Changes in affinity of Na+- and K+-transport ATPase for divalent cations during its reaction sequence

Previous experiments (Fukushima, Y., and Post, R.L. (1978) J. Biol. Chem. 253, 6853-6862) demonstrated that the Ca x phosphoenzyme of sodium- and potassium-transport adenosine triphosphatase gradually becomes stable after dissociation of Ca2+ in the presence of a chelating agent such as 1,2-cyclohexylenedinitrilo-tetraacetic acid. In the present study, we investigated whether the ADP- and K+-sensitive forms of the Ca x phosphoenzyme show different affinities for divalent cations. Our findings were as follows. (a) As the concentraion of Na+ was increased during phosphorylation of the enzyme with ATP at pH 7.4 and 0 degrees C, both the sensitivity to ADP and the amount of calcium-free phosphoenzyme increased in parallel. (b) For this Na+-dependent change, kidney enzyme required higher concentrations of Na+ than did brain enzyme. (c) In addition, the rate of dissociation of Ca2+ from the ADP-sensitive Ca x phosphoenzyme was faster than that from the K+-sensitive phosphoenzyme. It was thus concluded that Ca2+ binds to the ADP-sensitive phosphoenzyme less tightly than to the K+-sensitive phosphoenzyme.