Nucleotide requirements for sodium‐sodium exchange catalysed by the sodium pump in human red cells

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Abstract
1. Resealed red cell ghosts containing a variety of nucleoside triphosphates, or a mixture of tri‐ and diphosphates, were allowed to lose 24Na into a 10 m M‐K medium or a K‐free high‐Na medium in the presence and absence of ouabain.2. Only ATP supported a ouabain‐sensitive Na:K exchange in the 10 m M‐K medium, or a ouabain‐sensitive Na:Na exchange in the K‐free medium.3. Because of ATPase and adenylate kinase activity, it is difficult to control the levels of ATP and ADP inside resealed red cell ghosts. Some control was achieved by incorporating into the ghosts an ATP regenerating system consisting of creatine phosphate and creatine kinase.4. Measurements of 24Na efflux from ghosts prepared in this way showed that ADP is required for the ouabain‐sensitive exchange of Na that occurs in K‐free high‐Na media, but not for the normal exchange of Na for K that occurs in K‐containing media.5. The significance of these findings is discussed.