The lactose analog GalNAcβ1→4Glc is present in bovine colostrum -: Enzymatic basis for its occurrence

We have isolated from bovine colostrum the lactose analog GalNAc beta 1-->4Glc. The enzymatic basis for its occurrence was studied by assaying the activities of GlcNAc beta-R beta 4-N-acetylgalactosaminyltransferase (beta 4-GalNAcT) and GlcNAc beta-R beta 4-galactosyltransferase (beta 4-GalT) in primary milk and several lactating bovine mammary gland fractions. As the beta 4-GalNAcT, which appears to be tightly membrane bound, is induced by the milk protein alpha-lactalbumin (alpha-LA) to act on Glc, it is concluded that beta 4-GalNAcT is responsible for the synthesis of GalNAc beta 1-->4Glc in the gland. The comparatively low level (15-20 mg/l) at which this disaccharide is produced may be due to the relatively poor interaction of beta 4-GalNAcT with alpha-LA as well as to the fact that alpha-LA does not inhibit the action of the enzyme on N-acetylglucosaminides. (C) 1999 Federation of European Biochemical Societies.