KINETICS OF INHIBITION OF NAK-ATPASE BY MG2+, K+, AND VANADATE

This paper describes the kinetics of inhibition of Na+- and K+-stimulated adenosine triphosphatase (NaK-ATPase) by Mg2+, K+, and vanadate in combination and the reversal of this inhibition by Na+. Inhibition required the simultaneous occupation of inhibitory sites for Mg2+, K+, and vanadate. Inhibitory sites for Mg2+ and K+ are distinct from their sites for activation of NaK-ATPase. Mg2+ increased the apparent affinity for K+ at its inhibitory site and vice versa. Mg2+ also increased the apparent affinity for vanadate. The occupation of inhibitory sites for Mg2+ and vanadate obeyed Michaelis-Menten kinetics, but the occupation of the inhibitory site for K+ was a sigmoid function of K+ concentration. It appears that at least two K+ ions may be required to inhibit. Cations which can substitute for K+ as activators of NaK-ATPase also substituted as inhibitors in the presence of Mg2+ and vanadate (e.g., Tl+, Rb+, Cs+, and NH4+). The relative affinities of these cations for the activation site and the inhibitory site ranked in about the same order. This result suggests a similarity between sites for activation and inhibition by K+. In the presence of inhibitory concentrations of Mg2+, K+, and vanadate, Na+ freely occupied its site for activation of NaK-ATPase, but the activity generated was limited by the extent of inhibition by Mg2+, K+, and vanadate. At higher Na+ concentrations, Na+ occupied a low-affinity site to antagonize inhibition. Occupation of this low-affinity site was markedly depressed at higher K+ concentrations, and it appeared that Na+ antagonized inhibition by displacing K+ from its inhibitory site. Inhibition of NaK-ATPase by Mg2+, K+, and vanadate could reflect a mechanism for regulation of NaK-ATPase in vivo. © 1979, American Chemical Society. All rights reserved.