Biosynthesis of lymph and plasma lipoprotein apoproteins by isolated perfused rat liver and intestine.

The ability of rat intestine and liver to synthesize the various apoproteins of plasma lipoproteins was investigated. After the individual isolated organs were perfused with blood containing [(3)H]lysine, chylomicrons plus very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL) were isolated from the perfusates and the intestinal lymph. After lipoprotein delipidation, apoproteins were separated by polyacrylamide gel electrophoresis and the (3)H content was determined. Livers incorporated [(3)H]lysine into all apoprotein bands of VLDL and HDL. The (3)H content was greater in large proteins that remained in the stacking gel (group I, predominantly beta-apoprotein) than in proteins with apparent molecular weights near 50,000 (group II) or in the smaller peptides (molecular weights near 10,000, group III). In the intestine, (3)H was incorporated into group I and, in larger amounts, into group II apoproteins of lymph VLDL. No labeled VLDL appeared in the perfusate. (3)H was also incorporated into group II apoproteins of lymph and perfusate HDL. Significantly, no [(3)H]lysine was found in the group III peptides of any lymph or intestinal perfusate lipoproteins. Since these peptides were always present in VLDL from mesenteric lymph collected in vivo, the results suggest that nascent VLDL of gut origin acquires group III peptides from other lipoproteins that penetrate lymph from plasma.