FLUORESCEINYL]THIOUREIDO]ETHYL]CARBAMOYL]ADENOSINE 5'-TRIPHOSPHATE AND ITS CR(H2O)(4) AND CO(NH3)(4) COMPLEX DERIVATIVES ARE NEW FLUORESCENT TOOLS FOR LABELING ATP BINDING-SITES OF NA+ K+-ATPASE/

2'(3')-O-[N- [2- [3- [5-fluoresceinyl] thioureido] ethyl] carbamoyl] a denosine 5'-triphosphate (FEDA-ATP), a spectroscopic tool used for stu dying skeletal muscle myosin ATPase subfragment 1, was applied to Na+/ K+-ATPase (EC 3.6.1.37). In contrast to the myosin subfragment, we fou nd that FEDA-ATP is not a substrate for Na+/K+-ATPase. On the other ha nd, FEDA-ATP showed an affinity for both the low (E-2, K-d = 200 mu M) and the high (E-1, K-d = 22 mu M) affinity ATP-binding sites. When th e microscopic affinities of FEDA-ATP were used for calculating the mac roscopic affinity in the overall reaction according to K-i = (K-dE1K- dE2)(1/2), the experimentally measured inhibition constant of 66 mu M was obtained. To evoke irreversible binding inhibitors, FEDA-ATP was t ransferred in its chromium(III) and cobalt(III) complex analogs, which are suitable tools for labelling the ATP binding sites of Na+/K+-ATPa se in a specific way.