The control of membrane-bound Ca2+ by ATP

Abstract 1. 1. It is known that the viscosity of venous blood is greater than that of arterial blood, and that an ATP-depleted erythrocyte loses its deformability. In order to test the hypothesis that these effects may be explained by an intracellular chelation of membrane-bound Ca 2+ by ATP, the effect of ATP on the membrane-bound Ca 2+ of erythrocyte ghost membranes was examined by adding 45 Ca 2+ and ATP to the isolated membranes. 2. 2. The membrane-bound Ca 2+ was reduced to zero at 1.5 mM ATP, pH 7. 3. 3. A complete adsorption isotherm for ATP at 21° indicated that there was a very small binding component of about 540 molecules of ATP per single erythrocyte membrane which became saturated at about 10 −7 M ATP. The majority of the ATP binding sites, however, did not indicate any saturation up to 10 −3 M ATP. 4. 4. 10 −5 M Ca 2+ or Mg 2+ depressed the membrane-bound ATP, but increased it at 1 mM. 5. 5. ATP binding was increased at low pH. 6. 6. Calculations indicated that the oxygenated erythrocyte would have a cytoplasmic free ATP level of 1.45 mM, and that the ATP concentration in the erythrocyte of venous blood (60% oxygenated) would be 0.94 mmole per l water. The level of membrane-bound Ca 2+ , therefore, would tend to be higher in the deoxygenated cell, explaining its greater rigidity and the higher viscosity of venous blood.