5'-p-fluorosulfonylbenzoyladenosine as an ATP site affinity probe for Na+, K+-ATPase

We have investigated the suitability of 5'-p-fluorosulfonylbenzoyladenosine (FSBA) as an ATP site affinity probe for the canine kidney Na+, K+-ATPase. The purified enzyme is slowly inactivated by this compound in suitable buffers, losing about half of its activity over a two-hour period. The rate of inactivation is more rapid in 0.1 M KCl than in 0.1 M NaCl. Low concentrations of ATP protect the enzyme against inactivation, with half-maximal effects at 4 microM ATP in 0.1 M NaCl and 350 microM ATP in 0.1 M KCl. ADP also protects against FSBA inhibition, but AMP is ineffective when present at 100 microM levels. This pattern is consistent with the previously described nucleotide specificity of the Na+, K+-ATPase. Addition of protective amounts of ATP after inactivation has occurred does not restore enzyme activity, indicating that inhibition is irreversible. Measurement of the concentration-dependence of FSBA inactivation suggests an apparent Kd for binding of this compound well above 1 mM, the solubility limit of the analog. This finding is reinforced by the failure of 1 mM FSBA to compete effectively with ATP for the high-affinity ATP site of the enzyme. Nevertheless, attachment of the analog to this site is indicated by its ability to prevent [3H]-ADP binding in proportion to the number of sites it has inactivated. Studies with [3H]-FSBA show that about 1 mole of the analog attaches specifically to the alpha subunit per mole of enzyme inactivated. A similar amount of nonspecific labeling also occurs with negligible effect on enzyme activity. These findings suggest that FSBA may be useful in probing the topography of the high-affinity ATP binding site of the Na+, K+-ATPase and related enzymes.