TY - JOUR T1 - Nucleotide Specificity of the Na<sup>+</sup>-Stimulated Phosphorylation and [<sup>3</sup>H]Ouabain-Binding Reactions of (Na<sup>+</sup> + K<sup>+</sup>)-Dependent Adenosine Triphosphatase JF - Molecular Pharmacology JO - Mol Pharmacol SP - 256 LP - 263 VL - 8 IS - 2 AU - T. TOBIN AU - S. I. BASKIN AU - T. AKERA AU - T. M. BRODY Y1 - 1972/03/01 UR - http://molpharm.aspetjournals.org/content/8/2/256.abstract N2 - ATP, CTP, and ITP supported a rapid initial rate of [3H]ouabain binding to rat brain (Na+ + K+)-ATPase, up to 4 times that supported by ADP. These nucleotides also prevented the rapid initial incorporation of the [γ-32P]phosphate of [γ-32P]ATP into the enzyme. [γ-32P]Cytidine triphosphate labeled the enzyme to the same extent as [γ-32P]ATP, suggesting that nucleotides other than ATP phosphorylate this enzyme. In support of this, the phosphorylation from [γ-32P]CTP required Mg2+, was stimulated by Na+, and was not observed in the presence of K+, and the label incorporated into the enzyme from [γ-32P]CTP turned over at the same rate as that from [γ-32P]ATP. In other experiments the initial rates of hydrolysis of ATP, ITP, UTP, and ADP in the absence of added K+ matched the initial rates of [3H]ouabain binding supported by these substrates. The results suggest that these substrates give rise to sufficient phosphoenzyme to account for the initial rates of [3H]ouabain binding supported by them. Concentrations of ADP sufficient to inhibit the phosphorylation of this enzyme by ATP also inhibited the initial rate of the ATP-supported binding of [3H]ouabain, suggesting that ADP can bind to this enzyme without stimulating [3H]ouabain binding. ACKNOWLEDGMENTS The authors wish to thank Mrs. Annie Han and Mr. Roxy So for excellent technical assistance. ER -