Cytoplasmic K+ effects on phosphoenzyme of Na,K-ATPase proteoliposomes and on the Na+-pump activity.
Three phosphorylated reaction intermediates (EP) of Na,K-ATPase, and ADP-sensitive K+-insensitive EP (E1P), an ADP- and K+-sensitive EP (E*P), and a K+-sensitive ADP-insensitive EP (E2P), have been discovered at present. By using Na,K-ATPase proteoliposomes (PL) prepared from the electric eel enzyme, we found in this study that E*P existed even in the presence of K+ on both sides of the PL and that there was a sidedness difference in K+ sites between E*P and E2P. Cytoplasmic K+ (K+cyt) accelerated the conversion of E*P to E2P but did not dephosphorylate the E2P. Although the extracellular K+ accelerated the dephosphorylation of E2P, it did not interact with E*P directly. This K+cyt effect was also verified by the activation of Na+-pump in the Na+-K+ exchange mode. In the presence of K+cyt, both the ATP hydrolysis and Na+ uptake rates of the PL containing K+ inside vesicles increased sigmoidally with the concentrations of ATP and cytoplasmic Na+ (Na+cyt). However, in the absence of K+cyt, these Na+-pump reactions in PL containing K+ inside vesicles had only a hyperbolic curve. These results imply that the E*P to E2P conversion is one of the rate-limiting steps of the Na+-pump in the presence of a high concentration of ATP and that K+cyt may control this reaction step by enhancing the conversion rate of E*P to E2P.
Yoda
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Department of Pharmacology, University of Wisconsin Medical School, Madison 53706.
Yoda
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D000818:Animals; D003600:Cytosol / Q000378:metabolism; D004593:Electrophorus; D007700:Kinetics; D008081:Liposomes; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology*; D011485:Protein Binding; D011510:Proteolipids / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D008081:Liposomes; D011510:Proteolipids; C020485:proteoliposomes; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium