysis showed differences in polypeptide compositions. Specifically, globulin and avenin proteins were present in all samples, however, ultrasonic treatments (UB and HP) released vromindoline, a starch-bound protein, while tryptophanin, a lipid-bound protein, was released after high power ultrasonic (HP) treatment. The second objective of this work was to determine the physical, chemical, and biological properties of the proteins after hydrolysis with proteases of various specificities. Proteins from CTL, UB, HP samples were hydrolyzed by three proteases (Flavourzyme, Papain, and Alcalase) and then characterized based on their sulfhydryl (SH) content, hydrophobicity and zeta potential. Data showed that HP ultrasonic treatments decreased SH concentrations in Papain and Alcalase hydrolysates but increased it in the Flavourzyme hydrolysate. The surface charge (i.e. zeta potential) was dependent on both the protein extraction and the protease. Papain had a much weaker surface charge (-0.78 to -1.32 mV) than the other proteases (-3.67 to -9.17 mV). UB treatments increased hydrophobicity for Flavourzyme and Papain hydrolysates, while HP treatments decreased the hydrophobicity. To further evaluate the hydrolyzed proteins (i.e. hydrolysates), their biological activities were determined. It was found that in general, ultrasonic treatments reduced the antioxidant activity of the protein hydrolysates. For example, peroxyl radical scavenging activity was reduced by UB and HP sonication; however, HO• scavenging activity was reduced by HP and not UB sonication. In the antidiabetic tests, hydrolysates were evaluated for their ability to modulate the activity of dipeptidyl peptidase-4 (DPP-4) and alpha-amylase, as well as the secretion of GLP-1, a hormone that stimulates insulin response in NCI-H716 cells. Ultrasonication did not affect DPP-4 inhibition; however, concerning proteases, Papain hydrolysates had substantially greater (49.7 -53.6%) DPP-4 inhibitory activity for all conditions. HP sonication slightly decreased the alpha-amylase inhibitory activity by 4.8 -7.2% compared to UB and control. A two-fold increase in hydrolysate concentration resulted in a 13.7 -76% increase in GLP-1 secretion, indicating a dose-dependent