The lipid intermediates arising during glycoprotein biosynthesis in liver microsomes

Abstract

Incubation of liver microsomes with GDP[ 14C]mannose leads to the formation of lipid-linked derivatives of [ 14C]mannose, a dolichol phosphate monosaccharide and dolichol pyrophosphate oligosaccharides. Standard procedures for separating these two types of compounds from each other were found to be deficient in that fractions thought to contain only dolichol pyrophosphate oligosaccharides are contaminated with dolichol phosphate mannose. This paper presents a column chromatographic procedure which conveniently separates the products of an 8 min labeling experiment into two components; dolichol phosphate [ 14C]mannose and a [ 14C]-mannose containing oligosaccharide which is also lipid bound. When this oligosaccharide is released from the lipid by hydrolysis and chromatographed on Sephadex G-50 or G-15 it gives a single peak with an indicated molecular weight of 1100. However, when this released oligosaccharide is chromatographed on concanavalin A Sepharose it is resolved into two peaks suggesting that there may be 2 oligosaccharide of approximately the same size but different structures. After brief periods of labeling with GDP[ 14C]mannose (5 s) an additional oligosaccharide of 3 to 4 sugar residues can be found in the dolichol pyrophosphate oligosaccharides fraction. Incubation of liver microsomes with UDP[ 14C]glucose or UDP[ 14C]galactose produces oligosaccharide components containing 7–8 sugar residues. Labeling of microsomes with UDP[ 14C]acetylglucosamine gives rise to three different components, including a lipid bound oligosaccharide containing 3–5 sugar residues.