Activity Changes and Catalytic Effect of Cathepsin L on Low-Salt Myosin Under High-Intensity Ultrasound

The present study aimed to investigate the effects of high-intensity ultrasound (HIU) on the activity and structure, as well as the catalytic characteristics of endogenous proteinase. Initially, the primary proteinase responsible for silver carp surimi modori was analyzed through the investigation of TCA-soluble peptides and sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) on surimi gels with different proteinase inhibitors. The results showed that cysteine proteinase was predominant. On this basis, cathepsin L (Cat L) was selected as a representative, and the corresponding changes in enzymatic characteristics under HIU were evaluated by enzyme activity, particle size, ultraviolet–visible spectra, and sulfhydryl (SH) contents. Moreover, TCA-soluble peptides, SDS-PAGE, and microstructures were conducted to explore the Cat L-catalyzed hydrolysis of myosin. HIU significantly reduced the particle size of Cat L ( P < 0.05) and destroyed its tertiary structures, as evidenced by the increased exposure of amino acids. Moreover, some SH groups embedded inside the active sites were exposed, leading to a marked decrease in Cat L activity ( P < 0.05). Furthermore, HIU suppressed the myosin hydrolysis induced by Cat L, as supported by reduced degradation of myosin heavy chain and a corresponding decrease in TCA-soluble peptides. Additionally, transmission electron microscope observations demonstrated that the microstructures of myosin induced by HIU-treated Cat L were less coarse. In summary, HIU destroyed the Cat L molecules, leading to a significant reduction in enzyme activity, which inhibited the deterioration of low-salt myosin gels induced by Cat L.