Caspase-3 does not enhance in vitro bovine myofibril degradation by μ-calpain

Tenderness is a key component of palatability, which influences consumers' perception of meat quality. There are a variety of factors that contribute to the tenderness of beef carcasses, including postmortem proteolysis. A more complete understanding of this biological mechanism regulating tenderness is needed to ensure consistently tender beef. Numerous reports indicate µ-calpain is primarily responsible for the degradation of proteins postmortem. Meanwhile, it has been shown that caspase-3 can cleave calpastatin, the inhibitor of µ-calpain. Therefore, the objective of this study was to determine if in vitro degradation of calpastatin by caspase-3 can enhance the postmortem breakdown of myofibrillar proteins by µ-calpain. Bovine semitendinosus muscles were excised from two carcasses 20 min postmortem. Muscle strips were dissected from the semitendinosus, restrained to maintain length, and placed in a neutral buffer containing protease inhibitors. Upon rigor completion, myofibrils were isolated from each strip, and sarcomere length was determined. Samples with similar sarcomere lengths were selected to minimize the effect of sarcomere length on proteolysis. Myofibrils were then incubated at 22°C with either µ-calpain, µ-calpain+calpastatin, µ-calpain+caspase-3+calpastatin, or caspase-3+calpastatin for 0.25, 1, 3, 24, 48, or 72 h at a pH of 6.8. Proteolysis of troponin T (TnT) and calpastatin was evaluated using SDS-PAGE and Western blotting techniques. Analysis of Western blots confirmed significant degradation of calpastatin by caspase-3 (P<0.05). Additionally, Western blots revealed intact calpastatin disappeared rapidly as a result of digestion by µ-calpain. Although caspase-3 did not significantly degrade TnT (P>0.05), all µ-calpain digestion treatments resulted in substantial TnT breakdown (P<0.05). Degradation of TnT did not differ between the µ-calpain+calpastatin and µ-calpain+caspase-3+calpastatin digestions (P>0.05). Results of this study indicate caspase-3 cleavage of calpastatin does not enhance in vitro degradation of TnT by µ-calpain.