Substrate inhibition of the (Na⁺, K⁺)-ATPase in the presence of excess Mg²⁺

1. 1. High concentrations of ATP inhibit completely the activity of (Na⁺, K⁺)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) prepared from sheep brain. 2. 2. The inhibition depends on the concentration of total ATP, i.e. complexed ATP + free ATP. 3. 3. The inhibition by high ATP concentrations persists in the absence of K⁺, and is then independent of the Na⁺ concentration between 2 and 140 mM Na⁺. 4. 4. Raising the K⁺ concentration at 20 mM Na⁺ increases the ATP concentration required for the maximal hydrolysis rate. 5. 5. The Hill number for the inhibition process is about three. 6. 6. The inhibition by ATP is temperature-dependent, in that as the temperature is increased. higher ATP concentrations are required for inhibition.