Identification of further elongation and branching of dimeric type 1 chain on lactosylceramides from colonic adenocarcinoma by tandem mass spectrometry sequencing analyses

Mammalian glycan chain elongation is mostly based on extending the type 2 chain, Galβ1-4GlcNAc, whereas the corresponding type 1 chain, Galβ1-3GlcNAc, is not normally extended. In a broader context of developing high sensitivity mass spectrometry methodologies for glycomic identification of Le^a versus Le^x and linear versus branched poly-N-acetyllactosamine (polyLacNAc), we have now shown that the dimeric type 1 glycan chain, as carried on the lactosylceramides of a human colonic adenocarcinoma cell line, Colo205, not only can be further extended linearly but can likewise be branched at C6 of 3-linked Gal in a manner similar to polyLacNAc. A combination of chemical and enzymatic derivatization coupled with advanced mass spectrometry analyses afforded unambiguous identification of a complex mixture of type 1 and 2 hybrids as well as those fucosylated variants founded exclusively on linear and branched trimeric type 1 chain. We further showed by in vitro enzymatic synthesis that extended type 1 and the hybrid chains can be branched by all three forms of the human I branching enzymes (IGnT) currently identified but with lower efficiency and stringency with respect to branching site preference. Importantly, it was found that a better substrate is one that carries a Gal site for branching that is extended at the non-reducing end by a type 2 and not a type 1 unit, whereas the IGnTs are less discriminative with respect to whether the targeted Gal site is itself β3- or β4-linked to GlcNAc at the reducing end.