Endocrine Abstracts

Advanced glycation end products modify protein structure and function in type I diabetes. Glycoproteins may be potentially modified through other mechanisms, particularly changes to insulin-dependent oligosaccharide components. We have assessed the use of immobilised lectins, characterising the oligosaccharides of a serum glycoprotein, human chorionic gonadotrophin (hCG). We wish to test the hypothesis that glycoprotein glycosylation may be modified in type I diabetes.

Blood was taken from pregnant women with and without type I diabetes at 10-12 weeks gestation. Partially purified hCG preparations obtained by ion-exchange chromatography were chromatographed on immobilised lectins; Concanavalin A (Con A) - ; Lens culinaris agglutinin (LCA) - and wheat germ agglutination (WGA) - Sepharose. HCG bound completely to Con A and WGA, being eluted specifically with 500 mM mannose or 500 mM N-acetyl-glucosamine (GlcNAc) respectively. Most hCG did not bind to LCA, only 18-27 % being specifically retained, eluting with 500 mM mannose. No difference between samples from diabetic and non-diabetic women was observed.

Further hCG preparations were chromatographed on WGA and eluted with a 0-15 mM GlcNAc gradient. HCG immunoactivity was detected at approximately 8 and 12 mM GlcNAc in samples from both diabetic and non-diabetic women. Samples from diabetic women also had hCG immunoactivity eluting at approximately 4 mM GlcNAc, not seen in control samples.

The lectin-binding pattern of hCG is consistent with the expression of oligosaccharides identified by other methods; i.e. core mannose residues (Con A+ve) and N-linked sequences containing GlcNAc and/or NeuNAc residues (WGA+ve) and a proportion expressing core fucosylated, bi- or tri-antennate oligosaccharides (LCA +ve). Variation in GlcNAc and/or NeuNAc content is more marked in hCG from diabetic women.

Lectin affinity chromatography is a useful method for characterizing serum hCG oligosaccharide expression. The data indicate that this expression may be modified in women with type 1 diabetes.