In order to investigate the effect of konjac glucomannan (KGM) on the gelling properties of shrimp myofibrillar protein (SMP), composite gel systems were prepared with different ratios to KGM and SMP (1:50, 1:20 and 1:10, m/m), named as SK50, SK20 and SK10, respectively. The surface hydrophobicity, endogenous fluorescence, turbidity, particle size, rheological properties, Fourier transform infrared (FTIR) spectra, protein composition and microstructure of the composite systems were investigated. The results showed that the surface hydrophobicity, turbidity and particle size of SMP and SMP-KGM composite systems increases with increasing KGM addition. Rheological analysis showed that shear thinning occurred in both SMP and SMP-KGM composite systems, and the storage modulus (G’) value of SK20 was the highest, indicating that the gel network in SK20 had the highest structural stability. FT-IR analysis showed the characteristic spectral bands of SMP and SMP-KGM composite systems were similar. SDS-PAGE analysis showed that addition of KGM did not significantly change the bands of SMP. After heating, the band densities of myosin heavy chain, paramyosin and actin were significantly decreased compared with the unheated group, indicating that the three proteins underwent aggregation during heating. Scanning electron microscope and fractal dimension analysis showed the formation of a dense and ordered gel with increasing KGM content. In general, adding an appropriate amount of KGM can improve the gel strength, and the product quality of shrimp surimi.

The purpose of this study is to efficiently prepare high-purity dipeptidyl peptidase-Ⅳ (DPP-Ⅳ) from beef kidney by sequential acid precipitation, ammonium sulfate fractionation and gel filtration chromatography. Under reducing conditions, the molecular mass of the purified protein was estimated to be 106 kDa with a purification fold of 169.9 and a yield of 9.5%. Twelve peptides with a total of 517 amino acid residues were identified from the DPP-Ⅳ by mass spectrometry. The matching degree of the 12 peptides with the amino acid sequence of bovine DPP-Ⅳ standard was 100%. The DPP-Ⅳ was found to be a dimeric glycoprotein (210 kDa), with an isoelectric point of 5.8. The secondary structure of the DPP-Ⅳ had a typical β-sheet conformation, and the thermal denaturation temperature was (72.7 ± 0.3) ℃. Metal ions such as Zn²⁺, Fe²⁺ and Fe³⁺ revealed a significant inhibitory effect on the activity of DPP-Ⅳ. The inhibition rate of 1 mmol/L Zn²⁺ on the enzyme’s activity was as high as 98%.

In this study, a novel esterase was purified from the liver of blue round scads via ammonium sulfate fractionation followed by a series of column chromatographies on Q-Sepharose, Q-HP, Phenyl Fast Flow and Superdex G75 with a recovery of 0.69% and 1150-fold purification factor. The esterase was a monomer with a molecular mass of 61.18 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Superdex G75 column chromatography. Peptide mass fingerprint analysis showed that the peptides showed a similarity of 100% to the predicted fatty acyl-CoA hydrolase precursor from Takifugu rubripes with 14 amino acid residues. The esterase showed an optimal activity at pH 8.0 and 50 ℃, and was comparably stable at pH 6.0-10.0 and 4-40 ℃. The metal ions K⁺, Zn²⁺, Cu²⁺, Al³⁺ and Fe²⁺ strongly suppressed the esterase activity while Ba²⁺, Ca²⁺, Mn²⁺, Co²⁺ and Mg²⁺ showed obvious activation effect on the enzyme’s activity. Moreover, the esterase exhibited strong tolerance against methanol, ethanol, acetone and isopropanol, as well as salt. Interestingly, the esterase preferably hydrolyzed esters with short carbon chain in aqueous solution, and exhibited hydrolytic activity toward long-chain esters in an emulsion system but not in aqueous solution. The results indicated that the esterase has both esterase and lipase activity.

This study was conducted in order to explore the changes in muscle quality of sea bass during cold storage. The physiological and biochemical changes of fish meat during storage at 4 ℃ were evaluated in terms of its pH, total bacterial count, total volatile basic nitrogen (TVB-N) content, K value and texture. Morphological changes of muscle were evaluated by Masson staining and immuno-histochemical staining. The enzymatic activity of matrix metalloproteinases (MMPs) was determined by hydrolyzing fluorescent peptide substrates. The results showed that the pH of sea bass muscle decreased firstly and then increased during cold storage. The total bacterial count, TVB-N content and K value increased with storage time, and exceeded the threshold on the 8^th day. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that there was little change in the protein composition in sea bass muscle after 6 days of storage, while the myosin heavy chain was significantly degraded on the 8^th day. Masson staining and immuno-histochemical staining of type Ⅰ collagen showed that the endomysium was gradually degraded during storage. On the 12^th day, an evident gap between type Ⅰ collagen and muscle fibers was observed. The activity of MMPs increased significantly with storage time while the hardness of fish muscle decreased gradually. Hence, the softening of fish muscle during cold storage is closely related to collagen decomposition, and type Ⅰ collagen degradation by MMPs may be the major reason for muscle softening after fish death.

In order to investigate the quality changes of Crassostrea gigas during cold storage at 0 and 4 ℃, the changes in the visual appearance of different tissues (adductor muscle, mantle, gills and visceral mass) during cold storage were observed. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to analyze the changes of total proteins in oyster tissues. The degradation of myosin heavy chain (MHC), actin, paramyosin (PM) and tropomyosin (TM) in adductor muscle was identified by Western blotting. The activity of endogenous serine protease and cathepsin B was measured using fluorescent spectrum. The quality change of adductor muscle during cold storage was evaluated in terms of its pH, K value and texture properties. The results showed that during cold storage, the texture of adductor muscle changed from tight to soft. Western blotting showed that MHC in adductor muscle began to degrade on days 3 and 1 for 0 and 4 ℃, respectively, indicating that low temperature is beneficial to reduce the rate of protein hydrolysis. However, PM, TM and actin did not change obviously. The activity of serine protease and cathepsin B in adductor muscle first rose and then decreased during cold storage. After cold storage for 11 days, the pH decreased from an initial value of 6.79 to 6.52 and 5.84 for 0 and 4 ℃, respectively. The K value exceeded the second degree of freshness range (40%) on days 9 and 5 for 0 and 4 ℃, respectively. The hardness and chewiness of adductor muscle peaked on day 1 and then decreased gradually, and elasticity decreased with storage time. The degradation of MHC in adductor muscle was most remarkable and closely relatedto the deterioration of oyster quality during cold storage. This study can provide a theoretical reference for oyster cold storage and preservation.

In order to improve the utilization of Haliotis discus hannai mantle, collagen purified from it was hydrolyzed by trypsin and pepsin to prepare angiotensin Ⅰ-converting enzyme (ACE) inhibitory peptides. The hydrolysate was purified by sequential ultrafiltration, Superdex^TM peptide 10/300 GL gel filtration chromatography and high performance liquid chromatography (HPLC). Three ACE inhibitory peptides, SGEVGQ, QRGPAGAQGPQ and GPPGPAGAR, were obtained . Among them, GPPGPAGAR had the highest ACE inhibitory activity with a half maximal inhibitory concentration (IC₅₀) of 177.1 μmol/L. Molecular docking results showed that GPPGPAGAR mainly acted on the S₁ active pocket of ACE and the inhibitory mode was similar to that of lisinopril. GPPGPAGAR still showed high ACE inhibitory activity after simulated gastrointestinal digestion. This study will provide a reference for deep processing of abalone mantle and the development of ACE inhibitory peptides.