The effect of calmodulin on the phosphoprotein intermediate of Mg2+-dependent Ca2+-stimulated adenosine triphosphatase in human erythrocyte membranes.
The effect of calmodulin on the formation and decomposition of the Ca2+-dependent phosphoprotein intermediate of the (Mg2+ + Ca2+)-dependent ATPase in erythrocyte membranes was investigated. In the presence of 60 microM-Ca2+ and 25 microM-MgCl2, calmodulin (0.5-1.5 microgram) did not alter the steady-state concentration of the phosphoprotein, but increased its rate of decomposition. Higher calmodulin concentrations significantly decreased the steady-state concentration of phosphoprotein. Calmodulin (0.5-1.7 microgram) increased Ca2+-transport ATPase activity by increasing the turnover rate of its phosphoprotein intermediate. Increasing the MgCl2 concentration from 25 microM to 250 microM increased the (Mg2+ + Ca2+)-dependent ATPase activity, but decreased the concentration of the phosphoprotein intermediate. Similarly to calmodulin, MgCl2 increased the turnover rate of the Ca2+-transport ATPase complex (about 3-fold). At the higher MgCl2 concentration calmodulin did not further affect the decomposition of the phosphoprotein intermediate. It was concluded that both calmodulin and MgCl2 increase the turnover of the Ca2+-pump by enhancing the decomposition of the Ca2+-dependent phosphoprotein intermediate.
D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000097:blood / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010750:Phosphoproteins / Q000097:blood / Q000378:metabolism*
D002135:Calcium-Binding Proteins; D002147:Calmodulin; D010750:Phosphoproteins; D000252:Calcium-Transporting ATPases; D008274:Magnesium