Abstract
Soy protein isolate was hydrolysed with gastrointestinal enzymes, pepsin and pancreatin. The hydrolysate (SPH) was sequentiaUy fractionated using ultrafiltration (UF), gel filtration (GF) and high performance liquid chromatography (HPLC) and tested at each stage to select the bioactive fractions. All fractions were tested for antioxidant activity by the peroxide value and TBARS methods in a linoleic acid containing system. Compared to the untreated control the GF fraction (GFF36) significantly decreased (p < 0.05) TBARS production to 0.02 μg/ml like synthetic BHT. Further purification with HPLC did not improve the antioxidant activity (TBARS 0.13 μg/ml). Investigation of the mechanism of antioxidant activity showed that SPH and the 2 kD fraction scavenged superoxide, hydroxyl radical and DPPH free radicals and chelated iron. SPH showed hydrogen peroxide scavenging activity. The 2, 5 and 10 kD fractions demonstrated copper chelating activity in a copper-pyridine pyrocatechol system. The 2 kD fraction also formed complexes with copper and catalysed ascorbic acid oxidation. Affinity chromatography purified fraction (P4 fraction) from SPH chelated copper. Additionally, 300 μg/ml P4 prevented copper dependent LDL oxidation (antioxidant activity 54.57%); however, this was less effective than the GFF 36 fraction which had 56.84 % antioxidant activity at only 10 μg/ml. All soya fractions exhibited ACE inhibitory activity, especially the UF and GF fractions containing smaller peptides. However, further purification by HPLC to three fractions (HPF15, 16 and 17) showed less activity than the parent GFF 57 fraction. The ACE inhibitory activity values for the SPH, 10 kD, 5 kD, 2 kD, GFF 57 and HPFs 15, 16 and 17 fractions were 17.70, 29.50, 43.66, 58.87, 84.68, 32.18, 44.07 and 20.86 % respectively. Fractions from soy protein isolate comprising potent peptides and possibly polyphenols demonstrated antioxidant, antihypeliensive and anticancer properties in vitro; these fractions could be developed further as neutraceuticals.