Kinetic studies of a (Na+, K+, Mg2+) ATPase in sugar beet roots, 3: a proposed model for the (Na+, K+) activation and its significance for field properties [sodium, potassium]

A microsomal (Na+ + K+ + Mg-2+)ATPase preparation from sugar beet roots was used. The activation by simultaneous addition of Na+ and K+ at different levels was examined in terms of steady state kinetics. The observed data can be summarized in the following way: 1. The apparent affinity between the enzyme and the substrate MgATP depends on the ration between Na+ and K+. At low Na+ concentration (below 5 mM), the apparent K-m decreases with increasing concentrations of K+ (1-20 mM). At 5 mM Na+, the K+ level does not change the apparent K-m, while at Na+ levels above 10 mM, the apparent K-m between enzyme and substrate increases with increasing concentration of K+. 2. When the MgATP concentration is kept constant, homotropic cooperativity (concerning one type of ligand) and heterotropic cooperativity (concerning different types of ligands) exist in the activation by Na+ and K+. The Na+ binding is cooperative with different K-m values and Hull coefficients (n) in the presence of low and high concentration of K+. At low Na+ level (5 mM), a negative cooperativity exists for Na+ (n-Na 1) which is more pronounced in the presence of high [K+]. When the concentration of Na+ is raised the negative cooperativity disappears and turns into a positive one (n-Na 1). Only K+ binding in the presence of low [Na+] shows cooperativity with a Hill coefficient that reflects changes from negative to positive homotropic cooperativity with increasing concentrations of K+ (n-K 1 to n-K 1). In the presence of [Na+] 10 mM, the changes in n-K are insignificant. 3. A model is proposed in which one or two different K sites and one or two Na sites control the catalytic activity, with multiple interactions between Na+, K+ and MgATP. 4. In the presence of Na+ ( 10 mM), K+ is probably bound to two K sites, one of which translocates K+ through the membrane by an antiport Na+/K+ mechanism. This could be connected with an elevated K+ uptake in the presence of Na+ and could therefore explain some field properties of sugar beets.