| The plastein reaction is an enzymatic process which could be simply seemed as areversal of usual protein degradation. During the plastein reaction a mixture ofhigher-molecular, protein-like substances formed from lower-molecular peptides.Plastein reaction could be used for enhancing protein’s nutritive values and functionalproperties, supplementing essential amino acid and improving of hydrolysates’ flavor.ACE inhibitory peptides could be an ideal drug for the treatment of hypertension for itcan control blood pressure through inhibiting angiotensin converting enzyme (ACE).Recently, research found that the ACE inhibitory activity of hydrolysate could beimproved by the modification of plastein reaction, this is a new way to improve theactivity of ACE inhibitory peptides. However, the mechanism of plastein reaction hasnot been elucidated till now despite the important theoretical bases can provide for theapplication of plastein in food, cosmetic and chemistry industries. So the purpose ofthis paper is to clarify the mechanism of plastein reaction of hydrolyzed Acaudinamolpadioidea with papain.The main contents of this study are as follows:1. The effect of modification by plastein reaction on the ACE inhibitory activityof Acaudina molpadioidea hydrolysates was studied. Results indicated that the ACEinhibitory activity of Acaudina molpadioidea protein hydrolysates decreased from0.44mg/mL to0.12mg/mL after being modified by plastein reaction. A novel ACEinhibitory peptide was isolated from modified product using different methodsincluding ultrafiltration, Sephadex G-15gel filtration and RP-HPLC. The purifiedACE inhibitory peptide was sequenced as NQNFVQYTTNT with an IC50value of3.67μmol/L, and this sequence was not found in the peptides database whilesearching by Mascot, which inferred that it was a new sequence compounded by plastein reaction. The accurate sites of ACE that the peptide bound to weredetermined through the computer assisted software SYBYL.2. Partial properties of the peptides modified by plastein reaction wereresearched. The stability of the modified peptides to simulated gastrointestinaldigestion was tested in vitro, results showed that the ACE inhibitory activities of thepeptides could be enhanced by digestive enzyme. Toxicity test showed no toxic to theMDCK cells when concentration was less than10mg/mL, and low concentrationcould promote the cells’ growth. UV spectrum analysis proved that changes could beobserved in the binding of ACE and peptides.3. The structure characterization of Plastein was researched. The FTIR spectraand CD spectrum showed that secondary structure, such as random curl and β-foldwere generated during plastein reaction. The denaturation temperature and enthalpyΔH of hydrolysate changed significantly after plastein reaction by the analysis ofthermodynamic properties, and the denaturation temperature increased from129.86℃to134.18℃, while the ΔH changed from21.8J/g to79.7J/g, indicating that strongforces were formed in plastein. The X-ray diffraction spectrum showed that newsubstance structure was formed in plastein reaction. The result of scanning electronmicroscopy showed that the plastein had obvious structure, similar to a completeprotein spatial structure.4. The mechanism of plastein reaction was research. From effects of sealing freeterminal groups’ content on plastein reaction, hydrophobicity and amino acidcomposition of plastein, we concluded that there were several processes in theplastein reaction. These processes contained new peptide bond forming (condensationand transpeptide), this not only proved why plastein reaction could be applied toenhancing the functional properties of protein, but also improved the ACE inhibitoryactivity of Acaudina molpadioidea hydrolysate. Moreover, physical interaction, suchas hydrophobic interaction, hydrogen bond and elecstatic interaction also played animportant role in plastein. Through these processes that plastein jelly was formed. |
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