Susceptibilities of Various Myofibrillar Proteins to Muscle Serine Protease1

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Abstract
The ability of serine protease of skeletal muscle to degrade native myofibrillar proteins, such as myosin, actin, troponin, tropomyosin, α-actinin, and M-protein from rabbit skeletal muscle was studied. The amino acids or peptides liberated from these proteins by the protease were determined fluorometrically using o-phthalaldehyde. The order of their susceptibilities at a molar ratio of the serine protease to substrate of 1: 100 was: myosin≫troponin>tropomyosin > actin. a-Actinin and M-protein were not degraded. Sodium dodecyl sulfate-polyacryl-amide gel electrophoresis showed that the myosin heavy chain was degraded into two fragments, having molecular weights of 100, 000 and 88, 000, whereas the light chains were scarcely degraded. The serine protease degraded troponin-T rapidly and troponin-I slowly, but did not degrade troponin-C. Tropomyosin was degraded rapidly into two components with molecular weights of 21, 500 and 19, 000. Actin was degraded slowly, but no liberated fragment could be detected.