Abstract
The technique of laser Doppler electrophoresis was applied for the study of the surface charge properties of (Na+,+)-ATPase containing microsomal vesicles derived from guinea-pig kidney. The influence of pH, the screening and binding of uni- and divalent cations and the binding of ATP show: (1) one net negative charge per protein unit with a pK = 3.9; (2) deviation from the Debye relation between surface potential and ionic strength for univalent cations, with no difference in the effect of Na+ and K+; (3) Mg2+ binds with an association constant of Ka = 1.1 · 102M−1 while ATP binds with an apparent Ka = 1.1 · 104M−2 for 1 mM Nacl, 0.2 mM KCI, 0.1 mM MgCl2, 0.1 mM Tris-HCI (pH 7.3). The binding is weaker at higher Mg2+ concentrations. There is no ATP binding in the absence of Mg2+. In addition, the average vesicle size derived from the linewidth of the quasi-elastic light scattering spectrum is 203.7 ± 15.2 nm. In the presence of ATP a reduction in size is observed.
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