Muscle-type phosphorylase activity present in muscle cells cultured from three patients with myophosphorylase deficiency.

Skeletal muscle fibers cultured from three patients whose mature fibers are deficient in glycogen myophosphorylase (EC 2.4.1.1) were shown to become rather mature, to have no excessive glycogen accumulation, and to develop signifcant myophosphorylase activity. That activity was characterized electrophoretically and immunologically and shown to be muscle phosphorylase rather than a genetically different type, thereby demonstrating true "rejuvenation" in culture of an enzyme genetically programmed ultimately to be deficient.