RELATIONSHIPS BETWEEN PHOSPHATIDYLCHOLINE, PHOSPHATIDYLETHANOLAMINE, AND SPHINGOMYELIN METABOLISM IN CULTURED OLIGODENDROCYTES

In most cell types the major pathway of sphingomyelin synthesis is the direct transfer of the phosphocholine head group from phosphatidylcho line to ceramide catalyzed by the enzyme L-acylsphingosine:phosphatidy lcholine phosphocholinetransferase (SM synthase; EC 2.7.8.-). Although this pathway has been demonstrated in brain tissue, its quantitative importance has been questioned. An alternative biosynthetic pathway fo r sphingomyelin synthesis in brain tissue has been proposed, viz., the direct transfer of phosphoethanolamine from phosphatidylethanolamine to ceramide, followed by methylation of the ethanolamine moiety to a c holine group. We have evaluated various possible biosynthetic pathways of sphingomyelin synthesis in rat spinal cord oligodendrocytes, the m yelin-forming cells of the CNS, by labeling cells in culture with radi olabeled choline, ethanolamine, or serine. Our results indicate that, in oligodendrocytes, most of the phosphocholine for the biosynthesis o f sphingomyelin is provided by phosphatidylcholine, which is predomina ntly derived from de novo synthesis. No evidence was found for the ope ration of the alternative pathway via ceramide-phosphoethanolamine. Fu rthermore, our results indicate that a small pool of phosphatidylcholi ne is provided by methylation of phosphatidylethanolamine, which in tu rn is formed preferentially by decarboxylation of phosphatidylserine.