Proteolytic activity of isolated lamb calpains on myofibrils under the conditions of pH, Ca2+ concentration and temperature existing in postmortem muscle.

The two calcium-activated neutral proteinases (calpains I and II) and their specific inhibitor were isolated by ion exchange chromatography in DEAE-Sephacel from lamb skeletal muscle (longissimus dorsi). Their proteolytic activities were then determined using myofibrils as substrate. The Ca2+ requirements were different for each form of the enzyme: calpain I needed only 50 mumol Ca2+ for half-maximal activity, while the other isoenzyme, calpain II, needed 1,000 mumol Ca2+ for reaching 50% of its maximum activity. Both calpains showed a relevant activity in the pH range 5.5-6.5 (over 40% of maximum activity found at pH 7.5). With regard to the effect of temperature, both isoenzymes retained about 25% of their activity at 25 degrees C with a temperature reduction down to 4 degrees C. It is concluded that calpain I is an active protease under conditions similar to that prevalent in lamb meat during postmortem storage.