Specificity and Interactions at the Cationic Sites of the Axonal (Na+, K+)‐Activated Adenosinetriphosphatase

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Abstract
Specificity at the Mg2+ site has been investigated with P‐nitrophenylphosphate and dinitrophenylphosphate as substrates. The interaction parameters between the enzyme and its effectors at the Mg2+ site have been determined. Ni2+ and Cd2+ can be added to the series of known Mg2+ analogues. Tb3+ also inhibits the enzymatic activity and appears as a promising tool for further studies since it interacts with the (Na+,K+)‐ATPase in a relatively high affinity process. Interactions between the Mg2+ and Na+ sites have been studied by the P‐nitrophenylphosphatase activity. Na+ decreases the apparent affinity of the enzyme for Mg2+ without strong modification of the cooperativity index. Kinetic parameters for ATPase, P‐nitrophenylphosphatase and dinitrophenylphosphatase activations by K+ analogues have been determined and the pH dependence of the apparent affinity studied. Heterotropic interactions between K+ and Na+ sites have been studied with K+, Tl+ and NH+4. Three substrates have been used. Firstly, with dinitrophenylphosphate, increasing Na+ concentration changes the positive cooperativity for K+ to a negative one, then back to a positive cooperativity. Simultaneously, the apparent affinity increases then decreases. Secondly, with ATP, an increase of the Na+ concentration decreases the apparent affinity for K+ and changes the cooperativity from a negative to a positive one. Thirdly, with P‐nitrophenylphosphate, interactions resemble those obtained with dinitrophenylphosphate. When added at concentrations which stimulate the activity, ATP imposes its type of heterotropic interactions between Na+ and K+ sites.