Antihypertensive capacity of defatted soft-shelled turtle powder after hydrolysis by gastrointestinal enzymes

Soft-shelled turtle (SST) is a high-value animal cultivated in Asia, possessing many mystical folk curative properties in traditional Chinese medicine. This study investigated the effects of enzymatic hydrolysate derived from defatted SST powder on inhibition of angiotensin I-converting enzyme (ACE) activity and on antihypertensive effect of spontaneously hypertension rats (SHR). The SST powder showed limited inhibition of ACE (IC50 = 16.7 mg/mL), while its enzymatic hydrolysate exhibited a fivefold increase in inhibition of ACE (IC50 = 3.2 mg/mL). The fraction of molecular weight (Mr) less than 1000 Da obtained through ultrafiltration exhibited the best inhibition of ACE (IC50 = 2.8 mg/mL). The fraction of Mr less than 1000 was then separated into six fractions by gel filtration, eluted with deionized water, and the ACE inhibitory activity from three fractions was analyzed. Fractions 4 and 5 with Mr at 560-600 Da and 440-480 Da had the strongest inhibition on ACE. Single oral administration of 500 mg/kg BW dose from enzymatic hydrolysates to SHR showed a noticeable decrease of systolic blood pressure (SBP) at the sixth and eighth hour after the administration when compared to 0 h (P < 0.05). The fractions with Mr over 5000 Da from enzymatic hydrolysate significantly lowered SBP after a single oral administration at a dose of 500 mg/kg BW in SHR. On the other hand, Mr less than 5000 and 1000 Da significantly lowered SBP after a single oral administration at a dose of 150 and 50 mg/kg BW. The results suggested that hydrolysates of defatted SST powder, produced with a gastrointestinal enzyme, showed inhibition of ACE activity and antihypertensive effect in SHR.