The biosynthesis of oligosaccharide-lipids. Formation of an alpha-1,2-mannosyl-mannose linkage.

An enzyme present in rabbit liver microsomes has been found to catalyze mannosyltransfer from GDP-mannose to exogenously added oligosaccharide-lipid acceptor resulting in the formation of an alpha-1,2-mannosyl-mannose linkage. Several lines of evidence suggest that the product is a heptasaccharide containing a single 1,6-linked branched glycosyl unit with 65% of the newly added mannosyl units in a terminal nonreducing position. The enzyme has been solubilized with non-ionic detergents and was partially purified on DEAE-cellulose and hydroxylapatite. The partially purified enzyme no longer catalyzed the formation of dolichol-P-mannose indicating a direct transfer from GDP-mannose to oligosaccharide-lipid acceptor.