Removal of rabbit psoas strips immediately after death and incubation in a saline solution containing 1 mM Ca2+ and 5 nM Mg2+ for 9 hr at 37°C and pH 7.1 causes complete Z-line removal but has no ultrastructurally detectable effect on other parts of the myofibril. Z lines remain ultrastructurally intact if 1 mM 1,2-bis-(2-dicarboxymethylaminoethoxy)-ethane (EGTA) is substituted for 1 mM Ca2+ and the other conditions remain unchanged. Z lines are broadened and amorphous but are still present after incubation for 9 hr at 37°C if 1 mM ethylenediaminetetraacetate (EDTA) is substituted for 1 mM Ca2+ and 5 mM Mg2+ in the saline solution. A protein fraction that causes Z-line removal from myofibrils in the presence of Ca2+ at pH 7.0 can be isolated by extraction of ground muscle with 4 mM EDTA at pH 7.0–7.6 followed by isoelectric precipitation and fractionation between 0 and 40% ammonium sulfate saturation. Z-line removal by this protein fraction requires Ca2+ levels higher than 0.1 mM, but Z lines are removed without causing any other ultrastructurally detectable degradation of the myofibril. This is the first report of a protein endogenous to muscle that is able to catalyze degradation of the myofibril. The very low level of unbound Ca2+ in muscle cells in vivo may regulate activity of this protein fraction, or alternatively, this protein fraction may be localized in lysosomes.

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