title: 5-Iodoacetamidofluorescein-labeled (Na,K)-ATPase. Steady-state fluorescence during turnover.
abstract: The fluorescence of (Na,K)-ATPase labeled with 5-iodoacetamidofluorescein was studied under turnover conditions. At 4 degrees C the hydrolysis of ATP is slowed sufficiently to permit study of the effects of Na+, K+, and ATP on the steady-state intermediates. With Na+ and Mg2+ (Na-ATPase conditions), addition of ATP produces a 7% drop in signal that reverts back to the initial, high fluorescence after a steady state of several minutes. K-sensitive phosphoenzyme is formed under these conditions, indicating that the fluorescence signal during the steady state is associated with E2P. Under (Na,K)-ATPase conditions (Na+, K+, Mg2+), micromolar ATP produces a steady-state signal that is 25% lower than the initial fluorescence, with no detectable phosphoenzyme formed. This low-fluorescence intermediate, which is also formed by adding K+ to enzyme in the Na-ATPase steady state described above, resembles the state produced by adding K+ directly to enzyme under equilibrium conditions, i.e. E2K. The K0.5(K+) for the fluorescence decrease and for keeping the enzyme dephosphorylated are nearly identical, indicating that the fluorescence change accompanies K+-dependent dephosphorylation. High ATP increases the steady-state fluorescence during the (Na,K)-ATPase reaction; while oligomycin produces still another steady-state fluorescent intermediate. These last two intermediates may be associated with the formation of E2P and E1P, respectively.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D004285:Dogs; D005452:Fluoresceins; D005453:Fluorescence; D005456:Fluorescent Dyes; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009840:Oligomycins / Q000494:pharmacology; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D005452:Fluoresceins; D005456:Fluorescent Dyes; D009840:Oligomycins; C015296:5-iodoacetamidofluorescein; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D011188:Potassium