A novel angiotensin converting enzyme inhibitory peptide derived from proteolytic digest of Chinese soft-shelled turtle egg white proteins.

In this study, soft-shelled turtle (Pelodiscus sinensis) egg white (SSTEW) proteins were digested by thermolysin and the resulting small peptides were further fractionated by reverse phase chromatography. Peptides with angiotensin I-converting enzyme inhibitory (ACEI) activity from these fractions were screened. A lysozyme-derived peptide, IW-11, from the fraction with the most effective ACEI was identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and its purified form showed effective ACEI activity in vitro (IC50=4.39±0.31μM). The Lineweaver-Burk plots indicated that the inhibition towards ACE caused by this peptide is a competitive inhibition. The molecular docking study further revealed that the ACEI activity of IW-11 is mainly attributed to the formation of hydrogen bonds between the N-terminal residue of IW-11 and the S1 pocket (Ala354 and Tyr523) and the S2' region (His513 and His353) of ACE. Moreover, the digestion parameters were further optimized and the target peptide (82% purity) was readily obtained (15% yield) without any cumbersome purification procedure. Notably, lysozyme C is the most abundant protein in SSTEW, which implies that an efficient production of this ACEI peptide from SSTEW is promising. BIOLOGICAL SIGNIFICANCE: Inhibition of ACE has proven to be an effective strategy in prevention and treatment of hypertension and related diseases. Unlike typical synthetic ACE inhibitors which exert well described side effects, food-derived peptides with ACE inhibitory activity may be safer alternatives for hypertension treatment. In this study, we comprehensively identified peptides derived from SSTEW digest using a proteomic approach. IW-11, which is derived from lysozyme, the most abundant protein in SSTEW, showed remarkable inhibition towards ACE. This peptide has been demonstrated to have a competitive inhibitory property which is able to bind to ACE active site and found to be a true inhibitor against ACE according to Lineweaver-Burk plots. Using an optimized thermolysin condition, IW-11 can be readily obtained without any complex purification step, which will benefit its further application to prevention or treatment of hypertension.