Regulation of the activity of a calcium-activated neutral protease during differentiation of skeletal myoblasts.

A calcium-activated neutral protease activity appears concomitantly with myotube formation during the differentiation of a cell line of rat skeletal myoblasts. Other proteases such as cathepsin D and plasminogen activator, however, do not show any changes in their activities. The appearance of the protease is not fusion dependent, as judged by assays of fusion defective myoblast mutants. The formation of the protease is suppressed along with differentiation in the presence of 5-bromodeoxyuridine. Undifferentiated myoblasts contain a potent inhibitor of the protease. The inhibitor, which is probably proteinaceous in nature, is lost during the differentiation of the cells into myotubes. This mode of regulation of an enzyme during differentiation seems so far to be an unique example of its kind.