STUDIES ON THE SYNTHESIS AND SECRETION OF SERUM LIPOPROTEINS BY RAT LIVER SLICES

Open Access
Publisher Website
Google Scholar
Abstract
Rat liver slices incubated in Krebs'' bicarbonate buffer containing 1-C14-acetate and 1-C14-leucine synthesized and secreted into the medium lipoproteins labeled both in the lipid and in the protein moieties. The lipoproteins were isolated by preparative ultracentrifuga-tion at 114,000 x G for 48 hr. at a solvent density of 1.21. After a lag period of 30 minutes the radioactivity in the lipids of the medium lipoproteins increased almost linearly for 4 hr. The specific radioactivity of the protein moiety of the medium lipoproteins continued to rise for 4 hr. The newly-synthesized lipoproteins in the density class 1.063-1.21 were shown to be identical with the normally circulating D 1.063-1.21 lipoproteins of the rat by using the "fingerprint" method of Katz, Dreyer and Anfinsen (J. Biol. Chem. 234 2897, 1959). Liver slices were in-cubated in serum with a complete mixture of C14 -labelled amino acids prepared from Chlorella grown in C14O2. The D 1.063-1.21 lipoproteins were isolated, delipidated, taken up in concentrated urea and digested with trypsin and chymotrypsin. The peptide products were chromatographed on paper in one direction, then separated by high-voltage electrophoresis in the perpendicular direction and located by staining with ninhydrin. An autoradiogram made of this "fingerprint" showed that each ninhydrin-positive spot was radioactive and that there was no radioactivity not associated with a peptide. Since the grossly stainable peptides were derived from the normal serum medium (no significant net amounts of new lipoprotein were added to the medium during incubation) this establishes the specific biosynthesis and secretion of D 1.063-1.21 or a1-lipoprotein by liver in vitro. The rate of 1-C14-leucine incorporation into the total lipoprotein fraction (D< 1.21) was measured in liver slices taken from cholesterol-fed, Triton-treated and control rats. Although cholesterol feeding depressed cholesterol synthesis by 90% and Triton injection increased it by 150% there were no significant changes in the rate of synthesis and secretion of the protein moiety of the lipoproteins. Liver slices from rats with nephrotic syndrome produced with anti-kidney serum synthesized both lipoprotein (D 1.21) at rates about twice normal. It is suggested that the hyper-lipemia of the nephrotic syndrome may thus be due to overproduction of both lipoprotein and albumin but with renal loss of the albumin.