Immunoreactivity and ouabain-dependent phosphorylation of (Na+ + K+)-adenosinetriphosphatase catalytic subunit doublets.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis with 6% polyacrylamide was used to resolve the 100-kDa catalytic (alpha subunit) polypeptide of (Na+ + K+)-adenosinetriphosphatase from various tissues. The catalytic subunit was identified on immunoblots with antisera against mouse brain catalytic subunit and lamb kidney holoenzyme. Immunoblots and Coomassie Blue-stained companion gels showed double species of the 100-kDa subunit in sucrose gradient fractions of mouse brain and kidney, bovine grey and white matter, purified lamb kidney and duck salt gland holoenzyme, electroplax microsomes, and NaI-extracted microsomes of goldfish and rat brain. The apparent molecular mass differences between the two species in each tissue all ranged between 5 and 8 kDa. Both forms in rat brain and lamb kidney enzyme contain common epitopes reactive with antibodies immunoaffinity-purified on either species from mouse brain. In addition, ouabain-dependent acid-stable inorganic phosphate incorporation was tested with mouse brain, lamb kidney, and electroplax enzyme. Ouabain-dependent phosphorylation was demonstrated in both species in lamb kidney and electroplax and in the larger of the two forms in mouse brain. These results suggest that double species of the phosphorylatable subunit are present generally in epithelial as well as excitable tissues and in fish and avian as well as mammalian species. Work is needed to elucidate their qualitative and quantitative characteristics in different tissues.