Gradient-polyacrylamide-gel electrophoresis of human milk serum separated three high-Mr glycoprotein bands. The properties of the components corresponding to the three bands (tentatively termed ‘Components C, B and A’ in their order of migration) were compared by staining with four monoclonal antibodies and lectins. Components B and C both reacted with the four antibodies, but Component A did not. Components B and C were stained with peanut (Arachis hypogaea) agglutinin (PNA) and wheat (Triticum)-germ agglutinin (WGA), Component A being stained with soya-bean (Glycine max) agglutinin as well as PNA and WGA. These results suggest that Components B and C were related molecules, whereas Component A was markedly different from them. The reactivities of Components B and C were the same as those of PAS-0, a high-Mr periodate/Schiff (PAS)-positive glycoprotein previously isolated from human milk fat-globule membrane (MFGM). Component C, whose electrophoretic mobility was the same as PAS-0, could have been a soluble form of PAS-0. A high-Mr glycoprotein having the same properties as Component A was also observed in MFGM. The amino acid composition of the isolated Component A was significantly different from that of Component C and PAS-0, high threonine and serine contents being characteristic of Component A. The carbohydrate content of Component A was 65-80%, and was much higher than that of Component C and PAS-0. Fucose, galactose, N-acetylglucosamine, N-acetylgalactosamine and sialic acid were each detected as constituent sugars of Component A. Component A represents, therefore, a new high-Mr glycoprotein species in human milk serum and MFGM. Since these glycoproteins were high-Mr mucin-like glycoproteins, the names ‘HM glycoprotein-A’ and ‘HM glycoprotein-C’ were proposed for Component A and Component C (PAS-O) respectively.

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