Properties of the Na+, K+-stimulated adenosine triphosphatase system associated with the plasma membrane of pig thyroid glands.

The enzymatic properties of plasma membrane-bound Na+, K+-ATPase [EC 3.6.1.3], isolated with high specific activity and in good yield from pig thyroid cells, were examined. The enzyme activity required the presence of both Na+ and K+ at physiological concentrations; it exhibited high sensitivity to K+ and an absolute requirement for Na+. It showed highly specific requirement for Mg2+ and ATP. The apparent Km for ATP was 0.14 mM under the assay conditions. Arrhenius plots had a point of inflection at about 22 degrees C, activation energies being 24.2 kcal/mol at 5-22 degrees C and 19.0 kcal/mol at 22-40 degrees C. In addition to ouabain, the ATPase was strongly inhibited by fluoride and the SH-blocking reagent, PCMB. Iodide and TSH had no appreciable effect on the enzyme activity.