Intestinal mucins from normal subjects and patients with cystic fibrosis. Variable contents of the disulphide-bound 118 kDa glycoprotein and different reactivities with an anti-(118 kDa glycoprotein) antibody

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Abstract
1. A specific antibody was developed against the disulphide-bound 118 kDa glycoprotein of human intestinal mucin and used to establish an e.l.i.s.a. Fourteen purified mucins [eight normal (N) and six cystic fibrosis (CF)] had the same affinity for the antibody in the e.l.i.s.a., but their relative immunoreactivities varied widely (approx. 100,000-fold). In general, CF mucins were more antigenic than N mucins. 2. Variations (approx. 10-fold) were detected in the 118 kDa glycoprotein content of both N and CF mucins (assessed from Coomassie Blue-stained polyacrylamide gels), but these did not appear to be responsible for the differences in mucin immunoreactivity. 3. Variations (approx. 6-fold) were also observed in the size of the 118 kDa peak produced by N and CF mucins on Western blots. These were mostly due to differences in the 118 kDa glycoprotein content of mucins, although a small proportion resulted from changes in the number of antigenic determinants within individual 118 kDa glycoproteins. 4. After concanavalin A affinity chromatography of four reduced mucins (two N and two CF), purified 118 kDa glycoprotein was recovered in the bound fractions from the column, specifically eluted by methyl alpha-mannoside. 5. The amounts of 118 kDa glycoprotein isolated from the four mucins varied as predicted from the size of their 118 kDa bands on Coomassie Blue-stained gels. 6. Three 118 kDa glycoproteins (one N and two CF) showed almost identical reactivity in the e.l.i.s.a.; the fourth had fewer antigenic determinants. 7. Since differences in 118 kDa glycoprotein content and in the number of antigenic determinants within the 118 kDa glycoprotein did not account for variations in the reactivity of native mucins in the e.l.i.s.a., it appeared that accessibility of the 118 kDa glycoprotein to antibody binding may be critical in determining mucin immunoreactivity. This suggests that the three-dimensional conformation of CF mucins may differ from that of N mucins, leading to increased antigenicity.