ADP stimulates hydrolysis of the “ADP-insensitive” phosphoenzyme in Na+, K+-ATPase and Ca2+-ATPase.

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ADP-sensitive (E1P) and K+-sensitive (E2P) phosphoenzymes are sequentially formed intermediates in the reaction pathways catalyzed by the Na+,K+- and Ca2+-ATPases. The kinetics of dephosphorylation of these intermediates were examined by means of rapid quenching with acid at 21 degrees C. Under conditions favoring the formation of E2P (25 mM Na+ and O K+), addition of 5 mM ADP + 10 mM EDTA to the Na+,K+-ATPase phosphoenzyme produced a biphasic pattern of dephosphorylation. Both phases of phosphoenzyme decomposition were accompanied by approximately stoichiometric amounts of inorganic phosphate (Pi) release. The rate of decay of the rapid phase was 10 times faster than the rate of phosphoenzyme turnover under phosphorylating conditions indicating acceleration of E2P hydrolysis by ADP. Similar patterns of ADP-stimulated phosphoenzyme decay and Pi release were observed in the Ca2+-ATPase from sarcoplasmic reticulum phosphorylated at low (0.1 mM) Mg2+ in the absence of KCl. These results demonstrate that ADP can enhance the rate of E2P hydrolysis in the cases of the Na+,K+-ATPase and Ca2+-ATPase. As a consequence measurement of "ADP-sensitive EP" may overestimate E1P.