Mucin synthesis

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Abstract
A UDP-GlcNAc:R1-β1-3Gal(NAc)-R2 [GlcNAc to Gal(NAc)] β6-N-acetylglucosaminyltransferase activity from pig gastric mucosa microsomes catalyzes the formation of GlcNAcβ1-3(GlcNAcβ1-6)Gal-R from GlcNAcβ1-3Gal-R where -R is -β1-3GalNAc-α-benzyl or -β1-3(GlcNAcβ1-6)GalNAc-α-benzyl. This enzyme is therefore involved in the synthesis of the I antigenic determinant in mucin-type oligosaccharides. The enzyme also converts Galβ1-3Galβ1-4Glc to Galβ1-3(GlcNAcβ1-6)Galβ1-4Glc. The enzyme was stimulated by Triton X-100 at concentrations between 0 and 0.2% and was inhibited by Triton X-100 at 0.5%. There is no requirement for Mn2+ and the enzyme activity is reduced to 65% in the presence of 10 mM EDTA. Enzyme products were purified and identified by proton NMR, methylation analysis and β-galactosidase digestion. Competition studies suggest that this pig gastric mucosal β6-GlcNAc-transferase activity is due to the same enzyme that converts Galβ1-3GalNAc-R to mucin core 2, Galβ1-3(GlcNAcβ1-6)GalNAc-R, and GlcNAcβ1-3GalNAc-R to mucin core 4, GlcNAcβ1-3(GlcNAcβ1-6)GalNAc-R. Substrate specificity studies indicate that the enzyme attaches GlcNAc to either Gal or GalNAc in β(1-6) linkage, provided these residues are substituted in β(1-3) linkage by either GlcNAc or Gal. The insertion of a GlcNAcβ1-3 residue into Galβ1-3GalNAc-R to form GlcNAcβ1-3Galβ1-3GalNAc-R prevents insertion of GlcNAc into GalNAc. These studies establish several novel pathways in mucin-type oligosaccharide biosynthesis.

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