A 70000-molecular-weight protein isolated from purified pig gastric mucus glycoprotein by reduction of disulphide bridges and its implication in the polymeric structure

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Abstract
The glycoprotein of pig gastric mucus was isolated free of non covalently bound protein as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and equilibrium density-gradient centrifugation. After reduction with 0.2 M-mercaptoethanol, protein was released from the glycoprotein, which consisted of a major 70,000-MW component and a minor 60,000-MW component. The 70,000-MW protein fraction was separated from the reduced glycoprotein by density-gradient centrifugation in CsCl or by gel filtration. Analysis of the 70,000-MW protein fraction showed that, within the limits of the analysis, it was nonglycosylated and its amino acid analysis was quite different from that of the reduced glycoprotein, which is high in serine, threonine and proline. There was a ratio of one 70,000-MW protein/native glycoprotein molecule of 2 .times. 106 MW. Dissociation of the native glycoprotein into glycoprotein subunits (5 .times. 105 MW) by reduction or proteolysis results in the release or hydrolysis, respectively, of the 70,000-MW protein. A similar 70,000-MW protein is demonstrated in human gastric mucus glycoprotein. A structural role for the proteins in these mucus glycoproteins is proposed.