The Effects of 2H2O on Sodium plus Potassium Ion-stimulated Adenosine Triphosphatase of Rat Brain

Abstract

Abstract Deuterated water (2H2O) was found to inhibit the Mg2+-dependent Na+ + K+-stimulated adenosine triphosphatase of rat brain microsomal membrane preparations. At a concentration of 95% 2H2O, the inhibition observed varied between 30 and 52%; no inhibition of the basic Mg2+-stimulated component in the ATPase was observed. The inhibition of the Na+ + K+-stimulated ATPase was not time dependent and was completely reversed on washing the enzyme free of 2H2O. Kinetic analysis of the effects of 2H2O on the various parameters which influence the activity of the Na+ + K+-stimulated ATPase revealed that the inhibition was apparently noncompetitive with respect to ATP, competitive with respect to Na+, and uncompetitive with respect to K+. The K+-stimulated p-nitrophenyl phosphatase activity associated with the ATPase was stimulated by 2H2O in a competitive manner with respect to K+. Based on these data it is suggested that Na+ activates the Na+ + K+-stimulated ATPase prior to its phosphorylation by ATP and that the presence of H2O at the Na+ site has an important functional role for the activation of the enzyme by Na+. A second locus of action of 2H2O is suggested to be on E2-P (the presumed conformation of the phosphorylated enzyme which is hydrolyzed in the presence of K+) prior to the action of K+ on it. Possibly this results in a change in this phosphorylated form so that its affinity for K+ is increased.