Metabolic fate of chylomicron phospholipids and apoproteins in the rat.

To study the metabolic fate of chylomicron phospholipid and apoproteins, 15 mg of doubly labeled ([(3)H]leu, [(32)P]phospholipid) rat mesenteric lymph chylomicrons were injected as an intravenous bolus into conscious rats. The specific radioactivity, composition, pool size, and morphology of the plasma lipoproteins were determined after 2-60 min. After injection of chylomicrons, there was a rapid transfer of radioactivity into high density lipoproteins (HDL). At peak specific activity in HDL (2-5 min), 35% of injected apoprotein and 25% of phospholipid radioactivity were recovered in HDL (d 1.063-1.21 g/ml), with smaller recoveries in other lipoproteins and liver. There was an initial rapid rise of (32)P specific activity in HDL and d 1.02-1.063 lipoproteins (low density lipoproteins [LDL]), but whereas LDL specific activity subsequently converged with that of d < 1.02 lipoproteins, HDL specific activity decayed more rapidly than LDL or d < 1.02 lipoproteins. Lipolysis of chylomicrons was associated with a transfer of phospholipid mass into LDL and HDL. At 5 min, 80% of injected triglyceride had been lipolyzed and there was a significant increase in phospholipid mass in LDL and a smaller increase in HDL. At 10 min, the mass of phospholipid in LDL had returned towards control values, and there was a further increase in phospholipid mass in HDL, which suggested phospholipid transfer from LDL to HDL. In donor lymph chylomicrons (3)H-radioactivity was present in apoprotein (apo)B, apoA-I, and apoA-IV, but only radioactivity of apoA-I and apoA-IV were transferred to HDL. Transfer of radioactivity was associated with loss of mass of apoA-I and apoA-IV from the fraction that contained the chylomicron remnants (d < 1.02). With injection of 15 mg chylomicron, there was a small but insignificant increase in the relatively large pool of HDL apoA-I. However, 60 min after injection of 250 mg of human or rat intestinal chylomicrons into the rat, there was a significant increase in HDL apoA-I that resulted from acquisition of a major fraction of the chylomicron apoA-I. After injection of chylomicrons, phospholipid vesicles were observed by negative stain electron microscopy in the LDL and HDL ultracentrifugal fractions, especially in the LDL. Upon addition of an osmotically active compound, cellobiose, vesicles were observed as flattened particles with a double lipid bilayer thickness ( congruent with 100 A). To validate further the identity of these particles, chylomicrons were injected into rats with [(3)H]glucose, and the recipient rats' plasma was fractionated by chromatography on 6% agarose. Trapping of [(3)H]glucose occurred in the void and LDL regions of the column, and vesicular particles were identified in these column fractions by negative stain electron microscopy. Catabolism of chylomicrons is associated with a rapid transfer of phospholipid, apoA-I, and possibly apoA-IV into HDL. Chylomicron phospholipid appears to give rise to vesicles which are probably incorporated into preexisting HDL. Chylomicron surface components may be an important source of plasma HDL.