1. K+ did not affect the Mg2+-dependent transphosphorylation but markedly increased the Na+-stimulated ADP–ATP exchange rate mediated by a microsomal fraction from guinea-pig kidney. 2. Rb+, Cs+, NH4+ and Li+ were equally effective in stimulating the Na+-dependent ADP–ATP exchange activity. 3. Treatment of the microsomal fraction with N-ethylmaleimide or increased concentrations of Mg2+ prevented stimulation of the Na+-dependent exchange reaction by K+. 4. Ouabain (2.5μm) inhibited ATP hydrolysis by 33% but did not decrease the K+-stimulated Na+-dependent ADP–ATP exchange rate. 5. A possible mechanism for stimulation of exchange activity by K+ is discussed.

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