Electrochemical Determination of Transmembrane Protein Na+/K+-ATPase and Its Cytoplasmic Loop C45

Electrochemistry of membrane proteins is complicated by the fact that the studied substances are poorly soluble or insoluble in aqueous environment. The solubilization of proteins using surfactants (detergents) affects the electrochemical analysis or even renders it impossible. In the present study, the electrochemistry of the transmembrane protein Na+/K+‐ATPase (NKA) and its water‐soluble isolated cytoplasmic loop C45 is described. The proteins were studied using adsorptive transfer cyclic voltammetry and square‐wave voltammetry on basal‐plane pyrolytic graphite electrode (PGE) as well as constant‐current chronopotentiometric stripping analysis on hanging mercury drop electrode (HMDE). The nonionic surfactant octaethylene glycol monododecyl ether (C12E8) was used for NKA solubilization. Under these conditions the oxidation currents of Tyr and Trp (peak Y: +0.55 V and peak W: +0.7 V, vs. Ag/AgCl/3 M KCl) and catalytic reduction currents (peak H: −1.8 V) of NKA and C45 loop can be observed. Using the experimental procedures suggested in this study, we were able to investigate the oxidation, reduction and adsorption of NKA and C45 at femtomole level without the necessity of labeling by electroactive markers or techniques based on protein immobilization within the lipid bilayer attached to the electrode surface.