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Degradation of myofibrillar proteins by extractable lysosomal enzymes and m-calpain, and the effects of zinc chloride.

A study was conducted to examine the effects that physiological levels of m-calpain (calpain requiring millimolar concentrations of Ca2+) extract and a lysosomal extract have on myofibrillar proteins in vitro, and the effects that zinc has on inhibiting proteolysis by these extracts. During a 22-h incubation period, the lysosomal extract degraded myosin heavy chain, alpha-actinin, desmin, troponin-I, and myosin light chains 1 and 2. The effectiveness of the lysosomal extract to degrade myofibrillar proteins was significantly affected by the presence or absence of EDTA. Zinc, which is a potent inhibitor of cysteine proteinases, prevented most, but not all, of the lysosomal extract-induced myofibrillar protein degradation. Incubation of myofibrils with m-calpain resulted in the hydrolysis of troponin-T, desmin, and a 58-kDa molecular weight protein, possibly vimentin, and 5 mM ZnCl2 completely blocked these changes. Results from this study indicate that the degradation by the lysosomal extract is far more extensive than the degradation that occurs with normal postmortem storage and that possibly a non-cysteine protease is present that is capable of hydrolyzing some myofibrillar proteins under this in vitro condition, because Zn2+ did not block all proteolysis. However, similar changes were induced by m-calpain incubation and postmortem storage.