Mucin synthesis. I. Detection in canine submaxillary glands of an N-acetylglucosaminyltransferase which acts on mucin substrates.

Canine submaxillary gland microsomes have been shown to catalyze the following reaction: UDP-GlcNAc + Gal beta 1-3GalNAc-X leads to Gal beta 1-3(GlcNAc)GalNAc-X + UDP where X is porcine or ovine submaxillary mucin polypeptide or a low molecular weight substituent. This activity was shown by mixed substrate experiments to be different from two other previously described glycoprotein N-acetylglucosaminyltransferases acting on N-glycosyl oligosaccharides and is the first N-acetylglucosaminyltransferase shown to act on mucin substrates. The transferase-catalyzed reaction proceeds at 70 to 80% of the optimum rate in the absence of added divalent cation or in the presence of 10 mM EDTA. The enzyme appeared to be unstable at 37 degrees C, but the reaction rate remained constant for at least 2 h at 25 degrees C. The enzyme showed a pH optimum of 7.0 and was stimulated 4-fold by 0.125% Triton X-100. Methylation analysis of product formed either with mucin acceptor or Gal beta 1-3GalNAc-alpha-O-benzyl indicated GlcNAc transfer to either carbon 4 or carbon 6 of the GalNAc residue of the acceptors.