LAUSR

Calpain is known to be the most important endogenous protease responsible for degrading myofibril protein, thus affecting meat quality. This study aimed to investigate the activation of calpain and its potential influence in fish after slaughter. Calpain activity, expressions of the calpain inhibitor and activator (calpastatin and UK114, respectively), and effects of influencing factors on calpain activity and on the necessary Ca2+ activation concentration was investigated. Calpain activity increased significantly during the first 5 h of storage followed by rapid decline (P < 0.05). Three calpastatin expression bands were detected, and the 120-kDa calpastatin was significantly down-regulated with storage time (P < 0.05), whereas the level of UK114 in tilapia muscle was significantly up-regulated during postmortem storage (P < 0.05). Calpastatin and UK114 showed opposite effects on calpain activity; a significant decrease and increase in calpain activity were observed when crude calpain was incubated with calpastatin and UK114, respectively (P < 0.05). Furthermore, a significant increase was observed in calpain activity following co-treatment with calpastatin and UK114 (P < 0.05), but lower than that of treatment with UK114. The calpain activity in the UK114-treated group was higher than that of the control and calpastatin with the same Ca2+ concentration. Calpastatin inhibited calpain activation, whereas UK114 promoted the activation of calpain by reducing the concentration of Ca2+ required. Therefore, we propose that UK114 may increase calpain activity by inhibiting that of calpastatin.