An investigation was undertaken to discover whether mucin purified from secretions in the lumen of rat small intestine differed in structure or composition from intracellular mucin purified from rat intestinal tissue. To do this, ligated loops were constructed in situ from previously washed intestinal segments and mucin purified separately from tissue homogenates or loop fluid. Secreted mucin (SM) differed from intracellular mucin (IM) by having a higher proportion of ‘minor’ mucin amino acids (aspartic acid, glutamic acid, glycine and alanine) and a lower proportion of ‘major’ amino acids (serine, proline and threonine). SM also contained less N-acetylgalactosamine and a small, but measureable, amount of mannose. Gel electrophoresis showed that SM penetrated the gel more readily and, unlike IM, gave a rather prominent, but diffuse, band having a midpoint position of Mr 200,000. After reduction both IM and SM gave rise to the putative ‘link’ component of Mr 118,000 and the 200,000-Mr band of SM disappeared. SM was included to a greater extent than IM on Sepharose CL-2B chromatography, suggesting a smaller size. With the use of CsCl-density-gradient ultracentrifugation of SM, a lighter species [buoyant density (rho) = 1.38 g/ml] enriched in the 200,000-Mr component, was separated from a heavier, more glycosylated, species (rho = 1.50 g/ml). Purified 200,000-Mr component had a composition identical with that of the 118,000-Mr ‘link’ component of IM, reacted in Western blots with an antibody specific for the 118,000-Mr ‘link’ component, and after reduction gave rise to a 118,000-Mr component on gel electrophoresis. Thus secreted mucin contains a 200,000-Mr component which appears to represent a disulphide-linked dimer of the previously described 118,000-Mr ‘link’ component of intracellular mucin.

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