HPLC analysis of discrete haptoglobin isoform N-linked oligosaccharides following 2D-PAGE isolation

Glycosylation is a common but variable modification that regulates glycoprotein structure and function. We combined small format 2D-PAGE with HPLC to analyse discrete human haptoglobin isoform N-glycans. Seven major and several minor haptoglobin isoforms were detected by 2D-PAGE. N-Glycans released from Coomassie-stained gel spots using PNGase were labeled at their reducing termini with 2-aminobenzamide. HPLC analysis of selected major isoform N-glycans indicated that sialic acid composition determined their separation by isoelectric focussing. N-Glycans from two doublets of quantitatively minor isoforms were also analysed. Although separation of each pair of doublets was influenced by sialylation, individual spots within each doublet contained identical N-glycans. Thus, heterogeneity in minor haptoglobin isoforms was due to modifications distinct from N-glycan structure. These studies describe a simple method for analysing low abundance protein N-glycans and provide details of discrete haptoglobin isoform N-glycan structures which will be useful in proteomic analysis of human plasma samples.