Glycoproteins and glycolipids of oxyntic cell microsomes I. Glycoproteins: Carbohydrate composition, analytical and preparative fractionation

Abstract

Isotopically-labeled sugars were incorporated into glycoproteins of isolated bullfrog gastric mucosa. The majority of the label was found in gastric microsomal fractions which were shown to contain membranes derived from the oxyntic cell tubular membrane system and were not significantly contaminated with mucus. The tubular membranes contained exceptionally large quantities of carbohydrate (approx. 260 μg/mg protein). Most of the sugar (73%) was associated with protein in the following molar ratios: hexose, 1.0: fucose, 0.42; hexosamine, 0.62; sialic acid, <0.02. The remaining sugar, predominantly hexose, could be extracted into lipid solvents and was presumably glycolipid. Gastric microsomes were dissolves in sodium dodecyl sulfate and subjected to acrylamide gel electrophoresis and Sephadex G-200 fractionation. The latter preparative procedure yielded several molecular weight classes, each of which contained different sets of proteins and/or glycoproteins; however, the molar ratios of the sugars found in the two carbohydrate containing classes were quite similar. Significant quantities of carbohydrate were also found in gastric microsomal fractions from other species, e.g. pig and rabbit. Furthermore, characteristic proteins and glycoproteins were not present in tadpole gastric microsomes until the later stages of metamorphosis when HCl secretory capability had been established. The above findings suggest that glycoproteins may play an important role in oxyntic cell functions; the possibility of a membrane protective role is discussed.