| Soybean protein is a quality vegetable protein. Because of its good functional properties, such as solubility, emulsibility, foamability and gelation capacity, it is widely applied in a variety of foods. Soybean protein hydrolysates with biological properties, like antioxidant activity and ACE inhibitory activity, can be obtained by hydrolyzing soybean protein isolate with some sort of protease.In our study, Fenton reaction and Plastein reaction were used to modify soybean protein hydrolysates respectively. The effect of Fenton reaction on DPPH radical scavenging activities of soybean protein hydrolysates, and the effect of Plastein reaction on ACE inhibitory activities of soybean protein hydrolysates were studied. The results obtained in our study are as follows:1. The modification of soybean protein hydrolysates by Fenton reaction.(1) Soybean protein isolate was hydrolyzed by Alcalase 2.4L FG, and hydrolysates with DH 9% were prepared. The IC50 value of the hydrolysates on DPPH radical was 2.92 mg/mL.(2) Soybean protein hydrolysates were modified by Fenton reaction. Ultraviolet absorption and Lowry assay were applied to estimate the effects of Fenton reaction on the hydrolysates. It was found that the amount of ultraviolet light absorbed at 280 nm was improved obviously after Feonton reaction. It was also found that the absorbance values of modified products at 750 nm by Lowry assay were enhanced.(3) The DPPH radical scavenging activities of modified products were studied. Their IC50 values obtained were 1.36～2.28 mg/mL, lower than that of original hydrolysates, which indicated that modification of soybean protein hydrolysates by Fenton reaction could improve their DPPH radical scavenging activities.(4) Variance analysis was performed using Excel 2003 sofeware. The results showed that Phe:H2O2 ratio had extremely significant effect on the properties of modified products (p<0.01), and reaction time had nonsignificant effect on the properties of modified products (p>0.05), which suggested that the additive amount of Fenton reagent was the main factor that impacted on the properties of modified products.2. The modification of soybean protein hydrolysates by Plastein reaction.(1) Soybean protein isolate was hydrolyzed by alcalase to prepare hydrolysates with ACE inhibitory activity as high as possible. It was found that the hydrolysates prepared by incubating of soybean protein isolate with alcalase for 4 h had the highest ACE inhibitory activity, whose IC50value was 1.45 mg/mL, and degree of hydrolysis was 16.6%.(2) The conditions of Plastein reaction to modify soybean protein hydrolysates were studied by response surface methodology with the decrease of free amino groups as response. The reaction temperature was fixed at 30℃, and the suitable conditions obtained were as follows: the concentration of the hydrolysates was 45% by weight, the addition level of alcalase was 275 U/g proteins, and reaction time was 3 to 4 h. The order of primary and secondary of the factors above impacting Plastein reaction was substrate concentration, reaction time, and E/S ratio.(3) Nine modified products with different reaction time were prepared and their ACE inhibitory activities were determined. The results showed that the IC50 values of modified products were in the range of 0.64 to 1.30 mg/mL, lowering than that of soybean protein hydrolysates (IC50 value equals to 1.45 mg/mL), which illuminated that their ACE inhibitory activities were improved. The modified product with 5 h Plastein reaction showed the highest ACE inhibitory activity, whose IC50 value was 0.64 mg/mL.(4) Soybean protein hydrolysates and three modified products were analyzed by size exclusion chromatography. The results showed that there were some larger peptides generated after Plastein reaction, and the peak area of larger peptides formed by Plastein reaction was in same order with the ACE inhibitory activity of the three modified products. A primary conclusion could be drawn that it was the formation of high molecular weight plasteins that improve the ACE inhibitory activity of modified products.
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