The red cell calcium pump.

Abstract

The ability of the human red cell to extrude Ca2+ ions against a gradient by a mechanism that depends directly on ATP (57) was taken at the time of its discovery as a curiosity. However, this Ca pump is an attribute of the plasma membrane not only in red cells from pigs, dogs, cattle, birds, and humans, but probably in all animal cells (59). The following deals with the Ca pump of human red cells. At saturating CaH concentration the cells transport some 10 mmoles per liter of cells per hour, which is 200-1000 times their passive permeability to Ca2+ (11, 27a). At the physiological permeability the pump maintains a gradient of almost 10,000 (58), such that the intracellular CaH concentra­ tion is < 4 X 10-7 M (27a, 63). In membranes isolated in Ca2+-containing media it presents itself as an ATPase (requiring free MgH ions) that is stimulated by Ca2+ with a K ag of' 1 ILM or less (6) depending on the pH; it has 2 ATP-accepting sites of vastly differing affinity (Ka�p 1-3.5 JLM; Ka�p 120-330 JLM) (33-35, 42, 65). The ATP and MgH sites are situated on the internal membrane surface, and inorganic phosphate is liberated internally (62). In Na+or K+-containing media the rate of transport and ATP hydrol­ ysis is 30-50% higher than in tris-Cl or choline-Cl media. The affini ties for