Heterogeneity of rat goblet-cell mucin before and after reduction

Goblet-cell mucin of rat small intestine was purified from mucosal scrapings by using centrifugation, Sepharose 4B and Sepharose 2B chromatography. The mucin was applied in low concentrations (1 microgram/track) to slab gels containing 0.5% agarose/2% (w/v) polyacrylamide, and bands were detected after electrophoresis by silver stain or by fluorography of 3H-labelled mucin. Before reduction the mucin contained three distinct components: a polymeric species at the top of the gel and two large glycoproteins of higher mobility. After reduction, the polymer disappeared, the two glycoproteins remained unchanged, and two glycopeptide bands of higher mobility appeared. In addition, a non-glycosylated, heavily stained peptide of mol.wt. 118000 was detected. The individual mucin components were partially separated on Sepharose 2B, 0.2M-NaCl/1% sodium dodecyl sulphate being used as eluant. Individual amino acid and carbohydrate analyses suggested that the glycosylated components, despite their differences in size, had identical profiles. The 118000-mol.wt. peptide had a very different amino acid profile, with much less serine, threonine and proline. Glycine and aspartic and glutamic acids comprised 34% of the total amino acids. Thus the 'native' mucin is a heterogeneous structure containing at least two non-covalently associated glycoproteins plus polymeric material. The latter is stabilized by disulphide bonds and consists of several glycopeptides of different size as well as a 'link' peptide of mol.wt. 118000.