Role of Proteinases and Protein Turnover in Muscle Growth and Meat Quality

Even a cursory review of the role of proteases in both muscle growth and in meat quality would require a lengthy treatise that could easily become too discursive to be either informative or easily understood. Besides, the role of proteases in meat quality (i.e., meat tenderness) has been clarified significantly during the last 15 years. A large amount of evidence has accumulated to indicate that the Ca2+dependent proteinases (hereafter, called the calpains; pcalpain for the proteinase requiring micromolar Ca2+ concentrations for activity and m-calpain for the proteinase requiring millimolar Ca2 + concentrations for activity) are responsible for most, if not indeed practically all, of the tenderization that occurs during postmortem storage of muscle at 2" to 4°C (Table 1). This evidence was reviewed a number of years ago by Goll et al. (1983), and the more recent studies in this area have been summarized in an excellent review by Koohmaraie (1991). Perhaps the most compelling evidence indicating that the calpains are responsible for postmortem tenderization is the unvarying finding that very little proteolysis occurs in muscle during postmortem storage; functional actomyosin can be prepared from muscle after 13 days of postmortem storage at 2" to 4"C, and electron micrographs show that muscle retains its characteristic Aand I-band structure after long periods of postmortem storage (Table 1 ). Consequently, the proteases involved in postmortem tenderization must be very specific and limited in their ability to degrade muscle proteins. The calpains are unique among the known proteases in that they do not degrade actin, myosin, or a-actinin (Goll et al., 1991a). SDSPAGE of postmortem muscle samples have consistently shown that neither actin, myosin, nor a-actinin are degraded during postmortem storage (see Bandman and Zdanis, 1988, as an example of one of these studies). It has been well documented that skeletal muscle Z-disks are degraded to varying degrees during postmortem storage (Table 1 and Henderson et al., 1970), and it is surprising, therefore, that a-actinin, a major Z-disk protein, is not degraded during