Surface carbohydrates of the milk fat globule membrane.

Abstract

Treatment of bovine milk fat globules with pronase resulted in the release of a soluble sialoglycopeptide fraction which was chromatographed on Sephadex G-50 as a single peak. Fractionation of this fraction by ion-exchange chromatography allowed the separation of two sialoglycopeptides; the sialic acid-poor (SP) and the sialic- acid-rich (SR) glycopeptides. Both fractions were found to be homogeneous by SDS-PAGE and by gel filtration on Sephadex G-75. Antisera were raised against both fractions and their contribution to the antigenicity of the bovine milk fat globule membrane (MFGM) was briefly studied. The carbohydrate composition of both glycopeptide fractions was determined by gas liquid chromatography. Preliminary amino acid analysis showed that the major amino acids in the SR fraction were serine and threonine whereas the dominant amino acid in the SP fraction was found to be asparagine. The accessibility of specific carbohydrate residues on the surfaces of intact and neuraminidase treated bovine and human milk fat globules was compared by means of lectin-based agglutination assay and by the use of the Fluorescence-Activated Cell Sorter. The presence of galactose, mannose and/or glucose and N-acetyl-glucosamine, but not fucose was detectable on both bovine and human globules. The major differences between bovine and human milk fat globules were the presence on human globules of high amounts of Arachis hypogaea receptor (Thomsen-Freideriech antigen), none of which was apparently masked by sialic acid and the presence of unsubstituted N-acetylgalactosamine on bovine but not on human milk fat globules. Pooled bovine, but not human, milk fat globules were found to be agglutinated by relatively high dilutions of the K99 adhesin (antigen) isolated from E. coli B41. Parallel studies of K99-induced agglutination of bovine globules and sheep erythrocytes were carried out by using bovine MFGM-derived sialoglycopeptides and a range of monosaccharides as specific inhibitors. The results suggest that N-acetylgalactosamine-containing carbohydrate complexes are the most likely candidates for the K99 receptor on the surfaces of bovine globules and sheep erythrocytes.

Date of Award	1981
Original language	English
Awarding Institution	University of Bath