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J Physiol Vol 235, Issue 2 pp 551-569
Copyright © 1973 by The Physiological Society
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Dependence on calcium concentration and stoichiometry of the calcium pump in human red cells

 H. J. Schatzmann

1. In resealed human red cells loaded with Ca-EGTA buffer solutions it was found that the intracellular free Ca2+ concentration for half saturation of the Ca transport system (which pumps Ca out of the cell) is equal to or smaller than 4 x 10-6 M and thus closely agrees with the dissociation constant of the Ca + Mg activated membrane ATPase.

2. The maximal rate of Ca transport from resealed cells to medium was found to be 0·148 ± 0·009 µmole/ml. cells.min at 28° C.

3. The rate of Ca transport was unaffected by a variation of the extracellular Ca2+ concentration from 3·10-7 to 5·10-3 M.

4. Evidence is presented making it probable that the stoichiometric relation between Ca transported and ATP hydrolysed is 1:1 rather than 2:1.

5. As the Ca transport is quite rapid even at half saturation and the passive leak for Ca negligible in intact cells it can be predicted that the steady-state cellular Ca2+ concentration must be low, most probably less than 10-6 µmole/ml. cells. Transport from cells containing 5·10-7 µmole/ml. into blood plasma is thermodynamically compatible with the normal plasma Ca2+ concentration and the normal cellular ATP, ADP and Pi content.

6. Treatment with the mercurial PCMBS in the cold for 15 hr allows to load red cells with 1 µmole Ca/ml. cells without destroying their ability to transport Ca after removal of the mercurial.

7. It is shown that at high cellular Ca concentrations (0·1-3 µmole/ml. cells) about 50% of the total is free Ca2+ on account of binding mainly to dialysable cell constituents.






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