Biosynthesis of plasma lipoproteins. Incorporation of 14C-glucosamine by cells and subcellular fractions of rat liver.

The incorporation of glucosamlneJ4C into plasma lipoproteins has been measured in the intact rat, in liver slices, and in the liver microsome fraction. The labeling of lipoprotein was studied in microsome subfractions, both in viva with glucosammeJ4C and in vitro with UDP-N-acetylglucosamineW. After isolation of lipoproteins with the aid of carrier plasma, labeled lipoproteins were found in smooth endoplasmic reticulum and Golgi membranes, but not in rough endoplasmic reticulum or ribosomes. Approximately 100 times as much smooth membrane protein as Golgi membrane protein was required to isolate the same total amount of labeled lipoprotein, in uivo as well as in vitro. High density lipoproteins had 3 times as much total label as low density lipoproteins, which may be related to the larger amount of the former molecules found in plasma. It is concluded that carbohydrate is attached to the lipoprotein after it has left the ribosome and that the Golgi apparatus plays a major role in this process.