In order to utilize upstream chum salmon as a component of nutraceutical food, their defatted muscle proteins were hydrolyzed with 5% thermolysin. The resulting hydrolysate showed high inhibitory activity against angiotensin I-converting enzyme (inhibitory concentration 50 = 27.9 protein g/mL) in vitro. A significant reduction of systolic blood pressure was observed when 500 and 2000 mg/kg of body weight were orally administered into spontaneously hypertensive rats. Angiotensin I-converting enzyme inhibitory peptides contained in the hydrolysate were isolated with various chromatographs. These 6 active peptides were Trp residuecontaining dipeptides: Trp-Ala, Val-Trp, Trp-Met, Met-Trp, Ile-Trp, and Leu-Trp. The inhibitory concentration 50 values of these dipeptides ranged from 2.5 M to 277.3 M.

We isolated Phe-Leu as an angiotensin I-converting enzyme (ACE) inhibitor from hydrolysate of chum salmon muscle. The IC 50 value of this peptide was 13.6 lm, and it showed non-competitive inhibition. The reverse sequence dipeptide Leu-Phe also showed ACE inhibitory activity. However, Leu-Phe is much less inhibitory than Phe-Leu with an IC 50 value of 383.2 lm. In addition, the inhibition mode was competitive. To investigate the relationship between dipeptide sequence and ACE inhibition properties, we further measured ACE inhibitory activity and inhibition mechanism using six Trp-containing dipeptides, which had been identified from the same salmon muscle hydrolysate as ACE inhibitory peptides in a previous study. Peptides with Trp as the C-terminal residue, Ala-Trp, Val-Trp, Met-Trp, Ile-Trp, Leu-Trp showed non-competitive inhibition. On the other hand, reversed sequence peptides with Trp at the N-terminal were competitive inhibitors, except Trp-Leu. These results indicate that the sequence of ACE inhibitory dipeptides can affect both inhibitory potency and the inhibition mechanisms.

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