The role of Ca 2+-dependent proteases (calpains) in post mortem proteolysis and meat tenderness

Abstract

This manuscript summarizes research results from our laboratory regarding the role of endogenous proteases in post mortem proteolysis resulting in meat tenderization. Proteolysis of key myofibrillar proteins is the principal reason for ultrastructural changes in skeletal muscle associated with meat tenderization. Proteases should have the following characteristics to be considered as possible candidates for bringing about post mortem changes: i) to be located within skeletal muscle cells; ii) to have access to the substrate ie, myofibrils); and iii) to be able to hydrolyze the same proteins that are degraded during post mortem storage. Of the proteases located within skeletal muscle cells and thus far characterized, only calpains have all of the above characteristics. Numerous experiments conducted in our laboratory have indicated that the calcium-dependent proteolytic system (calpains) is responsible for post mortem proteolysis. Some of this evidence includes: 1) incubation of muscle slices with buffer containing Ca 2+ accelerates post mortem proteolysis; 2) incubation of muscle slices with Ca 2+ chelators inhibits post mortem proteolysis; 3) infusion or injection of carcasses with a solution of calcium chloride accelerates post mortem proteolysis and the tenderization process such that post mortem storage beyond 24 h to ensure meat tenderness is no longer necessary; 4) infusion of carcasses with zinc chloride, a potent inhibitor of calpains, blocks post mortem proteolysis and the tenderization process; and 5) feeding a ß-adrenergic agonist to lambs results in a reduction of the proteolytic capacity of the calpain system, which leads to a decreased rate of post mortem proteolysis and produces tough meat. Based on these results, we have concluded that calpains are the main proteolytic system responsible for post mortem proteolysis, and that one of the main regulators of calpains is their endogenous inhibitor, calpastatin.