Na,K-ATPase inhibitor from guinea pig brain is not ouabain-like.

An acetone HCl extract of guinea pig brain reported to have ouabain-like activity was prepared. The extract was column-chromatographed on Sephadex G-10 and fractions were tested for ion content and "apparent ouabain-like" activity, i.e., ability to displace ouabain from its binding site and to inhibit Na,K-ATPase under conditions that optimize one or the other assay. "Apparent ouabain-like" activity eluted with the salt peak. When the ability of these fractions to inhibit ouabain binding was correlated with their potassium concentrations, the displacement could be entirely accounted for by the potassium content. The Na,K-ATPase inhibiting activity of these salt peak fractions was demonstrable in a Tris assay medium but not in a histidine (metal chelator)assay medium. Thorough desalting of fractions with "apparent ouabain-like" activity abolished this activity. Additional Na,K-ATPase inhibitory activity appeared in fractions distinct from the salt peak. However, these fractions did not display ouabain displacing ability. It is concluded that the "apparent ouabain-like" activity of the guinea pig brain extract is not truly ouabain-like and can be explained by interfering ions.