Phosphatidylcholine synthesis for incorporation into membranes or for secretion as plasma lipoproteins by Golgi membranes of rat liver.

Phosphatidylcholine, the major phospholipid of very low density lipoproteins, is packaged with triglyceride in the Golgi cisternae. CTP-phosphocholine cytidyltransferase and CDP-choline phosphotransferase activities of Golgi subfractions were higher than those of rough or smooth microsomes measured under the same conditions, indicating that phosphatidylcholine synthesis can occur in Golgi membranes. Consistent with this, the specific activity of phosphatidylcholine of Golgi membranes rose more rapidly than that of rough and smooth microsomes after injection of [14C]choline in vivo. The specific activity of the Golgi content phosphatidylcholine (non-membrane fraction) remained low. The S-adenosylmethionine phosphatidylethanolamine methyltransferase activity of Golgi subfractions was also higher than that of rough or smooth microsomes. After injection of [3H]methyl-labeled methionine in vivo, the specific activity of phosphatidylcholine of the Golgi membranes rose in parallel with that of the rough and smooth microsomes. The specific activity of the Golgi content phosphatidylcholine rose above that of the Golgi membranes and exhibited a different pattern, suggesting that this pathway may selectively label phosphatidylcholine which is secreted as lipoproteins. These observations indicate that the Golgi membranes have the enzymes necessary for synthesis of phosphatidylcholine, and incorporation of lipid precursors indicates that synthesis of phosphatidylcholine by Golgi membranes occurs in vivo.