Molecular Pharmacology, Vol 20, 631-636, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Inhibition of Phospholipid Methyltransferase(s) from Rat Liver Plasma Membranes by Analogues of S-Adenosylhomocysteine

¹ Department of Pharmacology, University of Bergen School of Medicine, Bergen, Norway

The kinetics of the enzymatic incorporation of [³H]methyl groups from S-adenosyl-L-[methyl-³H]methionine into phospholipids of isolated plasma membranes from rat liver were determined. Physiological pH (7.4) and the presence of Mg²⁺ (1 mM) favored the incorporation of the first methyl group into phosphatidylethanolamine to form phosphatidyl-N-monomethylethanolamine, whereas pH 10.0 favored the incorporation of two methyl groups into phosphatidyl-N-monomethylethanolamine to form phosphatidylcholine. The apparent K_m values for S-adenosylmethionine were 4.1 µM (pH 7.4) and 92 µM (pH 10.0). The inhibition of phospholipid methylation by S-adenosylhomocysteine and some S-adenosylhomocysteine analogues known to be effective in other biological systems was determined. Linear competitive inhibition was observed with S-adenosylhomocysteine and some analogues, and among these compounds S-tubercidinylhomocysteine was particularly effective. The inhibitor constants at pH 7.4 (in the presence of Mg²⁺) were 0.5 µM (S-adenosylhomocysteine), 0.3 µM (S-tubercidinylhomocysteine), 1.5 µM (S-3-deazaadenosylhomocysteine), 15 µM (S-adenosyl-D-homocysteine), and 20 µM (sinefungin). 5'-Deoxy-5'-S-isobutyl-thioadenosine and the sinefungin metabolite A9145C did not inhibit phospholipid methylation. Similar results were obtained at pH 10.0, except that the inhibition constants were about 10-fold higher. The results are discussed in relationship to the biological properties of these analogues of S-adenosylhomocysteine.

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ACKNOWLEDGMENTS The authors wish to thank Ivar Bergesen (Lilly Laboratories), Dr. James Coward, and Dr. Edgar Lederer for generous gifts of the AdoHcy analogues used in this work.