RESUMEN
A 3-fold higher concentration of "endogenous digitalis" is found in the serum of patients with essential hypertension than in the serum of normotensives, whose concentration was determined in 22 normotensive probands by an receptor assay using isolated (Na+ + K+)-ATPase as 76.3 +/- 9.3 nM ouabain equivalents. Since the concentration of "endogenous digitalis" was 7-19 fold higher in 2 patients, who had become uremic due to a malignant hypertension and since their serum levels fell 3-fold by hemodialysis, the hemofiltrate was used for partial purification of the substance. This was possible by hydrophobic chromatography on Amberlite XAD-2, octadecyl-coated silica gel, anion exchange chromatography and affinity chromatography on membrane-bound (Na+ + K+)-ATPase. The partially purified substance behaved like the material described by Cloix et al. (1985) Biochem. Biophys. Res. Commun. 131:1234-1240 and competed with digoxin for digoxin antibodies. Ascorbic acid isolated on the search for an inhibitor of (Na+ + K+)-ATPase from beef brain inhibited [3H]ouabain binding due to a decrease of the ouabain binding sites by a reduction of a group within the ATP binding site of the enzyme. Unsaturated fatty acids claimed to be "endogenous digitalis (Tamura et al. [1985] J. Biol. Chem. 260:9672-9677)" also lowered the capacity of the cardiac glycoside binding site but did not compete with ouabain.