Identification of an amino acid substitution in human alpha 1 Na,K-ATPase which confers differentially reduced affinity for two related cardiac glycosides.

The ouabain-resistant cell line H1C1 displays a 30-fold differential of reduced sensitivity to the structurally related cardiac glycosides digoxin and digitoxin (Baker, R. M. (1976) in Biogenesis and Turnover of Membrane Macromolecules (Cook, J.S., ed) pp. 93-103, Raven Press, New York). Since these ligand congeners differ only by the presence of a hydroxyl group at C-12 of digoxin we predicted that the H1C1 phenotype must reflect a mutation which alters the binding site of the cardiac glycoside receptor (Na,K-ATPase). Complementary DNA encoding the alpha 1 Na,K-ATPase was prepared from H1C1 cell total RNA by reverse transcription-coupled polymerase chain reaction and these cDNAs were cloned. Sequence analysis of the reverse transcriptase-polymerase chain reaction clones revealed several independent isolates containing a G > A transition at nucleotide 332 of the propeptide coding sequence, generating the amino acid substitution C108Y. The ability of this substitution to confer differential sensitivity for digoxin and digitoxin was tested and confirmed by expressing a human alpha 1 C108Y-Na,K-ATPase in wild type HeLa cells and assaying for inhibition of cell growth and inhibition of Na,K-ATPase activity. Phenylalanine or alanine substitutions of this cysteine also confer this pattern of ligand discrimination. Ouabain-resistant Na,K-ATPase substitutions, at positions other than Cys-108 failed to exhibit differential sensitivity indicating that this ligand discrimination is unique to Cys-108 substitutions rather than a general property of cardiac glycoside-resistant mutants. It is proposed that differential resistance of the C108Y receptor for these ligands is a consequence of altering two features of the ligand-receptor interaction; one, a disruption of a common hydrogen bond resulting in general loss of affinity for cardiac glycosides and the other, formation of a new H-bond between the C-12 hydroxyl of digoxin and the receptor, specifically augmenting the stability of this ligand-receptor complex.