The Cytosol of Human Erythrocytes Contains a Highly Ca2+-Sensitive Thiol Protease (Calpain I) and Its Specific Inhibitor Protein (Calpastatin)1

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Abstract
The cytosol of human erythrocytes was found to contain a Ca2+thiol protease (calpain) and its specific inhibitor (calpastatin) by DEAE-cellulose chromatography at pH 8.0, although no proteolytic activity toward casein was detected in the unfractionated hemolysate. The protease required only 40 pM Ca for 50 % activation, indicating that it belongs to the highly Ca2+sensitive type of calpain, namely, calpain I. It was not inactivated by heating at 58°C for 10 min at pH 7.2, the optimal pH for its action on casein. The inhibitor comprised major and minor components, calpastatin H (Mr=280,000) and calpastatin L (Mr =48,000). Both were heat-stable proteins which were readily inactivated by tryptic digestion. The inhibition of erythrocyte calpain by erythrocyte calpastatin H or L was not due to sequestering of Ca from the reaction medium by the inhibitor protein. The calpain preparation preferentially digests bands III and IVa of human eryth rocyte membrane proteins, with little or no cleavage of the bands corresponding to spectrin.