On the reaction sequence of the K + -dependent acetyl phosphatase activity of the Na + pump

Probing energy coupling in the yeast plasma membrane H+-ATPase with acetyl phosphate

The energy-rich compound acetyl phosphate (ACP) was examined as a substrate for energy-linked reactions by the yeast plasma membrane H+-ATPase. The hydrolysis of ACP was sensitive to inhibition by vanadate with an IC50 approximately 1 microM, which is comparable to the level obtained in the presence of ATP. A Km of 8.29 +/- 0.65 mM for the hydrolysis of ACP was approximately 10-fold higher than that obtained for ATP, while Vmax values of 8.66 +/- 0.29 and 7.23 +/- 0.34 micromol Pi mg(-1) min(-1) were obtained with ATP and ACP, respectively. ACP formed a phosphorylated intermediate that was efficiently chased with hydroxylamine. Both ACP and ATP effectively protected the enzyme from trypsin-induced inactivation and formed identical tryptic digestion patterns, suggesting that ACP mimics the formation of conformational intermediates induced by ATP. However, unlike ATP, ACP was unable to drive proton transport by H+-ATPase. In addition, a pma1-S368F mutant enzyme that is highly insensitive to inhibition by vanadate in the presence of ATP was largely sensitive to vanadate in the presence of ACP. These results are interpreted in terms of a reverse, short-circuit pathway of the normal P-type ATPase kinetic pathway, in which the formation of E2P by-passes the E1P high-energy intermediate. In this pathway, ACP favors the formation of an E2P conformational state, which can interact with classical inhibitors like vanadate, but possesses insufficient free energy to drive proton transport by the H+-ATPase.

The effect of dimethylsulfoxide on the substrate site of Na+/K(+)-ATPase studied through phosphorylation by inorganic phosphate and ouabain binding

To obtain further information on the role of H2O at the substrate site of Na+/K(+)-ATPase, we have studied the enzymes reaction with P(i) and ouabain in 40% (v/v) Me2SO (dimethylsulfoxide). When the enzyme (E) was incubated with ouabain (O) for 5 min in a 40% (v/v) Me2SO-medium with 5 mM MgCl2 and 0.5 mM KCl (but no phosphate), ouabain was bound (as EO). Subsequent incubation with P(i) showed that E, but not EO, was rapidly phosphorylated (to EP). Long-time phosphorylation revealed that EO is also phosphorylated by P(i) albeit very slowly (t1/2 about 60 min) and that binding of ouabain to EP also is very slow. The EOP complex is stable, i.e., the t1/2 for the loss of P(i) is >> 60 min in contrast to about 1 min in water. These results in 40% Me2SO are distinctly different from what would be obtained in a watery milieu: ouabain would bind slowly and inefficiently in the absence of P(i), and ouabain would catalyse phosphorylation from P(i) rather than retard it. Equilibrium binding of [3H]ouabain to E and EP in water or 40% Me2SO confirmed these observations: Kdiss in water is 11 microM and 12 nM for EO and EOP, respectively, whereas in Me2SO they are 112 nM and 48 nM. It is suggested that the primary effect of the lowered water activity in 40% Me2SO is a rearrangement of the substrate site so that it also in the absence of P(i) attains a transition state configuration corresponding to the phosphorylated conformation. This would be sensed by the ouabain binding site and lead to high affinity ouabain binding in the absence of P(i).

Some total and partial reactions of Na+/K+-ATPase using ATP and acetyl phosphate as a substrate

Acetyl phosphate, as a substrate of (Na+ + K+)-ATPase, was further characterized by comparing its effects with those of ATP on some total and partial reactions carried out by the enzyme. In the absence of Mg2+ acetyl phosphate could not induce disocclusion (release) of Rb+ from E2(Rb); nor did it affect the acceleration of Rb+ release by non-limiting concentrations of ADP. In K+-free solutions and at pH 7.4 sodium ions were essential for ATP hydrolysis by (Na+ + K+)-ATPase; when acetyl phosphate was the substrate a hydrolysis (inhibited by ouabain) was observed in the presence and absence of Na+. In liposomes with (Na+ + K+)-ATPase incorporated and exposed to extravesicular (intracellular) Na+, acetyl phosphate could sustain a ouabain-sensitive Rb+ efflux; the levels of that flux were similar to those obtained with micromolar concentrations of ATP. When the liposomes were incubated in the absence of extravesicular Na+ a ouabain-sensitive Rb+ efflux could not be detected with either substrate. Native (Na+ + K+)-ATPase was phosphorylated at 0 degrees C in the presence of NaCl (50 mM for ATP and 10 mM for acetyl phosphate); after phosphorylation had been stopped by simultaneous addition of excess trans-1,2-diaminocyclohexane-N,N,N',N' tetraacetic acid and 1 M NaCl net synthesis of ATP by addition of ADP was obtained with both phosphoenzymes. The present results show that acetyl phosphate can fuel the overall cycle of cation translocation by (Na+ + K+)-ATPase acting only at the catalytic substrate site; this takes place via the formation of phosphorylated intermediates which can lead to ATP synthesis in a way which is indistinguishable from that obtained with ATP.

Acetyl phosphate can act as a substrate for Na+ transport by (Na+ + K+)-ATPase

Experiments using liposomes with (Na+ + K+)-ATPase incorporated showed that in the presence of extravesicular Mg2+, acetyl phosphate was able to stimulate Na+ uptake when the liposomes contained Na+ or choline and were K+-free; this acetyl phosphate-dependent Na+ transport was similar to the ATP-dependent transport observed with 0.003 mM or 3 mM ATP. When the intravesicular solution contained K+, there was an ATP-dependent Na+ uptake which was large with 3 mM ATP and small (about the size seen in K+-free liposomes) with 0.003 mM ATP; in this case, although acetyl phosphate produced a slight activation of Na+ transport, the effect was not statistically significant. All ATP and acetyl phosphate-stimulated Na+ transport disappeared in the absence of extravesicular Mg2+ or in the presence of ouabain in the intravesicular solution. These results are consistent with the hypothesis that, at the concentration used, acetyl phosphate can replace ATP in the catalytic but not in the regulatory site of the (Na+ + K+)-ATPase and active Na+ transport system. This suggests that as far as the early stages of the pump cycle are concerned the role of ATP is simply to phosphorylate.

Specific effects of spermine on ouabain-sensitive and potassium-dependent phosphatase activity of kidney plasma membranes. Specificity of the potassium sites

Specific inhibition of ouabain-sensitive and K+-dependent p-nitrophenyl-phosphatase activity of rabbit kidney plasma membranes by spermine (N,N'-bis(3-aminopropyl)-1,4-butanediamine) was characterized kinetically. 1. Inhibition by spermine was competitive with K+. The Ki for spermine was 31 micronM in the presence of 1 mM Mg2+. 2. Excess Mg2+ inhibited the ouabain-sensitive phosphatase activity in competition with K+. The Ki for Mg2+ was 2.6 mM. 3. Increasing Mg2+ concentrations reduced the spermine inhibition. This could be observed at Mg2+ concentrations higher than that of K+. 4. In the absence of inhibition by Mg2+, spermine was noncompetitive with Mg2+ which was essential for the ouabain-sensitive phosphatase activity. This could be observed at Mg2+ concentrations lower than that of K+. 5. Although Ca2+ was a strong inhibitor of the ouabain-sensitive phosphatase activity in the presence of K+, it produced a small stimulation of the activity in the absence of K+. Approximately 0.1 mM Ca2+ gave the maximum stimulation. 6. The observed Ca2+- and Mg2+-dependent phosphatase activity was inhibited strongly by ouabain and by spermine. The half-maximal inhibition concentrations of ouabain and spermine were 0.1 and 63 micronM, respectively. It is likely that Mg2+, Ca2+ and spermine bind to the same site as does K+.

Potassium activated phosphatase from human red blood cells. The effects of p-nitrophenylphosphate on carbon fluxes

1. When red cells are incubated in solutions containing p-nitrophenyl-phosphate (p-NPP), intracellular p-NPP quickly builds up reaching with a half-time of 3 min a concentration in cell water equal to one fourth the external concentration, which under the conditions used is the expected value for a divalent anion in Gibbs-Donnan equilibrium. Hence p-NPP added to the incubation media in red cells has quick access to the active centre of the membrane phosphatase which is located at the inner surface of the cell membrane.2. When p-NPP is added to the incubation media of ATP-free red cells or reconstituted ghosts, no ouabain-sensitive cation movements are detectable, suggesting that hydrolysis of p-NPP by the active transport system is unable to energize active ion translocation.3. When p-NPP concentration in the incubation media of ATP-containing cells is progressively raised, both ouabain-sensitive Na loss and ouabain-sensitive Rb uptake tend to zero along rectangular hyperbolae. For both movements inhibition is half-maximal at 77 mM external p-NPP (i.e. 19 mM internal p-NPP).4. p-NPP inhibits with equal effectiveness the Na:K and the Na:Na exchanges catalysed by the Na pump.5. The inhibitory effect of p-NPP cannot be attributed to the products of its hydrolysis, is inversely related to the intracellular ATP concentration and seems to be exerted at the inner surface of the cell membrane with an apparent affinity similar to that of the membrane phosphatase. These facts suggest that inhibition is mediated by the combination of p-NPP with the active centre of the membrane phosphatase.6. Apart from affecting the ouabain-sensitive cation movements, p-NPP increases the ouabain-resistant uptake and loss of both Na and Rb. This effect is about 4 times larger for Rb than for Na, and its kinetic analysis suggests that it is due to an increase in the passive permeability of the cell membrane.7. The increase in passive cation permeability upon addition of p-NPP cannot be attributed to the products of its hydrolysis. It seems to be due to the combination of p-NPP with a site which, like the active centre of the ouabain-resistant membrane phosphatase, faces the inner surface of the cell membrane, is unaffected by ATP and is half saturated by about 15 mM-p NPP.