The mannan-binding protein of human serum

Turner, Neil Richard (1983) The mannan-binding protein of human serum. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A previously unreported protein has been isolated from normal human serum and citrated plasma and has been designated mannan-binding protein (MBP) because of its Ca +-dependent binding to yeast mannans. The apparent molecular weight of MBP, estimated by SDS-PAGE, was about 30 x 103. Under non-dissociating conditions in aqueous solution MBP was highly aggregated. MBP is an acidic protein behaving similarly to serum album in an ion exchange chromatography using DE52.This protein appears to be the human equivalent of a mannan-binding protein found in rabbit serum, although the latter is present at much higher levels (400 pg per 100 ml cf 25 pg per 100 ml for human MBP). The published method for the isolation of MBP from rabbit serum was unsuitable for the human protein because of contamination of the product with antimannan antibodies and serum amyloid P-component (SAP). This contamination was partially overcome by the development of an isolation procedure based on the binding of MBP to the glucosyl residues of Sephadex G200. However, SAP was difficult to exclude completely as it was bound by MBP itself. During the project SAP was studied in some detail to determine its relationship to MBP. Using an improved procedure SAP was isolated in a biologically active but unusual, pentameric form. The pI of human SAP was 5.7 - 5.8. Previously unreported binding specificities for phosphate esters and carbohydrates such as L-fucose were identified. The recognition of phosphate esters such as phosphoryl choline emphasises the homology between SAP and other pentraxins. The binding of rabbit MBP to glycoproteins was specific for N-acetyl-Dmannosamine, N-acetyl-D-glucosamine and D-mannose residues. Binding to horseradish peroxidase, a mannose-rich glycoprotein, was particularly strong and was inhibited by serum from patients with suspected systemic Candidiasis although not by normal serum. This property could be valuable diagnostically. Previously, no suggestions had been made regarding the biological role of MBP in serum. However, MBP from both human and rabbit serum agglutinated yeast cells demonstrating that MBP is a lectin. Radio-labelled rabbit MBP bound specifically to yeasts and certain bacteria, even in the presence of normal serum. This suggests that MBP could have a role in defence against infection.

This record has no associated files available for download.