DEDO: A specific, fluorescent inhibitor for spectroscopic investigations of Na,K-ATPase

The interaction between the fluorescent ouabain derivative DEDO and purified renal Na,K-ATPase (isolated from different animal species) is investigated. Equilibrium binding studies provide a pK value of about 7.5 and a stoichoimetric coefficient of 1. Nonmodified ouabain exhibits the same affinity to the rabbit enzyme; the enzyme originating from the other sources binds DEDO 10 times less strongly than ouabain. Kinetic studies indicate that this is the consequence of a 10 times higher dissociation rate constant of the complexes formed with DEDO. The fluorescence emission intensity of DEDO is enhanced, being dependent on the enzyme source. The single decay time of DEDO is 3 ns in the absence and 21 ns in the presence of the rabbit enzyme and 14 ns in the presence of the pig renal enzyme. This result suggests that the fluorophore of DEDO is bound to a very hydrophobic environment of the enzyme. Further characterization of the static fluorescence spectra provides evidence for energy transfer between Trp residues of the enzyme and DEDO. Distance estimations suggest that one or two Trp residues are likely to be located in the proximity of the fluorophore.