Decrease of apparent calmodulin affinity of erythrocyte ( at low Ca2+ concentrations |
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Authors: | Birthe Foder Ole Scharff |
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Institution: | Department of Clinical Physiology, Finsen Institute, Strandboulevarden 49, DK 2100 Copenhagen φ Denmark |
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Abstract: | The calmodulin activation of the ( (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied in the range of 1 nM to 40 μM of purified calmodulin. The apparent calmodulin-affinity of the ATPase was strongly dependent on Ca2+ and decreased approx. 1000-times when the Ca2+ concentration was reduced from 112 to 0.5 μM. The data of calmodulin (Z) activation were analyzed by the aid of a kinetic enzyme model which suggests that 1 molecule of calmodulin binds per ATPase unit and that the affinities of the calcium-calmodulin complexes (CaiZ) decreases in the order of . Furthermore, calmodulin dissociates from the calmodulin-saturated Ca2+-ATPase in the range of 10?7–10?6 M Ca2+, even at a calmodulin concentration of 5 μM. The apparent concentration of calmodulin in the erythrocyte cytosol was determined to be 3 to 5 μM, corresponding to 50–80-times the cellular concentration of Ca2+-ATPase, estimated to be approx. 10 nmol/g membrane protein. We therefore conclude that most of the calmodulin id dissociated from the Ca2+-transport ATPase in erythrocytes at the prevailing Ca2+ concentration (probably 10?7 – 10?8 M) in vivo, and that the calmodulin-binding and subsequent activation of the Ca2+-ATPase requires that the Ca2+ concentration rises to 10?6 – 10?5 M. |
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Keywords: | Calmodulin affinity (Human erythrocyte) SDS sodium dodecyl sulfate HEDTA |
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