Abstract
The inhibitory characteristics of ouabagenin on beef brain and rat heart Na+, K+-adenosine triphosphatase (ATPase) preparations were compared to ouabain. The 150 for both compounds was 103 greater for rat heart enzyme than for beef brain enzyme (ouabain on rat heart = 6 x 10-5 M; ouabain on beef brain = 5 x 10-8 M; ouabagenin on rat heart = 2 x 10-4 M; ouabagenin on beef brain = 7 x 10-7 M). Ouabagenin reversibly inhibited both enzyme preparations. 3H-ouabain binding studies demonstrated the labile nature of the ouabagenin enzyme complex but suggested that ouabagenin interacted with the same binding site(s) as ouabain. Kinetic analysis revealed that ouabagenin is a noncompetitive inhibitor with respect to sodium and ATP. The effect of ouabagenin (brain enzyme) on potassium activation appears to be competitive at potassium < 10 mM but noncompetitive at higher concentrations. The data suggest that the high affinity of ouabain for some Na+, K+-ATPase preparations involves both the sugar and steroid portions of the molecule. The insensitivity of the rat heart Na+, K+-ATPase to cardiac glycosides cannot be explained solely by a lack of a "sugar binding site" in Na+, K+-ATPase.
Footnotes
- Received August 21, 1973.
- Accepted January 7, 1974.
- © 1974 by The Williams & Wilkins Co.
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THE NATURE OF THE TRANSPORT ADENOSINE TRIPHOSPHATASE-DIGITALIS COMPLEX. VII. CHARACTERISTICS OF OUABAGENIN-Na+, K+- ADENOSINE TRIPHOSPHATASE INTERACTION
