Assignment of charge movements to electrogenic reaction steps of Na,K-ATPase by analysis of salt effects on the kinetics of charge movements

Abstract.

Na,K-ATPase-enriched membrane fragments adsorbed to lipid bilayers were used to study electrogenic Na⁺ movements induced by enzyme phosphorylation when ATP was photo-released from inactive caged ATP, and simultaneously by externally applied alternating voltages which allowed the measurement of small ATP-induced membrane admittance changes. A detailed analysis of frequency dependence of the capacitance and conductance increments showed that the observed process consists of more than one electrogenic step. The frequency dependence could be described by the sum of two Lorentzian functions and a constant term. The faster process (~2000 s^–1) was assigned to the release of the first extracellular Na⁺ ion. The corner frequency of the slower Lorentzian (about 30 s^–1) coincided with the reciprocal exponential time constant of the falling phase of the transient current, which can be assigned to the conformational transition. Preferentially, the slower process showed a dependence on the ion concentration of choline salts with different anions. The effectiveness of the used chaotropic anions to decelerate the kinetics decreased in agreement with the Hofmeister series, I^–>Br^–>Cl^–. This observation matches their effect on the partition between two phosphoenzyme states of the Na,K-ATPase, as established previously.