Glycosylation of β-Lipoprotein and the Possible Role of Choline Derivatives

(1) The incorporation of [1-14C]glucosamine and UDP-N-acetyl-[1-14C]glucosamine into β-lipoprotein was studied with rat liver slices and subcellular particles, respectively. β-Lipoprotein released into the medium was isolated by a modified heparin precipitation technique. The lecithin–protein ratios of heparin precipitable β-lipoprotein from the medium of liver microsomes and of serum were found to be very similar. With microsomes a substantial incorporation was obtained which reached a maximum in 12 min and 27 °C. The enzymatic attachment of the amino sugar to the apoprotein was further indicated when the incorporation was correlated with the quantity of microsomal protein present.(2) As shown previously with other protein acceptors, the addition of choline derivatives caused a considerable stimulation in incorporation of the label for slices, microsomes, and purified rough microsomes. The effect of CDP-choline on glycosylation was not as great when a Golgi-rich membrane fraction was used, suggesting that the major stimulation occurred at the sites of lipoprotein synthesis rather than secretion.(3) The release of prelabelled β-lipoprotein from liver slices was not enhanced by the addition of phosphorylcholine or excess glucosamine to the medium, suggesting that increased carbohydrate coupling does not stimulate secretion.