Two types of inhibitor of phospholipid methylation in MOPC-31C cells with modified membrane phospholipids

Effect of modification of membrane phospholipid composition on the activity of phosphatidylethanolamine N-methyltransferase of Tetrahymena

The activity of phosphatidylethanolamine N-methyltransferase is less than 10% of control levels in microsomes prepared from the ciliate protozoan Tetrahymena thermophila whose phospholipid composition had been altered by being cultured on media containing phosphonic acids. The primary modification obtained is decreased levels of phosphatidylethanolamine (J.D. Smith and D.A. Giegel, Arch. Biochem. Biophys., 206, 420-423 (1981) and 213, 595-601 (1982)). The enzyme protein is present in these cells at normal levels since addition of the substrate phosphatidylethanolamine to the assay system restores enzyme activity of the lipid-modified microsomes to control levels, while the enzyme from control microsomes is not affected by added phosphatidylethanolamine. The microsomal enzyme is inhibited by the anionic phospholipids cardiolipin, phosphatidylglycerol, and phosphatidylinositol and by lysophosphatidylethanolamine while it is activated only by phosphatidylserine in addition to the substrates phosphatidylethanolamine and phosphatidyldimethylethanolamine. The added phosphatidylethanolamine acts directly as a substrate for the methyltransferase rather than acting by merely stimulating utilization of endogenous lipid since added phosphatidyl[14C]ethanolamine is directly converted to phosphatidylcholine. The results suggest that the technique of phosphonic acid-induced modification of lipid composition will be useful for the study of other membrane-bound enzymes.