CALPAIN (CA2+-DEPENDENT THIOL PROTEASE) IN ERYTHROCYTES OF YOUNG AND OLD INDIVIDUALS

Limited proteolysis by calpain (Ca2+ activated protease; EC 3.4.22.17) if believed to regulate the function of membrane enzymes and modify the behavior of membrane structural proteins. Calpain is activated by autolysis. The degradation of band 3 protein by mu-calpain is known to be enhanced in erythrocyte membranes from human individuals > 70 years old (old) as compared with that from individuals 20-30 years old (young). In tbe present study, monoclonal antibody to mu-calpain was used to study the behavior of calpain in erythrocytes of young and old individuals. Less calpain was found in erythrocyte cytosol and membranes from old than in those from young. Increasing the erythrocyte Ca2+ induced translocation of calpain to the cell membrane and autolysis of the enzyme. Alkylation of erythrocyte thiols also promoted translocation of calpain to the membrane, especially in the presence of Ca2+. When calpain was added to erythrocyte membranes, initial binding was greater and subsequent autolysis faster in old than in young individuals, possibly arising from alterations in cell membranes of old individuals. The enhanced calpain autolysis was accompanied by enhanced degradation of band 3 protein in the old. The results suggest that calpain in old individuals is translocated to the cell membrane and is activated by autolysis, resulting in degradation of certain membrane proteins and loss of calpain. Enhanced calpain-induced membrane proteolysis may play a role in abnormal cell destruction (e.g., shortening the life span of erythrocytes in the aged, neuronal degeneration, etc). The erythrocyte membrane provides a convenient model for the study of age-associated alterations in cell membranes and in calpain behavior.