Irreversible inhibition of Na-+ +K-+ -ATPase by strophanthidin 3,5-bid-p-benzoyl benzoate, a photochemical analogue of strophanthidin.

A photoactivatible analogue of strophanthidin, strophanthidin 3,5-bis-p-benzoyl benzoate (SBB), was synthesized and tested as a photoaffinity label for the cardiotonic steroid binding site of Na-+ +K-+ -ATPase. SBB inhibited rat brain Na-+ +K-+ -ATPase with an I50 of approximately 1 times 10-minus 5 M and displaced (3-H) ouabain from its specific binding site on this enzyme while the photoaffinity group, methyl p-benzoyl benzoate (me-pBB), alone was not effective. Ultraviolet photoactivation of SBB which had been specifically bound at the cardiotonic steroid binding sites of this enzyme produced 35% irreversible inhibition of enzyme activity. However, only slightly less irreversible inhibition was observed in the absence of cardiotonic site directed binding of SBB and photoactivation of me-pBB itself produced marked inhibition of the enzyme. It was concluded that the bulk of the photoactivated inhibition occurring with SBB does not involve the cardiotonic steroid binding site and that a substantial reduction in the concentration of non-specifically bound SBB is required to expose any site directed labeling.