External Na dependence of ouabain-sensitive ATP: ADP exchange initiated by photolysis of intracellular caged-ATP in human red cell ghosts

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Abstract
The intracellular ATP:ADP exchange reaction was initiated by photoreleased ATP following brief irradiation at 350 nm of human erythrocyte ghosts containing caged-ATP. The ouabain-sensitive component of the ensuing ATP:ADP exchange reaction shows a biphasic response to extracellular Na. External Na in the range 0-10 mM has an inhibitory effect while increasing concentrations beyond this range stimulate the rate of exchange in a roughly linear fashion up to 100 mM Na. These results represent the first direct demonstration of the sidedness of the effects of Na on this partial sequence in the overall enzyme cycle and bear a qualitative resemblance to the Na effects on the Na-ATPase which occur in the absence of intracellular ADP in human red blood cells.