| Popis: |
The Na, K-ATPase, which is found in the cells of all higher eukaryotes, utilizes ATP to transport Na+ and K+ across the cell membrane. For every three sodium ions transported out of the cell two potassium ions are transported in. The enzyme is composed of two subunits, a larger a subunit, which is thought to contain most of the catalytic sites, and a smaller a subunit which is required for the proper processing and maturation of the enzyme. Three isoforms exist for the a subunit (α1, α2 and α3) and two isoforms exist for the β subunit (β1 and β2) in mammalian cells (Lingrel et al., 1990; Sweadner, 1989; Takeyasu et al, 1989). An additional isoform, β3, exists in Xenopous (Good et al., 1990). The a1 isoform is found in all cells while α2 is the primary isoform found in skeletal muscle, but is also present in the heart and nervous system. Expression of the α3 isoform is limited to the heart and nervous system (Orlowski and Lingrel, 1988). The cDNAs and genes corresponding to these isoforms have been isolated and the mechanisms responsible for their differential expression are being investigated (Lingrel et al., 1990). The Na,K-ATPase is a member of the P-type family of ATPases, which also include the Ca-ATPases and the H,K-ATPases. These transport proteins are similar in structure and have in common an aspart/l phosphate intermediate during their catalytic cycle. Site-directed mutagenesis and expression (MacLennan, 1990; Clarke et al., 1990; Vilsen and Andersen, 1992; Andersen and Vilsen, 1992) studies have been carried out to define cation binding sites in the Ca-ATPase but, less information is available using this approach with the Na.K-ATPase. The Na,K- ATPase differs from the other P-type ATPases in that it is sensitive to cardiac glycosides, a class of drugs which are used in the treatment of congestive heart failure and certain arrhythmias. The sensitivity of the enzyme to these drugs provides a useful tool for investigating structure-function relationships. In this report we describe progress made toward identifying potential cation binding sites as well as amino acid residues that are involved in determining cardiac glycoside sensitivity. |
