Angiotensin-converting enzyme inhibition and free-radical scavenging properties of cationic peptides derived from soybean protein hydrolysates.

Soybean protein isolate was hydrolyzed consecutively with pepsin and pancreatin followed by separation of the digest on a SP-Sepharose column to obtain angiotensin-converting enzyme (ACE)-inhibitory fractions. Three of the five fractions obtained inhibited ACE activity, with values of the concentration of hydrolysate fraction that inhibited 50% of ACE activity of 1.09, 0.42, and 0.25 mg/ml for fractions 2, 3, and 4, respectively. Kinetic analysis showed that fractions 2, 3, and 4 are non-competitive, competitive and mixed inhibitors of ACE, respectively. Fluorescence emission of ACE was quenched along with a red shift in wavelength of emission maxima in the presence of the protein hydrolysate fractions. Fraction 3 was the most potent free-radical scavenger, while fraction 4 had almost no scavenging effect. We concluded that fraction 3 had the most potential to be used as a bioactive ingredient because of its strong ACE-inhibitory and free-radical scavenging properties.