Assembly of rat hepatic very low density lipoproteins in the endoplasmic reticulum

The intracellular site of assembly of hepatic very low density lipoproteins has been investigated. Two endoplasmic reticulum fractions and Golgi vesicles (relatively free from endosomal contamination) were isolated from rat liver and the luminal contents were released. The apoB-containing entities were separated from the lumen of the endoplasmic reticulum and Golgi vesicles by an immunoaffinity isolation procedure. The amount of each lipid moiety (triacylglycerols, cholesterol plus cholesteryl esters and phospholipids) associated with a unit mass of apoB was shown to be very similar in the luminal contents isolated from each of the three fractions. Moreover, the apoB-containing particles that were isolated from the endoplasmic reticulum and Golgi were mainly present in a fraction of density < 1.02 g/ml. The average diameter of these lipoprotein particles was shown by negative staining electron microscopy to be in the size range of very low density lipoproteins and low density lipoproteins. The size and composition of the intracellular lipoproteins were very similar to those of both very low density lipoproteins isolated from the culture medium from rat hepatocytes and plasma very low density lipoproteins. The conclusion from this study is that the full complement of lipids associate with apoB at an early stage during the secretory pathway, in the endoplasmic reticulum.