μ-Calpain is essential for postmortem proteolysis of muscle proteins

The objective of this investigation was to test the hypothesis that μ-calpain is largely responsible for postmortem proteolysis of muscle proteins. To accomplish this objective, we compared proteolysis of known muscle proteins in muscles of wild type and μ-calpain knockout mice during postmortem storage. Knockout mice (n = 6) were killed along with control mice (n = 6). Hind limbs were removed and stored at 4°C. Muscles were dissected at 0, 1, and 3d postmortem and subsequently analyzed for degradation of nebulin, dystrophin, metavinculin, vinculin, desmin, and tropo-nin T. In a separate experiment, hind limb muscles from knockout (n = 4) and control mice (n = 4) were analyzed at 0, 1, and 3 d postmortem using casein zy-mography to confirm that μ-calpain activity was knocked out in muscle and to determine whether or not μ-calpain is activated in murine postmortem muscle. Cumulatively, the results of the first experiment indicated that postmortem proteolysis was largely inhibited in μ-calpain knockout mice. The results of the second experiment established the absence of μ-calpain in the muscle tissue of knockout mice and confirmed the results of an earlier study that μ-calpain is active in postmortem murine muscle. The results of the current study show that even in a species in which μ-calpain is activated to some extent postmortem, μ-calpain is largely responsible for postmortem proteolysis. This observation excludes a major role for any of the other members of the calpain family or any other proteolytic system in postmortem proteolysis of muscle proteins. Therefore, understanding the regulation of μ-calpain in postmortem muscle should be the focus of further research on postmortem proteolysis and tenderization of meat.