Is the red cell calcium pump regulated by ATP?

Abstract

THE very low physiological level of calcium in the human red cell is maintained by a powerful ATP-fuelled Ca extrusion pump which has been extensively studied¹. Recently evidence has been accumulating that the kinetic properties of the active Ca flux and associated (Ca + Mg) ATPase activity depend very much on the experimental conditions and/or the manner of preparation of the red cell membranes or resealed ghosts^2–4. The variability may reflect the nature or degree of interaction between the membrane Ca pump and a cytoplasmic activator protein described recently^5–7, which could be involved in regulation of physiological Ca levels. In previous work with whole red cells^8,9 or resealed ghosts¹⁰ the concentrations of the major Ca pump ligands, ATP and/or Ca, have been changing during the course of the reaction. Because of the possible importance of the ligand conditions in assessing the physiological functioning of the Ca pump, we have looked at (Ca + Mg)-dependent ATP hydrolysis in resealed ghosts with buffered ATP and Ca levels and at the active Ca flux with a buffered ATP level. The experiments reported here show that both the ATP hydrolysis and the Ca flux are activated with two distinct ATP affinities and this raises the possibility of regulation of the Ca pump by ATP in the region of the lower affinity.