Comparison of the substrate dependence properties of the rat Na,K-ATPase alpha 1, alpha 2, and alpha 3 isoforms expressed in HeLa cells.
The role of multiple isoforms for the alpha subunit of Na,K-ATPase is essentially unknown. To examine the functional properties of the three alpha subunit isoforms, we developed a system for the heterologous expression of Na,K-ATPase in which the enzymatic activity of each isoform can be independently analyzed. Ouabain-resistant forms of the rat alpha 2 and alpha 3 subunits were constructed by site-directed mutagenesis of amino acid residues at the extracellular borders of the first and second transmembrane domains (L111R and N122D for alpha 2 and Q108R and N119D for alpha 3). cDNAs encoding the rat alpha 1 subunit, which is naturally ouabain-resistant, and rat alpha 2 and alpha 3, which were mutated to ouabain resistance (designated rat alpha 2* and rat alpha 3*, respectively) were cloned into an expression vector and transfected into HeLa cells. Resistant clones were isolated and analyzed for ouabain-inhibitable ATPase activity in the presence of 1 microM ouabain, which inhibits the endogenous Na,K-ATPase present in HeLa cells (I50 approximately equal to 10 nM). The remaining activity corresponds to Na,K-ATPase molecules containing the transfected rat alpha 1, rat alpha 2*, or rat alpha 3* isoforms. Utilizing this system, we examined Na+, K+, and ATP dependence of enzyme activity. Na,K-ATPase molecules containing rat alpha 1 and rat alpha 2* exhibited a 2-3-fold higher apparent affinity for Na+ than those containing rat alpha 3* (apparent KNa+ (millimolar): rat alpha 1 = 1.15 +/- 0.13; rat alpha 2* = 1.05 +/- 0.11; rat alpha 3* = 3.08 +/- 0.06). Additionally, rat alpha 3* had a slightly higher apparent affinity for ATP (in the millimolar concentration range) compared with rat alpha 1 or rat alpha 2* (apparent K0.5 (millimolar): rat alpha 1 = 0.43 +/- 0.12; rat alpha 2* = 0.54 +/- 0.15; rat alpha 3* = 0.21 +/- 0.04) and all three isoforms has similar apparent affinities for K+ (apparent KK+: rat alpha 1 = 0.45 +/- 0.01; rat alpha 2* = 0.43 +/- 0.004; rat alpha 3* = 0.27 +/- 0.01). This study represents the first comparison of the functional properties of the three Na,K-ATPase alpha isoforms expressed in the same cell type.
Jewell
E A
EA
Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Ohio 45267-0524.
Lingrel
J B
JB
D000818:Animals; D015152:Blotting, Northern; D003001:Cloning, Molecular; D004351:Drug Resistance; D006367:HeLa Cells; D006801:Humans; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D016297:Mutagenesis, Site-Directed; D010042:Ouabain / Q000494:pharmacology; D011189:Potassium Chloride / Q000378:metabolism; D051381:Rats; D012965:Sodium Chloride / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D013379:Substrate Specificity
D007527:Isoenzymes; D008024:Ligands; D012965:Sodium Chloride; D010042:Ouabain; D011189:Potassium Chloride; D000254:Sodium-Potassium-Exchanging ATPase