title: Low affinity superphosphorylation of the Na,K-ATPase by ATP.
abstract: Pre-steady-state phosphorylation of purified Na,K-ATPase from red outer medulla of pig kidney was studied at 25 degrees C and an ample range of [tau-32P]ATP concentrations. At 10 microM ATP phosphorylation followed simple exponential kinetics reaching after 40 ms a steady level of 0.76 +/- 0.04 nmol of P/mg of protein with kapp = 73.0 +/- 6.5 s-1. At 500 microM ATP the time course of phosphorylation changed drastically, since the phosphoenzyme reached a level two to four times higher at a much higher rate (kapp greater than or equal to 370 s-1) and in about 40 ms dropped to the same steady level as with 10 microM ATP. This superphosphorylation was not observed in Na,K-ATPase undergoing turnover in a medium with Mg2+, Na+, and ATP, suggesting that it required the enzyme to be at rest. Superphosphorylation depended on Mg2+ and Na+ and was fully inhibited by ouabain and FITC. After denaturation the phosphoenzyme made by superphosphorylation had the electrophoretic mobility of the alpha-subunit of the Na,K-ATPase, and its hydrolysis was accelerated by hydroxylamine. On a molar basis, the stoichiometry of phosphate per ouabain bound was 2.40 +/- 0.60 after phosphorylation with 1000 microM ATP. The results are consistent with the idea that under proper conditions every functional Na,K-ATPase unit can accept two, or more, phosphates of rapid turnover from ATP.
authors.affiliation: Instituto de Química y Fisicoquímica Biológicas, Facultad de Farmacia y Bioquímica, Junín, Buenos Aires, Argentina.
authors.forename: R D
authors.initials: RD
authors.lastname: Peluffo
authors.forename: P J
authors.initials: PJ
authors.lastname: Garrahan
authors.forename: A F
authors.initials: AF
authors.lastname: Rega
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002412:Cations; D004591:Electrophoresis, Polyacrylamide Gel; D016650:Fluorescein-5-isothiocyanate; D007679:Kidney Medulla / Q000201:enzymology; D008274:Magnesium / Q000378:metabolism; D010042:Ouabain / Q000378:metabolism; D010766:Phosphorylation; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013552:Swine
chemical_list: D002412:Cations; D010042:Ouabain; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D016650:Fluorescein-5-isothiocyanate; D008274:Magnesium