Mucin synthesis. III. UDP-GlcNAc:Galβ1-3(GlcNAcβ1-6)GalNAc-R (GlcNAc to Gal) β3-N-acetylglucosaminyltransferase, an enzyme in porcine gastric mucosa involved in the elongation of mucin-type oligosaccharides

Abstract

Pig gastric mucosa microsomes have been shown to catalyze the following reaction: UDP-GlcNAc + Galβ1-3(GlcNAcβ1-6)-GalNAc-α-R → GlcNAcβ1-3Galβ1-3(GlcNAcβ1-6)GalNAc-α-R + UDP, where R is o-nitrophenyl or benzyl. The enzyme catalyzing this reaction has been named UDP-GlcNAc:Galβ1-3(GlcNAcβ1-6)GalNAc-R (GlcNAc to Gal) β3-N-acetylglucosaminyltransferase. The β3-GlcNAc-transferase does not act on Galβ1-3GalNAc-α-o-nitrophenyl. The β3-GlcNAc-transferase requires Mn2+ and Triton X-100 for optimal activity. The Vmax for the microsomal enzyme is 8.7 nmol/mg protein per hour and the Km values are 1.6, 0.9, and 1.2 mM for UDP-GlcNAc and the α-o-nitrophenyl and α-benzyl derivatives of Galβ1-3(GlcNAcβ1-6)GalNAc, respectively. Pig gastric mucosa microsomes catalyze the transfer of GlcNAc to lactose to form GlcNAcβ1-3Galβ1-4Glc, but fail to transfer GlcNAc to lactosyl ceramide, Galβ1-4GlcNAc, or Galβ1-4GlcNAc-β-benzyl.