Mechanisms by which Li+ stimulates the (Na+ and K+)-dependent ATPase.
The addition of LiCl stimulated the (Na+ + K+)-dependent ATPase activity of a rat brain enzyme preparation. Stimulation was greatest in high Na+/low K+ media and at low Mg-ATP concentrations. Apparent affinities for Li+ were estimated at the alpha-sites (moderate-affinity sites for K+ demonstrable in terms of activation of the associated K+-dependent phosphatase reaction), at the beta-sites (high-affinity sites for K+ demonstrable in terms of activation of the overall ATPase reaction), and at the Na+ sites for activation. The relative efficacy of Li+ was estimated in terms of the apparent maximal velocity of the phosphatase and ATPase reactions when Li+ was substituted for K+, and also in terms of the relative effect of Li+ on the apparent Km for Mg-ATP. With these data, and previously determined values for the apparent affinities of K+ and Na+ at these same sites, quantitative kinetic models for the stimulation were examined. A composite model is required in which Li+ stimulates by relieving inhibition due to K+ and Na+ (i) by competing with K+ for the alpha-sites on the enzyme through which K+ decreases the apparent affinity for Mg-ATP and (ii) by competing with Na+ at low-affinity inhibitory sites, which may represent the external sites at which Na+ is discharged by the membrane Na+/K pump that this enzyme represents. Both these sites of action for Li+ would thus lie, in vivo, on the cell exterior.
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology; D004789:Enzyme Activation / Q000187:drug effects; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology*; D008433:Mathematics; D008954:Models, Biological; D011188:Potassium / Q000494:pharmacology*; D011485:Protein Binding; D051381:Rats; D012964:Sodium / Q000494:pharmacology*
D008094:Lithium; D012964:Sodium; D000251:Adenosine Triphosphatases; D011188:Potassium