Histochemical studies of epithelial cell glycoproteins in normal rat colon

Abstract

Two general classes of glycoproteins have been identified in the colonic epithelial cells of the Sprague Dawley rat. Glycoproteins belonging to the first of these classes contain sialic acids both with and without side chaino-acyl substituents, abundanto-sulphate ester and ‘neutral sugars’ (hexose, 6-deoxyhexose or N-acetyl hexosamine residues) with oxidisablevicinal diols and are located in the goblet cells of the descending colon and in goblet cells populating the upper halves of the crypts of the ascending colon. In the descending colon, the sulphosialoglycoproteins in the goblet cells in the base of the crypts contain sialic acids without side chaino-acyl substituents. It appears that as these cells migrate up the crypts, there iso-acylation of the side chains of the sialic acids of the glycoproteins and an increase in the quantity of ‘neutral sugars’ without a corresponding increase in sialic acid. Glycoproteins with similar properties to those of the goblet cells of the upper halves of the crypts of the descending colon, but containing less sulphate, are found in the goblet cells of the upper half of the crypts of the ascending colon. The second general class of glycoproteins contain sialic acids all, or almost all of which, are substituted at position C8 and only relatively small quantities of sulphate. They are located in the mucous cells of the descending colon, the deep crypt secretory cells of the ascending colon and the columnar absorptive cell brush border. Glycoproteins of all three types contain neutral sugars but the periodate oxidation of those in the mucous cells is apparently prevented by the presence of ano-acyl ester substituent(s) located on theirvicinal diol(s). Such sugars have not previously been identified in mammalian epithelial glycoproteins; their presence, together with that of 8-o-acyl sialic acids, would account for the observed lack of PAS reactivity of the mucous cell glycoproteins. We propose that (i) there are two general biosynthetic sequences for colonic epithelial cell glycoproteins and (ii) the primary function of 8-o-acyl sialoglycoproteins is protective while that of the sulphated species is lubrication.