Molecular Pharmacology, Vol 15, 620-626, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

The Harmala Alkaloids: Evidence for Their Complex Inhibition of the K⁺-Acyl Phosphatase Reaction of Membranes

¹ Department of Pharmacology, University of Alberta, Edmonton, Alberta, T6G 2H7 Canada

Although the harmala alkaloids were originally reported to be specific inhibitors of the Na⁺-activation site of (Na⁺ + K⁺)-ATPase, their reaction is now known to be more complex as the K⁺-activation site of this enzyme is also thought to be involved. By using a fluorescent method of assay employing 3-O-methyl fluorescein phosphate as substrate, and by appropriate correction of the data for the fluorescence quenching of harmaline itself, we have confirmed that this alkaloid directly interacts with the K⁺ activation site of the acyl-phosphatase partial reaction of (Na⁺ + K⁺)-ATPase. However, calculation of the Hill coefficients for harmaline inhibition of K⁺-acyl phosphatase and for K⁺-activation of the system in the presence of harmaline suggests that activation and inhibition do not occur at the same site.