Effect of chaotropic anions on the sodium transport by the Na,K-ATPase

Abstract

The effect of choline iodide, bromide and chloride on the kinetics of the electrogenic sodium transport by the Na,K-ATPase was investigated in a model system of ATPase-containing membrane fragments adsorbed on the lipid bilayer membrane. The kinetic parameters of Na + transport were determined from short circuit currents after fast release of ATP from its caged precursor. The falling phase of the current transients could be fitted by a single exponential with the time constant, s 2 . Its temperature dependence allowed an estimation of the activation energy of the rate-limiting reaction step, the conformation transition E 1 /E 2 . Choline iodide and bromide caused a decrease of the activation energy as well as the overall rate of the process expressed as the pre-exponential factor A of the Arrhenius equation. If choline iodide or bromide were present on the cytoplasmic and extracellular sides of the protein, the temperature dependent changes were more pronounced than when present on the cytoplasmic side only. These results can be explained by an effect of the anions on water structure on the extracellular surface of the protein, where a deep access channel connects the ion-binding sites with the solution. Chloride ions also caused a deceleration of the electrogenic transport, however, in contrast to iodide or bromide, they did not affect the activation energy, and were more effective when added on the cytoplasmic side. This effect can be explained by asymmetric screening of the negative surface charges which leads to a transmembrane electric potential that modifies the ion transfer.

Hofmeister effect of anions on calcium translocation by sarcoplasmic reticulum Ca2+-ATPase

“…3 and Table 1 ). A similar explanation for the effect of bromide and iodide on ATP-dependent sodium translocation by Na + , K + -ATPase was proposed 31 . On the other hand, if the E 2 P conformation of SERCA is stabilized in the presence of the kosmotropic sulphate, lumenal release of Ca 2+ ions is favoured and an increase in the amount of the translocated charge is expected ( Fig.…”

“…We recall here that in the case of Na + , K + -ATPase-containing membrane fragments adsorbed on bilayer lipid membranes the amount of Na + -related charge transferred by the protein following an ATP concentration jump in the presence of 150 mM bromide or iodide was reduced to about 40% with respect to the charge obtained in the control measurement (150 mM NaCl), whereas a slight increase in the translocated charge was observed in the presence of 300 mM chloride 31 . The and Hasselbach also reported a 50% inhibition of ATP-dependent calcium uptake in native SR vesicles in the presence of 150 mM thiocyanate or 200 mM nitrate 33 .…”

“…It is known that various anions are able to affect partial and overall enzymatic reactions of the electrogenic ion pump Na + , K + -ATPase. The interaction of anions with Na + , K + -ATPase was characterized by biochemical and equilibrium fluorescence measurements, kinetic experiments using fluorescence dyes and electrical measurements on bilayer lipid membranes 25 26 27 28 29 30 31 . The order of anion effectiveness in influencing the Na + , K + -ATPase kinetics and transport activity follows the Hofmeister series.…”