Different responses of fish/microbial transglutaminase to salt and ultrasound: Implications for myosin cross-linking

This study investigated the differential responses of fish transglutaminase (FTGase) and microbial transglutaminase (MTGase) to NaCl concentrations and high intensity ultrasound (HIU) during myosin cross-linking. Increasing NaCl concentrations enhanced MTGase activity by 34.5% at 0.8 mol/L compared to the control without NaCl. This enhancement was accompanied by structural unfolding, as evidenced by increased UV absorption intensity, which indicated greater exposure of aromatic residues. In contrast, FTGase activity progressively declined with increasing NaCl concentrations, with minimal structural changes observed. Given its Ca²⁺-independent property, MTGase was used to explore the synergistic effect of HIU and NaCl. The combination of 400 W HIU and 0.3 mol/L NaCl induced the most pronounced structural changes in MTGase, which exposed some buried reactive sulfhydryl groups and elevated activity by 17.0%. Furthermore, HIU pretreatment of both enzymes enhanced their catalytic efficiency for myosin cross-linking, as evidenced by increased formation of ε-(γ-Glu)-Lys isopeptide bonds. Notably, while HIU-pretreated FTGase improved the water holding capacity (WHC) of myosin samples, HIU-pretreated MTGase likely induced excessive cross-linking, which paradoxically reduced WHC. Conversely, applying HIU directly to preformed enzyme-myosin complexes disrupted established cross-links. These findings provide a basis for optimizing transglutaminase applications in food processing using HIU.