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Dependence on calcium concentration and stoichiometry of the calcium pump in human red cells
Author(s) -
Schatzmann H. J.
Publication year - 1973
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1973.sp010403
Subject(s) - chemistry , calcium , mole , extracellular , egta , biophysics , stoichiometry , membrane transport , atpase , divalent , dissociation (chemistry) , dissociation constant , ion transporter , membrane , biochemistry , analytical chemistry (journal) , chromatography , enzyme , biology , receptor , organic chemistry
1. In resealed human red cells loaded with Ca‐EGTA buffer solutions it was found that the intracellular free Ca 2+ concentration for half saturation of the Ca transport system (which pumps Ca out of the cell) is equal to or smaller than 4 × 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 Ca 2+ 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 Ca 2+ 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 Ca 2+ concentration and the normal cellular ATP, ADP and P i 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 Ca 2+ on account of binding mainly to dialysable cell constituents.