Effects of ATP on the intermediary steps of the reaction of the (Na+ + K+)-ATPase. IV. Effect of ATP on K0.5 for Na+ and on hydrolysis at different pH and temperature.

The pH optimum for (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) depends on the combination of monovalent cations, on the ATP concentration and on temperature. ATP decreases the Na+ concentration necessary for half maximum activation, K0.5 for Na+ (Na+ + K+ = 150 mM), and the effect is pH and temperature dependent. At a low ATP concentration a decrease in pH leads to an increase in K0.5 for Na+, while at the high ATP concentration it leads to a decrease. K0.5 for ATP for hydrolysis decreases with an increase in pH. The fractional stimulation by K+ in the presence of Na+ decreases with the ATP concentration, and at a low ATP concentration K+ becomes inhibitory, this being most pronounced at 0 degrees C. The results suggest that (a) ATP at a given pH has two different effects: it increases the Na+ relative to K+ affinity on the internal site (K0.5 for ATP at pH 7.4, 37 degrees C, is less than 10 microM); it increases the molar activity in the presence of Na+ + K+ (K0.5 for ATP at pH 7.4, 37 degrees , is 127 microM), (b) binding of the cations to the external as well as the internal sites leads to pK changes (Bohr effect) which are different for Na+ and for K+, i.e. the selectivity for Na+ relative to K+ depends both on ATP and on the degree of protonation of certain groups on the system, (c) ATP involves an extra dissociable group in the determination of the selectivity of the internal site, and thereby changes the effect of an increase in protonation of the system from a decrease to an increase in selectivity for Na+ relative to K+.