Glycoproteins and glycolipids of oxyntic cell microsomes II. Glycopeptides and glycolipids

Abstract

Glycosylated compounds associated with the carbohydrate-rich tubular membrane system of the oxyntic cell were investigated. Two glycopeptide fractions, designated Peaks A and B, were isolated from pronase digests of bullfrog oxyntic cell microsomes. Molecular sieve chromatography and cellulose acetate electrophoresis revealed that, although somewhat heterogeneous, each peak was composed primarily of glycopeptides with similar molecular weights and net charge densities. Peak B glycopeptides had a mean molecular weight of about 6000 and contained 70% of the recovered carbohydrate in the following molar ratios: hexose, 1.00; N- acetylhexosamine , 0.71; fucose, 0.61; sialic acids, <0.03. Peak a glycopeptides were considerably larger (molecular weight approx. 100 000) and contained carbohydrates in molar ratios similar to those of Peak B. In both peaks galactose and N- acetylglucosamine , respectively, were the predominant hexose and amino sugar isomers. The glycolipid content of bullfrog oxyntic cell microsomes was assessed by qualitative and quantitative thin-layer chromatography. The most abundant glycolipids were monoglucosylceramides (0.098 mole/mole phospholipid) and monogalactosylceramides (0.046 mole/mole phospholipid). Small quantities of sulfatides and gangliosides were also present. A compilation of available data regarding the chemical composition of the microsomes revealed that these membranes resemble plasma membranes in having high molar ratios of cholesterol to phospholipid (approx. 1.0) and large quantities of carbohydrate (225 μg/mg protein). The possible significance of these compositional features in protecting the oxyntic cell is discussed.