title: The Na+/K+-ATPase reaction of human erythrocytes is not near equilibrium. A 31P-NMR study.
abstract: We have addressed the question of whether the Na/K+-ATPase in the human erythrocyte is in a state of near-equilibrium by varying the extracellular ratio of Na+ and K+ and following the cytosolic phosphorylation potential by 31P-NMR and by combined enzymatic colorimetric measurements. There was no correlation at room temperature between the extracellular Na+/K+ ratio and the cytosolic phosphorylation potential measured either by NMR or alternative methods. The cytosolic phosphorylation potential measured by NMR was 4100 +/- 1300 (S.E.) M-1 at an extracellular K+ concentration of 5.9 mM (Na+/K+ ratio of 24.3) and 2800 +/- 700 (S.E.) M-1 at 75 mM extracellular K+ (Na+/K+ ratio of 0.99). The chemically determined phosphorylation potential was 6400 +/- 1200 (S.E.) and 5000 +/- 700 (S.E.) M-1 at 5.9 and 75 mM extracellular K+, respectively. Omission of Ca2+ from the buffer solutions did not affect the results. A consistent finding in this study was that the NMR-determined value of ATP was about 10-20% lower than the value determined enzymatically on perchloric acid extracts. The inorganic phosphate (Pi) was fully NMR visible.
authors.affiliation: Department of Biochemistry A, Panum Institute, Copenhagen, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Petersen
authors.forename: E J
authors.initials: EJ
authors.lastname: Pedersen
authors.forename: B
authors.initials: B
authors.lastname: Quistorff
mesh_terms: D002138:Calibration; D004912:Erythrocytes / Q000201:enzymology*; D005110:Extracellular Space / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D010758:Phosphorus; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D011199:Potentiometry; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000097:blood*
chemical_list: D010758:Phosphorus; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium