Polymeric structure of a high-molecular-weight glycoprotein from bovine cervical mucus

Macromolecular properties and polymeric structure of mucus glycoproteins

Mucus glycoproteins (mucins) were isolated from cervical and gastric mucus as well as from chronic bronchitic sputum. The mucus gel was solubilized by slow stirring in 6M-guanidinium chloride supplemented with low-Mr proteinase inhibitors. Subsequent removal of non-mucin proteins and DNA was achieved with isopycnic density-gradient centrifugation. The cervical and the respiratory mucins were of similar size (Mr about 10 X 10(6) and 18 X 10(6)), respectively), whereas the gastric mucins were considerably larger (Mr about 45 X 10(6)). 'Subunits' isolated after disulphide bond cleavage were the same size for the three mucins, as were glycopeptides obtained after subsequent trypsin digestion of the subunits. Physical data suggest that the respiratory and gastric mucins conform to the model for the polymeric structure proposed previously for cervical mucins. The macromolecules are described as linear flexible chains behaving, in dilute solution, as random coils. We propose that mucus glycoproteins are considerably larger than hitherto recognized and that mucins of various origins are very similar in their macromolecular properties and polymeric structure.

Viscoelastic properties of solutions of ovine submaxillary mucin

The linear viscoelastic and rheological properties of high molecular weight ovine submaxillary mucin (OSM) solution have been investigated in terms of the Newtonian steady-flow viscosity [eta(gamma)], the complex oscillatory viscosity [eta*(omega)], and the storage and loss shear moduli [G'(omega) and G"(omega)]. It was observed that tau(gamma), eta*(omega), and G'(omega) are always higher when OSM is dissolved in 0.1M NaCl than when at the same concentration in 6M GdnHCl. This is consistent with previous observations that submaxillary mucins self-associate in 0.1M NaCl to form large aggregates, which are disrupted in 6M GdnHCl. As the OSM concentration increases, the appearance of a plateau shear modulus indicates the formation of a gel network in both solvents. The results suggest gelation involves specific intermolecular interactions, perhaps due to hydrophobic forces between interdigitated oligosaccharide side chains. The viscoelastic behavior of OSM solution at high concentration is thus similar to that reported in the literature for porcine gastric mucin (PGM). However, the OSM gels are mechanically weaker, having moduli that are an order of magnitude lower than those for PGM gels of comparable concentration. The oligosaccharide side chains of OSM consist of only 1-2 sugar units compared to 10-15 for PGM, but it appears that this is sufficient to allow for intermolecular interaction and the formation of weak gels.

Subunit interactions and physical properties of bovine gallbladder mucin

In this study the interaction of gallbladder mucin subunits was examined by gel permeation chromatography, quasielastic laser light scattering, and viscometry. Purified mucin eluted primarily in the void volume of a Sepharose 2B-Cl column, indicating a molecular weight above 2 x 10(6). Disaggregation of the mucin polymer resulted in the elution of glycoprotein in the included volume of the Sepharose 2B-Cl column. Gallbladder mucin had a hydrodynamic radius of 630 A that was independent of mucin concentration below 2 mg/ml. At mucin concentrations above 2 mg/ml, a concentration-dependent increase in both hydrodynamic radius and apparent viscosity of gallbladder mucin was observed. Mucin demonstrated a strong pH dependence in hydrodynamic radius and viscosity with the maximum occurring at approximately pH 5.5. These findings suggest that noncovalent interactions participate in bovine gallbladder mucin subunit associations. Furthermore, changes that occur in the physicochemical environment of the gallbladder during periods of stasis may enhance the viscoelastic properties of mucin and promote the accumulation of biliary sludge in the gallbladder.

Dynamic light scattering studies of porcine submaxillary mucin fractions in dilute solution at intermediate scattering vectors

We report dynamic light scattering measurements over a wide range of scattering vectors for fractionated samples of porcine submaxillary mucin (PSM) glycoproteins in two different solvents: 0.1M NaCl, and 6M GdnHCl. The relaxation spectrum has been successfully resolved into a slow mode corresponding to pure translational diffusion and a fast mode containing information on the relaxation times for intramolecular motion. Analysis of the slow mode permits a light scattering evaluation of the polydispersity of these high molecular weight mucin glycoprotein fractions. Determination of the longest intramolecular relaxation times tau 1 shows that these are much longer for the PSM fractions in 0.1M NaCl compared to 6M GdnHCl. These data are consistent with earlier studies showing that the chain conformation is the same in both solvents, but that in 0.1M NaCl, the PSM glycoprotein undergoes a self-association process that is end-to-end in nature. Since the tau 1 value is intimately related to the viscoelastic behavior of PSM solutions and gels, it is interesting to speculate that the end-to-end association process may be physiologically important.

The complexity of mucins

Mucins represent the main components of gel-like secretions, or mucus, secreted by mucosae or some exocrine glands. These high-molecular-weight glycoproteins are characterized by the large number of carbohydrate chains O-glycosidically linked to the peptide. The determination of mucin molecular weight and conformation has been controversial for several reasons: 1) the methods used to solubilize mucus and to purify mucins are different and 2) the molecules have a strong tendency to aggregate or to bind to other molecules (peptides or lipids). Recently, electron microscopy has shown the filamentous shape of most mucins and their polydisperse character which, in some secretions, might correspond to a polymorphism of the peptide part of these molecules. The recent development of high pressure liquid chromatography and high-resolution proton NMR spectroscopy has allowed major progress in the structural study of mucin carbohydrate chains. These chains may have from 1 to about 20 sugars and bear different antigenic determinants, such as A, B, H, I, i, X, Y or Cad antigens. In some mucins, such as human respiratory mucins, the carbohydrate chain diversity is remarkable, which raises many questions. Mucins are molecules located at the interface between mucosae and the external environment. The carbohydrate chain diversity might allow many interactions between mucins and microorganisms and play a major role in the colonization or the defense of mucosae.

Immunologically related multimeric forms of 30-40 kDa peptides associated with lung surfactant in various mammalian species

A comparative study of lung surfactant associated proteins was undertaken to determine which mammalian species would best serve as models for investigating alterations of the human lung surfactant system. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of purified surfactants in the presence of dithiothreitol revealed that surfactant invariably contains at least one peptide with molecular weight of 30 000-40 000. In the absence of disulfide reducing agents, the above peptides were in the form of high-molecular-weight proteins (greater than 400 kDa) in primates and cat, whereas in dog, rat and rabbit, the protein was a 72 kDa dimer. The 30-40 kDa peptide subunits were isolated from human, rat and dog surfactants and found to contain four or five residues of hydroxyproline. Antisera to either the human 34 kDa peptide or high-molecular-weight proteins reacted with the high-molecular-weight bands, the 34 kDa subunit and at least six intermediate disulfide-linked forms separated from purified human surfactant by electrophoresis under nonreducing conditions. Following electrophoresis in the presence of dithiothreitol, both antisera detected the 34 kDa peptide as well as other peptides ranging in molecular weight from 23 000 to 160 000. The isolated 34 kDa peptide readily reaggregated into disulfide-linked forms including 68 and 100 kDa complexes which were not reduced by 40 mM dithiothreitol. We conclude that the 34 kDa surfactant-associated peptide forms a complex system of monomeric and multimeric proteins, which varies among the species and could conceivably vary in distribution during lung development or disease.