Biosynthesis of β1,4‐ and β1,β1‐galactopyranosyl xylopyranosides in the mammary gland of lactating cow

Abstract

Lactose is a principal carbohydrate in nearly all species of mammalian milk. In order to examine the acceptor substrate specificity of lactose synthase in vivo, d-xylose as an acceptor substrate was injected into the jugular vein of a Holstein cow during lactation, then a milk sample obtained by milking. b1,b1-galactopyranosyl xylopyranoside, a nonreducing disaccharide, was separated from the bovine milk sample after elimination of reducing sugars, identified by fast-atom bombardment (FAB)-MS and 1 H-NMR analysis. A mixture of b1,b1-and b1,4-galactopyranosyl xylopyranoside fractions was also obtained by thin layer chromatography from the milk sample and elucidated by electrospray ionization (ESI)-MS and 1 H NMR analysis. Comparison of the integrated intensity of the products shows that the b1,b1 and b1,4 isomers are present in a ratio of 1.0 : 1.4, suggesting that d-xylose, transported from capillary blood across the plasma membrane of the mammary gland, was recognized by lactose synthase in its normal and reverse orientation owing to high symmetry of its structure. While the b1,4-isomer is known as a fragment of the linkage region between the protein and the polysaccharide chain of proteoglycans, the b1,b1-isomer has not been identified in vivo. Here, we demonstrate that galactosylation of d-xylose transported from the capillary blood can occur by lactose synthase catalysis in the mammary gland while the usual galactosylation of d-glucose proceeds. In addition, these results suggest that the possibility of endogenous occurrence of the b,b-trehalose type disaccharide in the mammary gland of lactating mammals may not be ruled out.

A Comparative Review of the Cell Biology, Biochemistry, and Genetics of Lactose Synthesis

“…The final step of lactose synthesis occurs within the Golgi apparatus, where lactose synthase (LS) joins glucose and UDP-galactose by a β-1-4 glycosidic bond [50][51][52]. Importantly, LS is a unique enzyme complex comprised of β-1,4-galactosyltransferase-1 (B4GALT1) and the mammary-specific modifier protein α-lactalbumin (LALBA), and requires close association with the uridine nucleotide cycle on the trans-Golgi [53]. Once lactose is produced by LS, it is then packaged into vesicles from the trans-Golgi and transported to the apical membrane for exocytosis.…”

Handbook of Carbohydrate-Modifying Biocatalysts

“…Wiemann et al (1994) could show that recognition of the acceptor substrate in the normal orientation apparently prevails, thus giving Galb1-4Xyl (13):Galb1-1bXyl(14) in a ratio of 2:1. These experiments were repeated by Hara and Suyama (2000) on a larger scale employing a lactating Holstein cow, and here a ratio of 13:14 = 4:1 was observed ( Fig. 13.3).…”

Enzymes of the Carbohydrate Metabolism and Catabolism for Chemoenzymatic Syntheses of Complex Oligosaccharides

“…Wiemann et al (1994) could show that recognition of the acceptor substrate in the normal orientation apparently prevails, thus giving Galb1-4Xyl (13):Galb1-1bXyl(14) in a ratio of 2:1. These experiments were repeated by Hara and Suyama (2000) on a larger scale employing a lactating Holstein cow, and here a ratio of 13:14 = 4:1 was observed (Fig. 13.…”