Disulphide bonds and the 118,000 Mr glycoprotein of human intestinal mucin

Purified intestinal mucins from eight normal subjects (N) and six patients with cystic fibrosis (CF) were compared for: (1) their content of free thiol groups (-SH) and disulphide bonds (S-S), (2) their immunoreactivity in a specific enzyme-linked immunoassay (ELISA) for the disulphide-bound 118,000 Mr glycoprotein and (3) their content of 118 kDa glycoprotein. Results showed considerable variability in the S-S/-SH content of individual mucin preparations but no differences were observed between N and CF samples. In the ELISA, all mucins were found to have identical antigenic determinants with the same affinity for the anti-118 kDa glycoprotein antibody but marked differences (ranging from 0.2 to 120,000 ng mucin) were observed in their relative immunoreactivities, indicating variable numbers of antigenic determinants. In general, CF mucins appeared to react more strongly in the immunoassay than N mucins. By SDS/polyacrylamide-gel electrophoresis under reducing conditions and densitometric analysis of Coomassie blue-stained gels, differences (up to six-fold) were also detected in the amount of 118 kDa glycoprotein in individual mucin preparations. However, this did not appear to be the cause of the variations in mucin immunoreactivity because not all strongly antigenic mucins contained large amounts of 118 kDa glycoprotein. Thus, accessibility of the 118 kDa glycoprotein to the antibody or the number of functional antigenic determinants within the 118 kDa glycoprotein may vary among different mucin preparations. The greater antigenicity of CF mucins may therefore reflect changes in their macromolecular conformation or in their 118 kDa glycoprotein.(ABSTRACT TRUNCATED AT 250 WORDS)