Electron microscopic and biochemical study of lipoprotein synthesis in the isolated perfused rat liver.

The isolated perfused rat liver was used to study the 300-800 A electron-opaque bodies which had previously been described in the liver cell Golgi apparatus, smooth endoplasmic reticulum, and space of Disse. When the perfusion medium was enriched with linoleate, the number and electron opacity of these particles increased markedly. Sequential biopsies showed that they appeared first in the smooth surfaced terminal ends of the rough reticulum, the smooth endoplasmic reticulum proper, and the Golgi apparatus and later in the space of Disse. After 60 min of perfusion, particles of the same size and shape as those in the liver cells could be isolated in large numbers from the d < 1.006 fraction of the perfusate. Control livers perfused with an identical medium but without linoleate did not show these changes. Puromycin markedly depressed the production of 300-800 A particles by livers perfused with an oleate-rich medium; however, it did not interfere with the formation of large cytoplasmic droplets of neutral fat. In keeping with these findings, puromycin blocked the incorporation of oleate-(14)C into lipoprotein triglyceride isolated from the perfusate, but did not interfere with the appearance of the labeled fatty acid in tissue triglyceride. Puromycin also blocked the incorporation of leucine-(3)H into both tissue protein and perfusate lipoprotein. We concluded that the 300-800 A particles observed are, in all likelihood, very low density lipoproteins and that their formation is blocked by puromycin, presumably through interference with the synthesis of their apoprotein.