Synthesis, modification, and flotation properties of rat hepatocyte apolipoproteins

M. Mahmood Hussain, Eleni E. Zanni, Mike Kelly, Vassilis I. Zannis*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

24 Citations (Scopus)

Abstract

We have studied apolipoprotein synthesis, intracellular modification and secretion by primary adult rat hepatocyte cultures using continuous pulse or pulse chase labeling with [35S]methionine, immunoprecipitation and two-dimensional isoelectric focusing/polyacrylamide gel electrophoresis. The flotation properties of the newly secreted apolipoproteins were studied by discontinuous density gradient ultracentrifugation and one- and two-dimensional polyacrylamide gel electrophoresis. These studies showed that rat hepatocyte apoE is modified intracellularly to produce minor isoproteins that differ in size and charge. One of these minor isoproteins represents a monosialated apoE form (apoE3s1). Similarly, apoCIII is modified intracellularly to produce a disialated apoCIII form (apoCIIIs2), whereas newly synthesized apoA-I and apoA-IV are not glycosylated and overlap on two-dimensional gels with the proapoA-I and the plasma apoA-IV form, respectively. Both ummodified and modified apolipoproteins are secreted into the medium. Separation of secreted apolipoproteins by density gradient ultracentrifugation has shown that 50% of apoE, 80% of apoA-I, and more than 90% of apoA-IV and apoCIII are secreted in a lipid-poor form, whereas apoB-100 and apoB-48 are 100% associated with lipids. ApoB-100 floats in the VLDL and IDL regions, whereas apoB-48 is found in all lipoprotein fractions. ApoE and small amounts of apoA-I, apoA-IV and apoCIII float in the HDL region. Small amounts of apoE and apoCIII are also found in the VLDL and IDL regions, and apoE in the LDL region. Ultracentrifugation of nascent lipoproteins in the presence of rat serum promoted flotation of apoA-I and apoA-IV in the HDL fraction and resulted in increased flotation and distribution of apoE and apoCs in VLDL, IDL and LDL regions. These observations are consistent with the hypothesis that intracellular assembly of lipoproteins involves apoB-48 and apoB-100 forms, whereas a large portion of apoA-I, apoCIII and apoA-IV can be secreted in a lipid-poor form, which associates extracellularly with preexisting lipoproteins.

Original languageEnglish
Pages (from-to)90-101
Number of pages12
JournalBiochimica et Biophysica Acta - Molecular and Cell Biology of Lipids
Volume1001
Issue number1
DOIs
Publication statusPublished - 23 Jan 1989
Externally publishedYes

Keywords

  • (Rat)
  • Apolipoprotein synthesis
  • Flotation property
  • Hepatocyte
  • Lipoprotein assembly

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