The roles of Golgi and endoplasmic reticulum in the synthesis and assembly of lipoprotein lipids in rat hepatocytes.

Microsomes, derived from the endoplasmic reticulum and Golgi fractions, were isolated from rat liver and separated into content and membrane subfractions by treatment with sodium carbonate or by passage through the French pressure cell. The microsomal content contained particles, which had a triglyceride molar concentration tenfold greater than that of phospholipid, cholesterol, and cholesteryl ester. In contrast, the Golgi content contained particles approximately 50% larger which contained proportionally less triglyceride. [3H]palmitic acid was initially incorporated into a pool of triglyceride, associated with the microsomal membrane, subsequently sequestered in the cisternal space, and then transferred to the Golgi region. [3H]palmitic acid was incorporated more slowly into phospholipid. The specific activity of the microsomal content phospholipid reached a level approximately twice that of the membrane, suggesting that newly synthesized phospholipid is preferentially sequestered. However, the specific activities of the Golgi membrane and content phospholipid were similar to that of the microsomal membrane. These observations suggest that the triglyceride to be secreted is sequestered into the cisternal space of the endoplasmic reticulum together with only small amounts of other lipids. The bulk of the phospholipid and cholesterol to be secreted is apparently added to the triglyceride-rich particles when these reach the Golgi cisternae.