Characterization of remnants produced during the metabolism of triglyceride-rich lipoproteins of blood plasma and intestinal lymph in the rat.
The metabolism of intravenously injected large and small chylomicrons from intestinal lymph and of very low density lipoproteins from blood plasma was studied in functionally eviscerated "supradiaphragmetic" rats. For studies with lymph lipoproteins, recipient animals were injected with 4-amino-pyrazolopyrimidine 18 h before injection of lipoprotein to prevent secretion of very low density lipoproteins into their blood plasma. In all cases, most of the triglycerides (labeled with 14C) were rapidly metabolized, whereas cholesteryl esters (labeled with 3H) persisted in the blood. Most of the cholesteryl esters remained in smaller "remnant" lipoproteins, less dense that 1.006, which retained an apparently spherical shape, as determined by electron microscopy of negatively stained preparations. Whereas the diameters and chemical compositions of large chylomicrons were substantially different from those of small chylomicrons and very low density lipoproteins, all remnants were similar in these respects. Average remnant diameters were 400-600 A and remnants were enriched in cholesteryl esters and in protein insoluble in tetramethylurea. In addition to triglycerides, remnants were depleted of phospholiarticle size, the composition of remnants, like that of their precursors, was consistent with the "pseudomicellar" model of lipoproteins, in which a core of nonpolar lipids is covered by a monolayer of polar lipids and protein. These results domonstrate the fundamental similarity of the initial step in the metabolism of triglyceride-rich lipoproteins from intestinal mucosa and liver and show that loss of triglycerides from the core of the particles is accompanied by removal of polar components from the surface.