Abstract
The Ca 2+ concentrations required for half-maximal activity of μ- and m-calpain purified from bovine skeletal muscle were tested using four different protein substrates and three different synthetic peptide substrates. Hammersten casein, the commonly used substrate for measuring μ- and m-calpain activity, required 2.5 μM Ca 2+ for half-maximal activity of μ-calpain and 290 μM Ca 2+ for half-maximal activity of m-calpain. When Hammersten casein was dialyzed against 8 M urea and 10 mM EDTA to remove all endogenous Ca 2+, it required 1.9 and 290 μM Ca 2+ for half-maximal activity of μ- and m-calpain, respectively. Rabbit skeletal muscle myofibrils and rabbit skeletal muscle troponin required 65 μM and 24 μM Ca 2+ for half-maximal activity of μ-calpain and 380 μM and 580 μM Ca 2+ for half-maximal activity of m-calpain, respectively. The three synthetic substrates tested, Suc-Leu-Tyr-MCA, Boc-Leu-Thr-Arg-MCA, and Suc-Leu-Leu-Val-Tyr-MCA, required 1.6 μM to 3.7 μM Ca 2+ for half-maximal activity of μ-calpain and 200 to 560 μM Ca 2+ for half-maximal activity of m-calpain.
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