Inhibitory effects of condensed tannins on angiotensin converting enzyme.

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Abstract
Effects of condensed tannins isolated from Rhei Rhizoma on the activities of angiotensin converting enzyme (ACE) and various proteases were examined in vitro. Among the various condensed tannins tested, procyanidin B-5 3, 3' di-O-gallate and procyanidin C-1 3, 3', 3''-tri-O-gallate strongly inhibited the activity of ACE. The concentration of procyanidin B-5 3, 3'-di-O-gallate required for 50% inhibition of ACE was 1.3×10-6 M. The inhibition of ACE by condensed tannins was reversible and non-competitive, according to dialysis and to Dixon plots. However, over one hundred times the concentration was required to inhibit activities of other proteases such as trypsin, chymotrypsin, leucine aminopeptidase, carboxypeptidase A and urinary kallikrein. These results suggest that the inhibitory effects of condensed tannins on the activities of ACE are specific.