UDP-GlcNAc concentration is an important factor in the biosynthesis of β1,6-branched oligosaccharides: Regulation based on the kinetic properties of N-acetylglucosaminyltransferase V

Abstract

Human β1,6-N-acetylglucosaminyltransferase V (GnT-V) was expressed by baculovirus-insect cell system, and the purified recombinant enzyme was kinetically characterized. The data obtained were used to establish the kinetic basis of the substrate specificity toward donor nucleotide sugars, and also revealed that Km values for the donors are much higher compared to those of other GlcNAc transferases, the kinetic properties of which have been reported. Because this exceptionally higher Km suggests that GnT-V is physiologically present at far from saturated conditions, it would appear that the production of β1,6-branched oligosaccharide, which is formed by GnT-V, could be regulated in vivo by the concentration of the donor, UDP-GlcNAc, as well as the expression levels of the enzyme. When B16 melanoma cells, which express high levels of GnT-V, were incubated with GlcNAc, the β1,6-branched oligosaccharide levels were increased, as judged by a lectin blot analysis, in conjunction with an increase in intracellular UDP-GlcNAc. These findings suggest that the level of UDP-GlcNAc can be a critical factor in the production of β1,6-branched oligosaccharides, for example, by tumor cells, which have been thought to be closely associated with tumor progression and metastasis.