The Role of Muscle Proteolytic Enzymes in Degradation of the Myofibril

It is now well established that proteins are continually synthesized and degraded in both developing and mature muscle cells. Consequently, the net rate of protein acretion in muscle is determined by the difference between the rate of protein synthesis and the rate of protein degradation in individual muscle cells. The importance of protein synthesis in determining rate and efficiency of muscle growth has been acknowledged for some time, however, it is only relatively recently that the importance of protein degradation in this process has been recognized. This recognition has come about primarily as a result of recent studies showing that the rate of protein degradation in the muscles of young, growing animals i s quite rapid (Garlick et al., 1976; Millward et al., 1975; Maruyama et al., 1978). For example, Garlick et al. (1976) have shown that in 75 Kg pigs up to 75% of the protein synthesized each day may be needed to replace degraded protein. Additionally, Maruyama et al. (1978) have shown that in young children in which growth rate has been altered by dietary manipulation, there i s a high negative correlation (-0.92) between fractional whole body growth rate and fractional rate of protein degradation. Thus, although there is disagreement among researchers as to the relative importance of changes in degradation rate in determining rate of muscle growth, there appears to be general agreement that protein degradation occurs in growing muscles and that it significantly decreases the rate and efficiency of muscle protein accretion in growing animals. Consequently, a reduction in the rate of muscle protein degradation without a concomitant decrease in the rate of muscle protein synthesis should result in an increase in both rate and efficiency of muscle growth. In view of the importance of protein degradation in determining rate and efficiency of muscle growth, it is extremely significant that a number of physiological factors that influence muscle size in the intact animal (e.g., hormones, nutri-