Identification of two major populations of mucins in respiratory secretions

Two populations of reduced subunits were present in the mucins purified from pooled normal secretions and asthmatic and chronic bronchitic sputa; their relative level differed between samples. To investigate the nature of this heterogeneity, an asthmatic respiratory mucin preparation from a single individual was reduced and alkylated with 14C-iodoacetamide. This preparation was analyzed by gel filtration, agarose gel electrophoresis, immunoblotting, rate-zonal- and density-gradient centrifugation, and HPLC ion-exchange- and reverse-phase chromatography. Two populations (A and B) of reduced mucin subunits and a high-M(r)protein-rich fraction were identified. Species A has the higher molecular mass, is slowest migrating on agarose electrophoresis, has longer oligosaccharide chains, and expresses the carbohydrate structure sialyl-Le(x). Species B has a lower molecular mass, migrates faster in agarose electrophoresis Species B has a lower molecular mass, migrates faster in agarose electrophoresis, has shorter chains, and does not express sialyl-Le(x). The two subunits have similar but not identical amino acid compositions and 14C-tryptic peptide maps indicating they have different protein cores. The anti-sialyl-Le(x) antibody selectively precipitated subunit A not only from the reduced but also from the nonreduced mucin preparation, demonstrating that subunits A and B are present in different intact mucins.