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ATP-enriched human red cells display high rates of Ca2+-dependent ATP hydrolysis (16 mmol . litre cells-1 . h-1) with a high Ca2+ affinity (K0.5 approximately 0.2 microM). The finding suggests a mechanism for regulation of cell Ca2+ levels, involving highly-cooperative stimulation of active Ca2+ extrusion following binding of calmodulin to the (Ca2+ +Mg2+)-ATPase.
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