Degradation of myofibrillar proteins by a calpain-like proteinase in the arm muscle of Octopus vulgaris

The effects of a calpain-like proteinase (CaDP) isolated from the arm muscl e of Octopus vulgaris on the myofibrils and myofibrillar proteins isolated from the same tissue were examined. Our studies clearly showed that treatme nt of intact myofibrils with CaDP in the presence of 5 mM Ca2+ results in t he degradation of the major myofibrillar proteins myosin, paramyosin, and a ctin. From the isolated alpha- and beta-paramyosins only beta-paramyosin is degraded by CaDP in the presence of 5 mM Ca2+ producing three groups of po lypeptides of 80, 75, and 60 kDa, respectively. The degradation rate depend s on the proteinase to substrate ratio, temperature, and time of proteolysi s and is inhibited by the endogenous CaDP inhibitory factor (CIF), as well as by various known cysteine proteinase inhibitors (E-64, leupeptin, and an tipain). From the other myofibrillar proteins examined myosin, but not acti n, is degraded by CaDP; myosin heavy chain (MHC, 200 kDa) is degraded by Ca DP producing four groups of polypeptides of lower molecular masses (155, 12 5, 115, and 102 kDa, respectively); the degradation rate depends on the inc ubation time and the proteinase to substrate ratio. Furthermore. CaDP under goes limited autolysis in the presence of both the exogenous casein and the endogenous beta-paramyosin producing two large active fragments of 52 and 50.6 kDa. respectively; CIF reversibly inhibits this CaDP autolysis.