RT Journal Article SR Electronic T1 The Harmala Alkaloids: Evidence for Their Complex Inhibition of the K+-Acyl Phosphatase Reaction of Membranes JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 620 OP 626 VO 15 IS 3 A1 SIMONSON, L. P. A1 CHARNOCK, J. S. YR 1979 UL http://molpharm.aspetjournals.org/content/15/3/620.abstract AB 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.