In this study, rapeseed protein isolate (RPI) was digested with various proteases to produce rapeseed protein hydrolysates (RPHs), which were then separated into different peptide fractions (<1, 1-3, 3-5, and 5-10kDa) by membrane ultrafiltration. Membrane fractionation showed that peptides with sizes <3 kDa had significantly (p<0.05) reduced surface hydrophobicity when compared to the RPHs and peptide fractions with sizes >3 kDa. In contrast, the <3 kDa peptides showed significantly (p<0.05) higher oxygen radical scavenging ability when compared to the >3 kDa peptides and RPHs. In vitro inhibition of angiotensin I-converting enzyme (ACE) was significantly (p<0.05) higher for the Thermolysin, Proteinase K and Alcalase RPHs when compared to the pepsin+pancreatin (PP) and Flavourzyme RPHs. The Alcalase RPH had significantly (p<0.05) higher renin inhibition among the RPHs, while with the exception of Thermolysin, the 5-10 kDa peptide fraction had the least renin-inhibitory ability when compared to the <5 kDa peptide fractions. Oral administration (100mg/kg body weight) of the RPHs and RPI to spontaneously hypertensive rats (SHR) showed the Alcalase RPH to be the most effective in blood pressure (BP) reduction (∼24 mm Hg) while Proteinase K RPH was the least effective (∼5 mm Hg) after 8h. However, the PP RPH had the most prolonged effect with BP reduction of ∼20 mm Hg after 24h of oral administration. We conclude that the strong BP-lowering ability of Alcalase and PP RPHs could be due to high resistance of the peptides to structural degradation coupled with high absorption rate within the gastrointestinal tract.
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