Lead actions on sodium-plus-potassium-activated adenosinetriphosphatase from electroplax, rat brain, and rat kidney

Inorganic lead ion, in micromolar concentrations, reversibly inhibits the sodium-plus-potassium-activated adenosinetriphosphatase (ATPase) and potassium-activated p-nitrophenylphosphatase (NPPase) activities of microsomal fractions from electric organ, rat kidney, and rat brain. In the presence of 3 mM MgC12 and 3 mM ATP, the concentrations of PbC12 producing half-maximal inhibition of the ATPase from these tissues are 4 X 10(-6) M, 20 X 10(-6) M, and 55 X 10(-6) M, respectively. The corresponding values for inhibition of the NPPase are 10(-6) M, 53 X 10(-6) M, and 22 X 10(-6) M. PbC12 also stimulates the phosphorylation by [gamma-32P]ATP of a microsomal protein from all three tissues in the absence of added sodium ion. This reaction was extensively studied with electroplax microsomes. In common with the well-known Na+-dependent phosphorylation of (Na+ + K+)-ATPase, the Pb2 -dependent reaction is inhibited by ouabain, specific for ATP, dependent on Mg2+, and yields and acid-stable phosphoprotein with a molecular weight of 98,000 in sodium dodecylsulfate. The Pb2+-dependent phosphoprotein, however, is not sensitive to K+. These observations are pertinent to the biochemistry and toxicity of inorganic lead in tissues and to the molecular mechanism of the cation transport enzyme.