The pre-steady-state hydrolysis of ATP by porcine brain (Na+ + K+)-dependent ATPase

Distinction between the intermediates in Na+-ATPase and Na+,K+-ATPase reactions. II. Exchange and hydrolysis kinetics at micromolar nucleotide concentrations

The ATP hydrolysis rate and the ADP-ATP exchange rate of (Na+ + K+)-ATPase from ox brain were measured at 10 microM Mg2+free and at micromolar concentrations of free ATP and ADP. (1) In the absence of K+, substrate inhibition of the hydrolysis rate was observed. It disappeared at low Na+ and diminished at increasing concentrations of ADP. This was interpreted in terms of free ATP binding to E1P. In support of this interpretation, free ATP was found to competitively inhibit ADP-ATP exchange. (2) In the presence of K+, substrate activation of the hydrolysis rate was observed. Increasing (microM) concentrations of ADP did not give rise to competitive inhibition in contrast to the situation in the absence of K+ (cf. 1, above). This was interpreted to show that at micromolar substrate, some low-affinity, high-turnover Na+ + K+ activity is possible, provided the Mg2+ concentration is low. (3) While small concentrations of K+ increased the hydrolysis rate (cf. 2) they decreased the rate of ADP-ATP exchange. To elucidate this phenomenon, parallel measurements of exchange and hydrolysis rates were performed over a wide range of ATP and ADP concentrations, with and without K+. If, in the presence and absence of K+, ADP (and ATP competing) are binding to the same phosphorylated intermediate for the backward reaction, it places quantitative restrictions on the ratio of rate constants with and without K+. The results did not conform to these restrictions, and the discrepancy is taken as evidence for the necessity for a bicyclic scheme for the action of the (Na+ + K+)-ATPase. (4) An earlier statement concerning the nature of the phosphoenzyme obtained in the presence of Na+ and K+ is amended.

Rat brain (Na+-K+)ATPase: modulation of its ouabain-sensitive K+-PNPPase activity by thimerosal

1. The (Na+ + K+) ATPase activity of a rat brain synaptic membrane preparation was inhibited by 10(-5) M thimerosal. 2. The ouabain inhibitable K+-PNPPase activity of thimerosal treated membranes was compared with that of untreated membranes with respect to sensitivity to temperature, ouabain, K+ and ATP. 3. All those kinetic characteristics were substantially altered by treatment with thimerosal.

The steady-state kinetic mechanism of ATP hydrolysis catalyzed by membrane-bound (Na+ + K+)-ATPase from ox brain. II. Kinetic characterization of phosphointermediates

(1) The kinetics of the phosphorylated enzymic intermediates of (Na+ + K+)-ATPase from ox brain, which are formed by incubation of the enzyme with 25 microM AT32P, 150 mM Na+ and 1 mM Mg2+, have been studied in dephosphorylation experiments at 1 degree C. The dephosphorylation of the 32P-labelled enzyme was initiated by addition of either 1 mM unlabelled ATP, 2.5 mM ADP or 1 mM unlabelled ATP + ADP in concentrations from 25 to 1000 microM. (2) In the absence of ADP the dephosphorylation curve was linear in a semilogarithmic plot almost from t = 0, whereas by addition of ADP a biphasic behaviour was obtained. The slope of the slow phase of dephosphorylation was virtually independent of the ADP concentration. (3) The results were analysed by the mathematical equation corresponding to the simplest possible model for the interconversion and breakdown of the phosphointermediates: (formula: see text) where alpha, beta, H and G are functions of all the rate constants and H and G furthermore are functions of the initial values for [E1P] and [E2P]. (4) The analysis confirmed the model and enabled the determination of all the rate constants. (5) k-1 was found to be equal to k'-1 + k"-1 . [ADP] indicating an ADP-independent 'spontaneous' dephosphorylation of E1P. The rate constant for this process was close to that for dephosphorylation of E2P, i.e., k'-1 congruent to k3. Also the value of k"-1 was determined. (6) k3 was found to be at least 10 . k-2. The implication of this for the role of the E1P to E2P transition in the Na+ + K+)-stimulated ATP hydrolysis will be discussed in detail in the following paper (Plesner, I.W., Plesner, L., Nørby, J.G. and Klodos, I. (1981) Biochim. Biophys. Acta 643, 483--494). (7) A refinement of the model, accounting for the effect of Na+ on the steady-state ratio between [E1P] and [E2P] is proposed: (formula: see text). At [Na+] = 150 mM as used here, E1P(Na) and E'1P are assumed to be in rapid equilibrium. (8) Comparison of our results with those of others underlines the general validity of the conclusions of the present paper.

Tryptophan fluorescence of (Na+ + K+)-ATPase as a tool for study of the enzyme mechanism

1. The protein fluorescence intensity of (Na+ + K+)-ATPase is enhanced following binding of K+ at low concentrations. The properties of the response suggest that one or a few tryptophan residues are affected by a conformational transition between the K-bound form E2 . (K) and a Na-bound form E1 . Na. 2. The rate of the conformational transition E2 . (K) leads to E . Na has been measured with a stopped-flow fluorimeter by exploiting the difference in fluorescence of the two states. In the absence of ATP the rate is very slow, but it is greatly accelerated by binding of ATP to a low affinity site. 3. Transient changes in tryptophan fluorescence accompany hydrolysis of ATP at low concentrations, in media containing Mg2+, Na+ and K+. The fluorescence response reflects interconversion between the initial enzyme conformation, E1 . Na and the steady-state turnover intermediate E2 . (K). 4. The phosphorylated intermediate, E2P can be detected by a fluorescence increase accompanying hydrolysis of ATP in media containing Mg2+ and Na+ but no K+. 5. The conformational states and reaction mechanism of the (Na+ + K+)-ATPase are discussed in the light of this work. The results permit a comparison of the behaviour of the enzyme at both low and high nucleotide concentrations.