Rapid release of 45Ca from an occluded state of the Na,K-pump

Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites: implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr(2+) and Ba(2+) did not compete with K(+) for ATPase (an extracellular K(+) effect). Sr(2+) and Ba(2+) did compete with Na(+) for ATPase (an intracellular Na(+) effect) and with K(+) for pNPPase (an intracellular K(+) effect). These results suggest that Ba(2+) or Sr(2+) can bind to the intracellular transport site, yet neither Ba(2+) nor Sr(2+) was able to activate pNPPase activity; we confirmed that Ca(2+) and Mn(2+) did activate. As another measure of cation binding, we observed that Ca(2+) and Mn(2+), but not Ba(2+), decreased the fluorescence of the FITC-labeled pump; we confirmed that K(+) substantially decreased the fluorescence. Interestingly, Ba(2+) did shift the K(+) dose-response curve. Ethane diamine inhibited Mn(2+) stimulation of pNPPase (as well as K(+) and Mg(2+) stimulation) but did not shift the 50% inhibitory concentration (IC(50)) for the Mn(2+)-induced fluorescence change of FITC, though it did shift the IC(50) for the K(+)-induced change. These results suggest that the Mn(2+)-induced fluorescence change is not due to Mn(2+) binding at the transport site. The drawbacks of models in which Mn(2+) stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn(2+) stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.

Extracellular terbium and divalent cation effects on the red blood cell Na pump and chrysoidine effects on the renal Na pump

We examined the effect of extracellular terbium (Tb(3+)) and divalent metal cations (Ca(2+), Sr(2+), and Ba(2+)) on (86)Rb(+) influx into rabbit and human red blood cells. We found that Tb(3+) at 15 and 25 microM was a non-competitive inhibitor of (86)Rb(+) influx suggesting that Tb(3+) is not binding to the transport site. This result reduces the usefulness of Tb(3+) as a potential probe for the E(out) conformation (the conformation with the transport site facing extracellularly). Ba(2+), Sr(2+) and Ca(2+), at concentrations >50 mM, had minimal effects on Rb(+) influx into red blood cells (1 mM Rb-out). This suggests that the outside transport site is very specific for monovalent cations over divalent cations, in contrast to the inside transport site. We also found that chrysoidine (4-phenylazo-m-phenylenediamine) competes with Na(+) for ATPase activity and K(+) for pNPPase activity suggesting it is binding to the E(in) conformation. Chrysoidine and similar compounds may be useful as optical probes of the E(in) conformation.

Entrance port for Na(+) and K(+) ions on Na(+),K(+)-ATPase in the cytoplasmic loop between trans-membrane segments M6 and M7 of the alpha subunit. Proximity Of the cytoplasmic segment of the beta subunit

Based on the following observations we propose that the cytoplasmic loop between trans-membrane segments M6 and M7 (L6/7) of the alpha subunit of Na(+),K(+)-ATPase acts as an entrance port for Na(+) and K(+) ions. 1) In defined conditions chymotrypsin specifically cleaves L6/7 in the M5/M6 fragment of 19-kDa membranes, produced by extensive proteolysis of Na(+),K(+)-ATPase, and in parallel inactivates Rb(+) occlusion. 2) Dissociation of the M5/M6 fragment from 19-kDa membranes is prevented either by occluded cations or by competitive antagonists such as Ca(2+), Mg(2+), La(3+), p-xylylene bisguanidinium and m-xylylene bisguanidinium, or 1-bromo-2,4, 6-tris(methylisothiouronium)benzene and 1,3-dibromo-2,4,6-tris (methylisothiouronium)benzene (Br(2)-TITU(3+)). 3) Ca(2+) ions raise electrophoretic mobility of the M5/M6 fragment but not that of the other fragments of the alpha subunit. It appears that negatively charged residues in L6/7 recognize either Na(+) or K(+) ions or the competitive cation antagonists. Na(+) and K(+) ions are then occluded within trans-membrane segments and can be transported, whereas the cation antagonists are not occluded and block transport at the entrance port. The cytoplasmic segment of the beta subunit appears to be close to or contributes to the entrance port, as inferred from the following observations. 1) Specific chymotryptic cleavage of the 16-kDa fragment of the beta subunit to 15-kDa at 20 degrees C (Shainskaya, A., and Karlish, S. J. D. (1996) J. Biol. Chem. 271, 10309-10316) markedly reduces affinity for Br(2)-TITU(3+) and for Na(+) ions, detected by Na(+) occlusion assays or electrogenic Na(+) binding, whereas Rb(+) occlusion is unchanged. 2) Na(+) ions specifically protect the 16-kDa fragment against this chymotryptic cleavage.

Rapid inhibition of the K(+)-sensitive phosphoenzyme of Na+/K(+)-ATPase by (Z)-5-methyl-2-[2-(1-naphthyl)ethenyl]-4-piperidinopyridine

Membrane-bound Na+,K(+)-ATPase (0.1 mg/ml) was incubated with the K(+)-site-directed probe (Z)-5-methyl-2-[2-(1-naphthyl)ethenyl]-4-piperidinopyridine (AU-1421) (Takada, J. et al. (1990) Biochim. Biophys. Acta 1037, 373-379) at 37 degrees C for 30 min in the absence of ligands, then the Na(+)-dependent phosphorylation level was examined in the presence of 10 microM [32P]ATP at 0 degrees C. The level was decreased to 50% and 0% by about 50 microM and 100 microM AU-1421, respectively. Addition of 1 mM K+ during the treatment with AU-1421 resulted in complete maintenance of the phosphorylation level. When the preincubation was performed at 0 degrees C for 10 s, even 100 microM AU-1421 did not impair the phosphorylation. In contrast to the non-phospho form of the enzyme, the K(+)-sensitive phosphoenzyme formed from ATP was immediately inhibited by the addition of AU-1421 at 0 degrees C. The reactivity of the inhibited phosphoenzyme was restored by the addition of K+. About 1 mM K+ gave the same maximal reactivity in the presence of various fixed concentrations (8-41 microM) of AU-1421, but the apparent affinity for K+ decreased simply with the increase of AU-1421 concentration. From this simple competitive relationship, the apparent Ki value of AU-1421 for the phosphoenzyme was calculated to be 7.2 microM. Compared to the non-phospho form of the enzyme, the phospho form appears to be rather susceptible to AU-1421, probably because the K(+)-site of the phosphoenzyme is exposed to the extracellular aqueous phase.

(Z)-5-methyl-2-[2-(1-naphthyl)ethenyl]-4-piperidinopyridine (AU-1421), calcium ions and ethylenediamine as the K(+)-site directed probe for Na+/K(+)-ATPase

Recently, we have shown that a hydrophobic amine (AU-1421) produces an irreversible inactivation of Na+/K(+)-ATPase activity. This inactivation was prevented by K+ and its congeners. In this study, we examined the possibility of Ca2+ or ethylenediamine as a probe of the K+ occlusion center of Na+/K(+)-ATPase. The inactivation by AU-1421 was prevented by Ca2+ with an apparent high affinity (approximately 0.1 mM). Ca2+ protection was cancelled by high concentrations of ATP, ADP or Mg2+. Ca2+ and K+ were similar in these respects. Kinetic analyses of the above data indicated the presence of two AU-1421 occlusion sites on the enzyme, either one of which is susceptible to Ca2+ occlusion. Ethylenediamine also prevented the inactivation by AU-1421 or by C12E8 solubilization of the enzyme, suggesting that ethylenediamine, like K+, stabilized the enzyme. However, an apparent affinity of ethylenediamine (approximately 1.4 mM) was one order of magnitude lower than that of K+ (approximately 0.2 mM), and the protective manner did not show a simple competition. In addition, ethylenediamine binding was unaffected by ATP or ADP at a low affinity site, and antagonized K+ binding. From these results we concluded that ethylenediamine does not act like K+ or Ca2+ in protecting AU-1421 inactivation, since it can't stabilize the enzyme conformation as an E2 (K(+)-bound form).