NA,K-ATPASE - ISOFORM STRUCTURE, FUNCTION, AND EXPRESSION

An interesting feature of the Na,K-ATPase is the multiplicity of alpha and beta-isoforms. Three isoforms exist for the alpha-subunit, alpha-1, alpha-2, and alpha-3, as well for the beta-subunit, beta-1, beta-2, and beta-3. The functional significance of these isoforms is unknown, but they are expressed in a tissue- and developmental-specific manner. For example, all three isoforms of the alpha-subunit are present in the brain, while only alpha-1 is present in kidney and lung, and alpha-2 represents the major isoform in skeletal muscle. Therefore, it is possible that each of these isoforms confers different properties on the Na,K-ATPase which allows effective coupling to the physiological process for which it provides energy in the form of an ion gradient. It is also possible that the multiple isoforms are the result of gene triplication and that each isoform exhibits similar enzymatic properties. In this case. the expression of the triplicated genes would be individually regulated to provide the appropriate amount of Na,K-ATPase to the particular tissue and at specific times of development. While differences are observed in such parameters as Na+ affinity and sensitivity to cardiac glycosides. it is not known if these properties play a functional role within the cell. Site-directed mutagenesis has identified amino acid residues in the first extracellular region of the alpha-subunit as major determinants in the differential sensitivity to cardiac glycosides. Similar studies have failed to identify residues in the second extracellular region involved in cardiac glycoside inhibition. Further analysis of the enzymatic properties of the enzyme, understanding the regulated expression of the genes, and structure-function studies utilizing site-directed mutagenesis should provide new insights into the enzymatic and physiological roles of the various subunit isoforms of the Na,K-ATPase.