Studies on the transfer of phosphatidylcholine from unilamellar vesicles into plasma high density lipoproteins in the rat.

To investigate the metabolic disposition of phosphatidylcholine vesicles, unilamellar vesicles of egg phosphatidylcholine or cholesterol/phosphatidylcholine were injected intravenously into rats. Ten minutes after injection of 0.5 to 3.0 mg phosphatidylcholine vesicles, a major fraction of their radioactivity and mass had been incorporated into high density lipoproteins (HDL). Agarose gel chromatography showed that the phospholipid of vesicles was incorporated into the plasma HDL region, in association with release of a trapped marker, [3H]inulin. Density gradient ultracentrifugtion showed that the highest specific activity of phospholipid was present in HDL of d 1.08-1.12 g/ml, consisting of homogenous spherical particles by negative stain electron microscopy. With increasing cholesterol/phosphatidylcholine ratio of injected vesicles, there was progressively less incorporation of phospholipid into HDL and vesicles retaining [3H]inulin were re-isolated from plasma. Sixty minutes after injection of cholesterol/phosphatidylcholines vesicles, phospholipid appeared to be partly transferred into HDL and partly taken up by the liver. In summary, injection of unilamellar egg phosphatidylcholine vesicles results in a rapid incorporation of vesicle phospholipid into plasma HDL, primarily as a result of insertion of phospholipid into pre-existing HDL. This process is inhibited by a high content of vesicle unesterified cholesterol. These studies may have relevance to the mechanisms of transfer of phosphatidylcholine from chylomicrons into plasma.