The Incorporation of Amino Acids into Soluble Lipoproteins by Cell-free Preparations from Rat Liver*

The net synthesis of low density plasma lipoproteins has been shown in the perfused rat liver and in rat liver slices in work reported from this laboratory (2). Haft, Roheim, White, and Eder (3) demonstrated amino acid incorporation into low and high density plasma lipoproteins in the perfused liver, and Radding and Steinberg (4) reported similar observations in rat liver slices. The present experiments were designed to extend this work to cell-free systems in which a more detailed analysis of the biosynthesis of plasma lipoproteins may be expected. The synthesis of plasma albumin by rat liver microsomes has been found by several groups of investigators (5-7). It was found in the present work that whole homogenates (devoid of nuclei) and isolated microsomes incorporate a variety of labeled amino acids into both low and high density plasma lipoproteins. Mitochondria are also capable of synthesizing lipoproteins at low rates, but whether these are truly plasma lipoproteins remains to be determined. Evidence that the lipoproteins labeled by liver microsomes were plasma, and not tissue, lipoproteins was obtained by the use of specific antisera and by a modified “fingerprint” study of trypsin-chymotrypsin digests of the labeled protein.