Peanut lectin and anti-Ii antibodies reveal structural differences among human gastrointestinal glycoproteins

Human gastrointestinal glycoproteins (mucins), isolated by pepsin digestion from foetal stomachs and meconia, and from paired tumour and non-neoplastic mucosal samples of patients with gastric and colorectal carcinomas, were tested for precipitating reactions with peanut lectin (PNL) and four anti-carbohydrate antibodies (two anti-I, Ma and Low, and two anti-i, Den and Galli). There was remarkable correlation between reactivities with PNL and anti-I (Ma): both reagents reacted with non-neoplastic gastric glycoproteins of "non-secretors", but not with those of "secretors", and also with the majority of gastric tumour and meconium extracts regardless of secretor status. Colorectal tissue extracts (with the exception of one tumour extract) reacted with neither reagent. The various precipitating activities, and results of mild acid hydrolysis and affinity chromatography experiments, enable certain inferences to be made regarding the oligosaccharide moieties of gastrointestinal glycoproteins: (a) expression of PNL and anti-I (Ma) determinants in gastric glycoproteins is dependent on secretor status; (b) extracts reacting with PNL and anti-I (Ma) are mixtures of macromolecules: minor populations react with both reagents, or with PNL only; the major population lacks both determinants, or they are masked by other substitutions; (c) determinants reactive with anti-Ii sera other than anti-I (Ma) are less frequently expressed; and (d) colonic glycoproteins in their lack of PNL and Ii determinants. This suggests that there are structural differences in the oligosaccharide backbones of the two types of glycoprotein.