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.