Effect of ATP on Na:K affinity and catalytic activity of (Na+ plus K+)-activated enzyme system

Na+, K+-dependent adenosine triphosphate phosphohydrolase. Two types of kinetics

Kinetic analysis of hydrolytic activity of (Na+, K+)-ATPase purified from duck salt glands shows that several substrates are hydrolysed in different manners. UTP, GTP and ITP are hydrolysed in accordance with usual Michaelis kinetics with the single Km value and with no cooperatively (Hill coefficient, nH = 1), while CTP is hydrolysed, like ATP, in accordance with non-Michaelis kinetics with two Km values. Hydrolysis of the last two substrates in the range of the second Km is characterised by positive cooperativity with nH greater than 1.

Evidence for the existence of a monovalent cation-stimulated, magnesium-dependent adenosine triphosphatase activity in the isolated plasma membranes of amoebas of the slime mold dictyostelium discoideum

Evidence for a magnesium-dependent ATPase activity that can be stimulated by Na+ and K+, or equally by Na+ or K+ alone, has been found in the plasma membranes isolated from amoebas of the slime mold Dictyostelium discoideum when the membranes are isolated from cultures grown up to the stationary phase. This ATPase activity is scarcely inhibitable by ouabain or phlorizin, but is very sensitive to low concentrations of azide or thimerosal. When the plasma membranes are isolated from amoebas growing in logarithmic phase, this monovalent cation-stimulated Mg2+-dependent activity is barely detectable.

The effects of ATP on the interactions between monovalent cations and the sodium pump in dialysed squid axons

1. The efflux of Na in dialysed axons of the squid has been used to monitor the sidedness of the interactions of the Na pump with Na(+) ions, K(+) ions and ATP. The axons were under conditions such that most of the Na efflux went through the Na pump by means of a complete cycle of ATP hydrolysis.2. With 310 mm-K(i) (+), 70 mm-Na(i) (+) and 10 mm-K(+) artificial sea water (ASW) more than 97% of the Na efflux was abolished by removal of ATP. The efflux of Na was stimulated by ATP with a K((1/2)) of about 200 mum. This is similar to the K((1/2)) of 150 mum found for the ATP dependence of a ouabain-sensitive Na,K-ATPase activity in membrane fragments isolated from squid optical nerves.3. A 100-fold reduction in the ATP concentration (from 3-5 mm to 30-50 mum) increased the apparent affinity of the Na pump for K(o) (+) about 8-fold. In addition, the maximal rate of K(o) (+)-stimulated Na efflux was reduced by a similar factor. Analogous results were seen in axons dialysed with 310 mm-K(i) (+) or without K(i) (+).4. The relative effectiveness of external monovalent cations as activators of the Na efflux was a function of the ATP concentration inside the axon. With 3-5 mm-ATP the order of effectiveness was K(+) > NH(4) (+) > Rb(+). With 30-50 mum-ATP the sequence was NH(4) (+) >> K(+) >> Rb(+). These results were not affected by the removal of K(i) (+).5. When the ATP concentration was 3 mm and the Na(i) (+) concentration 70 mm, the removal of K(i) (+) produced a slight and reversible increase in the total efflux of Na (15%) and no change in the ATP-dependent Na efflux. When the ATP concentration was reduced to 30-50 mum, or the Na(i) (+) concentration lowered to 5-10 mm, the removal of K(i) (+) reversibly increased the total and the ATP-dependent efflux of Na. The largest increase in Na efflux was seen when both ATP and Na(i) (+) were simultaneously reduced. The ATP-dependent extra Na efflux resulting from the exclusion of K(i) (+) was abolished by 10(-4)m-ouabain in the sea waters.6. The increase in the ATP-dependent Na efflux observed in axons dialysed with 0 K(i) (+) + 10 mm-K(+) ASW was not seen in axons perfused with 310 mm-K(i) (+) + 450 mm-K(+) ASW. However, both experimental conditions gave rise to a similar (and small) ATP-independent and ouabain-insensitive efflux of Na. This indicates that the effects on the Na pump of removing K(i) (+) are not due to the simultaneous membrane depolarization. In addition, it suggests that K(i) (+) has an inhibitory effect on the Na pump, and that that effect is antagonized by Na(i) (+) and ATP.7. The present results are consistent with the idea that the same conformation of the Na pump (and Na,K-ATPase) can be reached by interaction with external K(+) after phosphorylation and with internal K(+) before rephosphorylation. This enzyme conformation produces an enzyme-K complex from which K(+) ions are not easily released unless high concentrations of ATP are present. This also stresses a non-phosphorylating regulatory role of ATP.

The ATP dependence of a ouabain-sensitive sodium efflux activated by external sodium, potassium and lithium in human red cells

The stimulation of ouabain-sensitive Na+ efflux by external Na+, K+ and Li+ was studied in control and ATP-depleted human red cells. In the presence of 5 mM Na+, with control and depleted cells, Li+ stimulated with a lower apparent affinity than K+, and gave a smaller maximal activation than K+. The ability of Na+, K+ and Li+ to activate Na+ efflux was a function of the ATP content of the cells. Relative to K+ both Na+ and Li+ became more effective activators when the ATP was reduced to about one tenth of the control values. At this low ATP concentration Na+ was absolutely more effective than K+.

Purification and characterization of (Na+, K+)-ATPase. V. Conformational changes in the enzyme Transitions between the Na-form and the K-form studied with tryptic digestion as a tool

1. Purified (Na+, K+)-ATPase consisting of membrane fragments was digested with trypsin. The time course of enzyme inactivation was related to the electrophoretic pattern of native and cleaved proteins remaining in the membrane. 2. Differences in both the inactivation kinetics and the cleavage of the large chain (mol. wt 98 000) allow distinction of two patterns of tryptic digestion of (Na+, K+)-ATPase seen with Na+ or K+ in the medium. 3. With K+, the inactivation of (Na+, K+)-ATPase is linear with time in semilogarithmic plots and the activity is lost in parallel with cleavage of the large chain to fragments with molecular weights 58 000 and 48 000. 4. With Na+, the inactivation curves are biphasic. In the initial phase of rapid inactivation, 50% of the activity is lost with minor changes in the composition of the large chain. In the final phase, the large chain is cleaved at a low rate to a fragment with a molecular weight of 78 000. 5. It is concluded that the regions of the large chain exposed in the presence of K+ are distinct from the regions exposed in presence of Na+ and that two conformations of (Na+, K+)-ATPase can be sensed with trypsin, a (t)K-form and a (t)Na-form. 6. Reaction of the (t)K-form with ATP cause transition to the (t)Na-form. Relatively high concentrations of ATP are required and Mg2+ is not necessary. Phosphorylation of (Na+, K+)-ATPase is accompanied by transition from the (t)Na-form to the (t)K-form. Previous kinetic data suggest that these conformational changes are accompanied by shifts in the affinities of the enzyme for Na+ and K+.