The present study reveals that partial proteolytic degradation of rat Muc 2 mucin can occur rapidly even in the presence of a battery of proteinase inhibitors. During the initial steps of purification from homogenates of intestinal scrapings, degradation was rapid, causing release of the entire 118 kDa C-terminal glycopeptide and, as shown by N-terminal sequencing, a large (200 kDa) N-terminal glycopeptide fragment. Degradation could be prevented by adding 6 M guanidinium chloride provided that its presence was maintained throughout every step of purification. Even after purification, however, the mucin was still vulnerable to partial proteolysis unless it was stored in guanidinium chloride at -20 °C. These findings imply that a potent proteinase contaminant remains tightly bound to the mucin through every step of purification, or else that the mucin has autocatalytic properties. Because the C- and N-terminal regions of secretory mucins are required for their assembly into linear mucin polymers that form functional gels, our findings emphasize that extreme care is required to purify structurally intact mucin molecules. They also imply that the specific degradation steps described here are likely to occur rapidly after mucins are secreted into the intestinal lumen and come into contact with the products of sloughed cells.

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