HUMAN-ERYTHROCYTE CA2+-MG2+-ATPASE - MECHANISM OF STIMULATION BY CA2+

In the presence of ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetic acid (EGTA), a detergent-solubilized preparation of human erythrocyte membrane Ca2+-Mg2+-ATPase was separated by gel filtration from its activator protein, designated as calmodulin; this makes the enzyme dependent on an exogenous calmodulin for maximum activity. In the presence of Ca2+, ATPase co-migrated with calmodulin, and the enzyme activity was independent of an exogenous calmodulin, suggesting that the enzyme and calmodulin form an active holoenzyme. Lowering the Ca2+ level dissociates the two proteins, returning the enzyme activity to its basal level. A time course experiment indicated that the effect of Ca2+ on the enzyme·calmodulin complex was immediate and reversible. Calmodulin exerts its effect on ATPase primarily by increasing its V. Thus, the mode of stimulation of human erythrocyte Ca2+-Mg2+-ATPase by calmodulin appears similar to that of Ca2+-dependent adenylate cyclase and phosphodiesterase. © 1979.