Comparison of calpain I and calpain II from carp muscle

1. The content of calpain II is 3.4 times more than that of calpain I when estimated by the elution profiles from a column of DEAE-cellulose. 2. Calpain I required 1 mM Ca2+ and calpain II required 5 mM Ca2+ to show the full activities. These data demonstrated that Ca2+-sensitivities of both calpains were lower than those of mammalian calpains, respectively. 3. The optimum caseinolytic activity was pH 7.2 for calpain I and pH 7.5 for calpain II. 4. The molecular weight of calpain I was estimated to be 110 k and that of calpain II to be 120 k by gel filtration. 5. Calpain I was much more heat-stable than calpain II around 50-60 degrees C. 6. Both calpains were sensitive to calpastatin, an endogenous inhibitor for calpain.