Separation of adenosine diphosphate–adenosine triphosphate-exchange activity from the cerebral microsomal sodium-plus-potassium ion-stimulated adenosine triphosphatase

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Abstract
A microsomal fraction from ox cerebral cortex catalysed [C14]ADP-ATP exchange at a speed similar to that at which it liberated Pi from ATP in the presence of Na+, K+ and Mg2+. Repeated washing the fraction with MgATP solutions solubilized most of the exchange activity. The exchange activity was accompanied by negligible adenosine-triphosphatase activity and was enriched by precipitation at chosen pH and by DEAE [diethylaminoethyl]-Sephadex. At no stage was its activity affected by Na+, K+ or ouabain. The washed microsomal fraction was exposed to a variety of reagents; a sodium iodide-cysteine treatment increased both adenosine-triphosphatase and exchange activities, as also did a synthetic zeolite. Preparations were obtained with exchange activities less than 3% of their Na+-plus-K+-stimulated adenosine-triphosphatase activity. Some contribution to the residual exchange activity was made by an adenylate kinase. Thus over 95% of the microsomal ADP-ATP-exchange activity does not take part in the Na+-plus-K+-stimulated adenosine-triphosphatase reaction. Participation of some of the residual 3% of the ADP-ATP-exchange activity has not been excluded, but there appears no firm evidence for its participation in the adenosine triphosphatase; the bearing of this conclusion on mechanisms proposed for the Na+-plus-K+-stimulated adenosine triphosphatase is indicated.