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Abstract
This thesis investigated the antioxidant, antidiabetic and antihypertensive activities of peptides derived from rice bran proteins. Albumin, globulin, prolamin and glutelin were sequentially extracted from rice bran and the proteins were hydrolysed with four protease preparations, namely Alcalase, Neutrase, Flavourzyme and Protamex, for up to 240 min under their respective optimum conditions. The proteins were also subjected to an in vitro simulated digestion. Hydrolysate and digest samples were analysed for their antioxidant and α-amylase, α-glucosidase and angiotensin-converting enzyme (ACE)-inhibitory activities. Hydrolysis of the proteins resulted in significant increases in these bioactivities, which were generally correlated with the degree of hydrolysis. In general, highest bioactivities were found with albumin and glutelin hydrolysates and digests, followed by globulin, while prolamin samples showed the lowest activities. Of the four enzymes used, Alcalase and Protamex-catalysed hydrolysates generally had the highest activities while Flavourzyme-produced hydrolysates had the lowest activity. The hydrolysates and digests were fractionated by ultrafiltration and ion exchange chromatography, and the bioactivities of the individual fractions determined. The fractions with MW<3kDa had the highest bioactivities. The net electric charge of the peptides also had a significant influence on the bioactivities of the peptides, but the effect was not consistent for the different bioactivities. Liquid chromatography tandem mass spectrometry/mass spectrometry (LC-MS/MS) was able to identify dozens of peptides in the most active fractions and search against the BIOPEP database revealed that most of the peptides contained amino acid sequences with known bioactive activities. Enzyme kinetic study showed that inhibitions of the peptides against both α-glucosidase and ACE were of the non-competitive mode. The strong inhibitory activities of rice bran protein hydrolysates against these enzymes suggest that they have the potential to be developed into dietary and nutraceutical supplements or used as functional food ingredients for managing related disorders.