Kinetic studies on the ADP-ATP exchange reaction catalyzed by Na+, K+-dependent ATPase. Evidence for the K.S.T. mechanism with two enzyme-ATP complexes and two phosphorylated intermediates of high-energy type

Abstract

The kinetic properties of the [3H]ADP-ATP exchange reaction catalyzed by Na+, K+-dependent ATPase [EC 3.6.1,3] were investigated, using NaI-treated microsomes from bovine brain, and the following results were obtained. 1. The rates of the Na+-dependent exchange reaction in the steady state were measured in a solution containing 45 micronM free Mg2+, 100 mMNaCl, 80 micronM ATP, and 160 micronM ADP at pH 6.5 and 4-5 degrees. The rate and amount of decrease in phosphorylated intermediate on adding ADP, i.e., the amount of ADP-sensitive EP, were measured while varying one of the reaction parameters and fixing the others mentioned above. Plots of the exchange rate and the amount of ADP-sensitive EP against the logarithm of free Mg2+ concentration gave bell-shaped curves with maximum values at 50-60 micronM free Mg2+. Plots of the exchange rate and the amount of ADP-sensitive EP against pH also gave bell-shaped curves with maximum values at pH 6.9-7. They both increased with increase in the concentration of NaCl to maximum values at 150-200 mM NaCl, and then decreased rapidly with increase in the NaCl concentration above 200 mM. The dependences of the exchange rate and the amount of ADP-sensitive EP on the concentration of ADP followed the Michaelis-Menten equation, and the Michaelis constants Km, for both were 43 micronM. The dependence of the exchange rate on the ATP concentration also followed the Michaelis-Menten equation, and the Km value was 30 micronM. The amount of ADP-sensitive EP increased with increase in the ATP concentration, and reached a maximum value at about 5 micronM ATP. 2. The N+-dependent [3H]ADP-ATP exchange reaction was started by adding [3H]ADP to EP at low Mg2+-concentration. The reaction consisted of a rapid initial phase and a slow steady phase. The amount of [3H]ATP formed during the rapid initial phase, i.e. the size of the ATP burst, was equal to that of ADP-sensitive EP, and was proportional to the rate in the steady state. At high Mg2+ concentration, the rate of Na+-dependent exchange in the steady state was almost zero, and EP did not show any ADP sensitivity. However, rapid formation of [3H]ATP was observed in the pre-steady state, and the size of the ATP burst increased with increase in the KCl concentration. From these findings, we concluded that an enzyme-ATP complex (E2ATP) formed at low Mg2+ concentration is in equilibrium with EP + ADP, that the rate-limiting step for the exchange reaction is the release of ATP from the enzyme-ATP complex, that the ADP-insensitive EP (formula: see text) produced at high Mg2+ concentration is in equilibrium with the enzyme-ATP complex, and that the equilibrium shifts towards the enzyme-ATP complex on adding KCl. Actually, the ratio of the size of the ATP burst to the amount of EP was equal to the reciprocal of the equilibrium constant of step (formula: see text), determined by a method previously reported by us.