ATPase, Na, K

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

FIGURE 10.8 A schematic diagram of the Na, K -ATPase in mammalian plasma membrane. ATP hydrolysis occurs on the cytoplasmic side of the membrane, Na ions are transported out of the cell, and ions are transported in. The transport stoichiometry is 3 Na out and 2 in per ATP hydrolyzed. The specific inhibitor ouabain (Figure 7.12) and other cardiac glycosides inhibit Na, K -ATPase by binding on the extracellular surface of the pump protein. 

A minimal mechanism for Na, K -ATPase postulates that the enzyme cycles between two principal conformations, denoted Ej and Eg (Figure 10.11). El has a high affinity for Na and ATP and is rapidly phosphorylated in the presence of Mg to form Ei-P, a state which contains three oeeluded Na ions (occluded in the sense that they are tightly bound and not easily dissociated from the enzyme in this conformation). A conformation change yields Eg-P, a form of the enzyme with relatively low affinity for Na, but a high affinity for K. This state presumably releases 3 Na ions and binds 2 ions on the outside of the cell. Dephosphorylation leaves EgKg, a form of the enzyme with two... 

FIGURE 10.10 The reaction of tridated sodium borohydride with the aspartyl phosphate at the active site of Na, K -ATPase. Acid hydrolysis of the enzyme following phosphorylation and sodium borohydride treatment yields a tripeptide containing serine, homoserine (derived from the aspartyl-phosphate), and lysine as shown. The site of phosphorylation is Asp" in the large cytoplasmic domain of the ATPase. 

FIGURE 10.11 A mechanism for Na, K -ATPase. The model assumes two principal conformations, Ei and E9. Binding of Na ions to Ei is followed by phosphorylation and release of ADP. Na ions are transported and released and ions are bound before dephosphorylation of the enzyme. Transport and release of ions complete the cycle. 

Na, K -ATPase, a K ATPase Ca-+-ATPase, SR Ca" -ATPase, plasma membrane H -ATPase, yeast H" -ATPase, Neurospora... 

FIGURE 10.13 Some of the sequence homologies in the nucleotide binding and phosphorylation domains of Na, K -ATPase, Ca -ATPase, and gastric H, K -ATPase. (Adapted from j0rgensm, P. L., and Andersen, J. R, 1988. Structnral basis for Ei - E2 confoyinational transitions in Na, K -pnmp and Cc -pnmp proteins. Journal of Membrane Biology 103 95-120)... 

FIGURE 10.15 A mechanism for Ca -ATPase from sarcoplasmic reticulum. Note the similarity to the mechanism of Na, K -ATPase (see also Figure 10.11). ( Out here represents the cytosol In represents the lumen of the SR.)... 

J. C. Seou (Aarhus) discovery of the first molecular pump, an ion-transporting enzyme Na+-K+ ATPase. 

Role of Na" "/K "-ATPase as energy convertor (Nobel lecture) 98AG(E)2320. 

Additional cellular events linked to the activity of blood pressure regulating substances involve membrane sodium transport mechanisms Na+/K.+ ATPase Na+fLi countertransport Na+ -H exchange Na+-Ca2+ exchange Na+-K+ 2C1 transport passive Na+ transport potassium channels cell volume and intracellular pH changes and calcium channels. 

Cardiac glycosides (CG) are potent and highly specific inhibitors of the intrinsic plasma membrane Na+/K+-ATPase, also known as the sodium pump. They modulate electrophysiological properties of the heart and its contractile functions. 

Inhibition of the Na+/K+-ATPase leads to a loss of potassium and an increase of sodium within the cell. Secondary intracellular calcium is increased via the Na VCa -exchanger. This results in a positive inotropic effect in the myocardium, with an increase of peak force and a decrease in time to peak tension. Besides this, cardiac glycosides increase vagal activity by effects on the central vagal nuclei, the nodose ganglion and increase in sensitivity of the sinus node to acetylcholine. 

Cardiac Glycosides. Figure 1 Cardiac glycosides - schematic representation of the Na+/K+-ATPase. 

The pharmacological receptor of cardiac glycosides is the sarcolemmal Na+/K+-ATPase expressed on most eucaryotic membranes. It was characterised biochemically in 1957 by J. Skou, who was awarded with the Nobel Prize in chemistry in 1997. The sodium... 

Blanco G et al (1998) Isozymes of the Na+-K+-ATPase heterogeneity in structure, diversity in function. Am J Physiol 275(5 Part 2) F633-F650... 

ENaC mediates Na+ entry from the tubule lumen at the apical membrane and the Na+/K+ ATPase extrudes Na+ at the basolateral side. K+ channels are present on the basolateral and apical membrane. K+ channels at the apical membrane mediate K+ secretion into the tubular lumen. 

Na+/K+-ATPase sodium pump Sodium- and potassium-activated adenosine 5 -triphosphatase EC 3.6.1.37. 

Tlie Na+/K+-ATPase belongs to the P-type ATPases, a family of more than 50 enzymes that also includes the Ca2+-ATPase of the sarcoplasmic reticulum or the gastric H+/K+-ATPase. P-Type ATPases have in common that during ion transport an aspartyl phos-phointermediate is formed by transfer of the y-phosphate group of ATP to the highly conserved sequence DKTGS/T [1]. 

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