Apparent variations in the activation characteristics of human erythrocyte membrane Ca²⁺-pump ATPase may be caused by variable membrane permeability

Published studies of the Ca²⁺-pump ATPase of the human erythrocyte membrane record a variety of patterns of activation by Ca²⁺ and calmodulin and also suggest that activation by Ca²⁺-calmodulin is slow rather than immediate. We have re-analysed these points in various types of human erythrocyte membrane preparation of widely different permeability characteristics, both in the intact state and after being rendered fully permeable by saponin. The various membrane preparations initially showed very different patterns of activation, but when permeabilised with saponin they all exhibited identical characteristics: these included highly cooperative activation by Ca²⁺ with maximum activity at ∼ 1 μM-Ca²⁺ and high sensitivity to calmodulin. Activation of Ca²⁺-ATPase by Ca²⁺-calmodulin in freely permeable ghosts was immediate. We therefore conclude that the Ca²⁺-pump ATPase exhibits high sensitivity to Ca²⁺ and calmodulin and responds rapidly to Ca²⁺-calmodulin. Apparent evidence to the contrary seems likely to have been a result of misinter-pretation of data derived from studies of partially sealed erythrocyte ghosts in which the added activators, Ca²⁺ and calmodulin, did not have free access to the appropriate sites on the ATPase.