Effect of potassium on sodium-dependent adenosine diphosphate-adenosine triphosphate exchange activity in kidney microsomes.

Maximal activation of a Na+-dependent adenosine diphosphate-adenosine triphosphate exchange reaction mediated by kidney microsomes was achieved at 0.1 mu and 0.4 mu Mg++ in the presence of 1.5 mM and 32 mM Na+, respectively. K+ inhibited the Na+-dependent exchange rate when lower concentrations of Na+ and Mg++ were used and stimulated it in the presence of 0.4 mM Mg++ and 32 mM Na+. The inhibitory effect of K+ could be reversed either by increasing the Na+ concentration or decreasing that of K+. N-Ethylmaleimide treatment of microsomes abolished the stimulatory effect of Kf. Ouabain, alkaline pH, and lower temperatures less effectively decreased stimulation of Na+dependent exchange by K+. Mg++ and K+ appeared to exhibit competitive antagonism on (Na+ + K+)-sensitive transphosphorylation activity. It is proposed that the stimulation of Na+-dependent adenosine diphosphate-adenosine triphosphate exchange reaction by K+ may be mediated, in part, by slowing the conversion of one form of the phosphoenzyme El-P into another Es-P.