Goll D E, Thompson V F, Taylor R G, Christiansen J A
Muscle Biology Group, University of Arizona, Tucson 85721.
Biochimie. 1992 Mar;74(3):225-37. doi: 10.1016/0300-9084(92)90121-t.
Muscle protein degradation has an important role in rate of muscle growth. It has been difficult to develop procedures for measuring rate of muscle protein degradation in living animals, and most studies have used in vitro systems and muscle strips to determine rate of protein degradation. The relationship between results obtained by using muscle strips and rate of muscle protein turnover in living animals is unclear because these strips are in negative nitrogen balance and often develop hypoxic cores. Also, rate of protein degradation is usually estimated by release of labeled amino acids, which reflects an average rate of degradation of all cellular proteins and does not distinguish between rates of degradation of different groups of proteins such as the sarcoplasmic and the myofibrillar proteins in muscle. A number of studies have suggested that the calpain system initiates turnover of myofibrillar proteins, which are the major group of proteins in striated muscle, by making specific cleavages that release thick and thin filaments from the surface of the myofibril and large polypeptide fragments from some of the other myofibrillar proteins. The calpains do not degrade myofibrillar proteins to small peptides or to amino acids, and they cause no bulk degradation of sarcoplasmic proteins. Hence, the calpains are not directly responsible for release of amino acids during muscle protein turnover. Activity of the calpains in living cells is regulated by calpastatin and Ca2+, but the nature of this regulation is still unclear.
肌肉蛋白质降解在肌肉生长速率中起着重要作用。开发测量活体动物肌肉蛋白质降解速率的方法一直很困难，大多数研究都使用体外系统和肌肉条来确定蛋白质降解速率。使用肌肉条获得的结果与活体动物肌肉蛋白质周转速率之间的关系尚不清楚，因为这些肌肉条处于负氮平衡状态，并且常常形成缺氧核心。此外，蛋白质降解速率通常通过标记氨基酸的释放来估计，这反映了所有细胞蛋白质的平均降解速率，并且无法区分不同蛋白质组（如肌肉中的肌浆蛋白和肌原纤维蛋白）的降解速率。许多研究表明，钙蛋白酶系统通过进行特定的切割，从肌原纤维表面释放粗细肌丝，并从其他一些肌原纤维蛋白中释放大的多肽片段，从而启动横纹肌中主要蛋白质组——肌原纤维蛋白的周转。钙蛋白酶不会将肌原纤维蛋白降解为小肽或氨基酸，也不会导致肌浆蛋白的大量降解。因此，钙蛋白酶并不直接负责肌肉蛋白质周转过程中氨基酸的释放。活细胞中钙蛋白酶的活性受钙蛋白酶抑制蛋白和Ca2+的调节，但其调节的本质仍不清楚。
