title: (Na+ + K+)-ATPase: confirmation of the three-pool model for the phosphointermediates of Na+-ATPase activity. Estimation of the enzyme-ATP dissociation rate constant.
abstract: The dephosphorylation kinetics of acid-stable phosphointermediates of (Na+ + K+)-ATPase from ox brain, ox kidney and pig kidney was studied at 0 degree C. Experiments performed on brain enzyme phosphorylated at 0 degree C in the presence of 20-600 mM Na+, 1 mM Mg2+ and 25 microM [gamma-32P]ATP show that irrespectively of the EP-pool composition, which is determined by Na+ concentration, all phosphoenzyme is either ADP- or K+-sensitive. After phosphorylation of kidney enzymes at 0 degree C with 1 mM Mg2+, 25 microM [gamma-32P]ATP and 150-1000 mM Na+ the amounts of ADP- and K+-sensitive phosphoenzymes were determined by addition of 1 mM ATP + 2.5 mM ADP or 1 mM ATP + 20 mM K+. Similarly to the previously reported results on brain enzyme, both types of dephosphorylation curves have a fast and a slow phase, so that also for kidney enzymes a slow decay of a part of the phosphoenzyme, up to 80% at 1000 mM Na+, after addition of 1 mM ATP + 20 mM K+ is observed. The results obtained with the kidney enzymes seem therefore to reinforce previous doubts about the role played by E1 approximately P(Na3) as intermediate of (Na+ + K+)-ATPase activity. Furthermore, for both kidney enzymes the sum of ADP- and K+-sensitive phosphoenzymes is greater than E tot. In experiments on brain enzyme an estimate of dissociation rate constant for the enzyme-ATP complex, k-1, is obtained. k-1 varies between 1 and 4 s-1 and seems to depend on the ligands present during formation of the complex. The highest values are found for enzyme-ATP complex formed in the presence of Na+ or Tris+. The results confirm the validity of the three-pool model in describing dephosphorylation kinetics of phosphointermediates of Na+-ATPase activity.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000201:enzymology; D027682:Cation Transport Proteins; D002417:Cattle; D004557:Electric Organ / Q000201:enzymology; D004593:Electrophorus; D007668:Kidney / Q000201:enzymology; D007700:Kinetics; D008433:Mathematics; D008956:Models, Chemical; D011188:Potassium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013552:Swine
chemical_list: D027682:Cation Transport Proteins; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; C021838:sodium-translocating ATPase; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium