Immunofluorescent localization of a Ca2+-dependent neutral protease in hamster muscle

A Ca2+-dependent neutral thiol protease is widely distributed in mammalian tissues and has been implicated in muscle protein turnover. As a first step to understanding the physiological function of this enzyme, its cellular and subcellular localization has been studied in hamster cardiac and skeletal muscle by means of indirect immunofluorescence. The results showed that the Ca2+-dependent protease is located either at the plasma membrane or in the connective tissue matrix that surrounds the muscle cels. The uniform distribution along membranes and around capillaries indicated that the protease is not confined to connective tissue cells. This evidence, together with the Ca2+-dependence of the enzyme, suggests a distribution throughout the extracellular connective tissue of the muscle. In this position, it would seem unlikely that the enzyme could participate in protein turnover within muscle cells in normal tissue.