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Title: Sarcomere Length Influences u-calpain Mediated Proteolysis

item Bowker, Brian

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2009
Publication Date: 3/27/2009
Citation: Weaver, A., Bowker, B.C., Gerrard, D. 2009. Sarcomere Length Influences u-calpain Mediated Proteolysis. Journal of Animal Science. 87:2096-2103.

Interpretive Summary: Tenderness is an economically important and complex meat quality trait that is difficult to predict and/or control for both meat processors and consumers. Beef tenderness is influenced by the degree of muscle shortening during rigor mortis development and by muscle protein degradation during meat storage. Unfortunately, the interacting effects of muscle shortening and protein degradation on tenderness are not well understood. In this study, muscle proteins were isolated from muscles with varying degrees of shortening and incubated with muscle proteases in vitro and the amount of protein degradation was measured. Data indicate that protein degradation was greater in muscles with less shortening. By showing that the degree of muscle shortening postmortem influences protein degradation associated with beef tenderization, these data help to decipher the underlying biological mechanisms that influence meat tenderness.

Technical Abstract: Muscle shortening and postmortem proteolysis both influence beef tenderness, but their interacting effects on tenderness are relatively unknown. Inherent myofibril structure and the extent of overlap between myosin and actin filaments are hypothesized to affect the availability of substrates for degradation by calpains. The objective of this study was to determine the influence of sarcomere length on the extent of calpain-induced proteolysis of bovine myofibrils in vitro. Bovine semitendinosus (ST) muscles were excised within 20 min postmortem and dissected into strips which were stretched and attached to applicator sticks or allowed slack to generate samples with different sarcomere lengths upon rigor completion. Samples were allowed to undergo rigor in a neutral pH buffer containing a protease inhibitor. Myofibrils were isolated and incubated at room temperature with exogenous µ-calpain at pH 6.8 for 0, 2, 60, 1440 or 2880 min. Purified troponin was subjected to the same digestion conditions. Proteolysis of troponin T (TnT) was monitored using SDS-PAGE and western blotting. Sarcomere length was greater (P<0.0001) in stretched versus shortened samples (2.99 µm ± 0.03 vs. 2.12 ± 0.03 µm respectively, means ± SE). Western blots for both stretched and shortened samples exhibited bands corresponding to intact TnT and TnT fragments. The abundance of intact TnT decreased (P<0.0001) with incubation time across both treatments. At 1440 and 2880 min, less (P<0.05) intact TnT was detected in samples with long sarcomeres. These data indicate proteolysis of TnT occurs to a greater extent in samples with longer sarcomeres possibly due to easier access of proteases to their targeted substrates. Degradation patterns of TnT were similar between myofibrils and purified troponin after incubation with µ–calpain. Therefore, it is unlikely that the mechanism by which proteolysis is limited in short sarcomeres involves an actomyosin-mediated interference of TnT.