Metabolism of chylomicron phosphatidylethanolamine in the rat.

10 min after intravenous injection of 32P-labelled chylomicrons 69% of the [32P]phosphatidylethanolamine, but only 34% of the [32P]phosphatidylcholine, was cleared from plasma. Over this time period the plasma phosphatidylethanolamine concentration that was increased after the chylomicron injection returned to normal, indicating that the rapid clearance of [32P]phosphatidylethanolamine was primarily due to metabolism and not due to exchange with tissue phospholipids. Also after chylomicron [32P]phospholipids had been transferred to high-density lipoproteins in vitro a greater fraction of [32P]phosphatidylethanolamine than of [32P]phosphatidylcholine was eliminated both in the intact rat and in the perfused rat liver. In the latter system the uptake of [32P]phosphatidylethanolamine was reduced by preperfusion with heparin to remove heparin-releasable hepatic lipase. Experiments with hepatocytes and non-parenchymal liver cells, which both contain only minor amounts of surface-bound hepatic lipase, did not demonstrate any rapid uptake of [32P]phosphatidylethanolamine by exchange with cellular phospholipids. This study shows that there is a mechanism for rapid clearance of chylomicron phosphatidylethanolamine, which maintains a low plasma phosphatidylethanolamine level (30-40 micrograms/ml) despite a significant transport of phosphatidylethanolamine with the intestinal lipoproteins. A major part of the rapid phosphatidylethanolamine metabolism occurs in the liver, where the activity of the hepatic lipase is likely to be of crucial importance.