Biosynthesis of the blood group P antigen-like GalNAc beta 1-->3Gal beta 1-->4GlcNAc/Glc structure: kinetic evidence for the responsibility of N-acetylglucosaminyl-transferase.

Previously we reported the occurrence of UDP-GalNAc:Gal beta 1-->4GlcNAc/Glc beta-1,3-N-acetylgalactosaminyltransferase activity in human plasma. Here, the donor substrate specificity of the enzyme partially purified from blood group O plasma was investigated by means of competition experiments with analogs of donor. The enzyme activity was found to be inhibited most strongly by UDP-GlcNAc among the nucleotide sugars tested, and UDP was the best inhibitor among test nucleotides. UDP-GlcNAc was much more inhibitory than UDP, whereas UDP-GalNAc was much less inhibitory than UDP. These results show that the donor-binding site of enzyme has a high affinity for UDP-GlcNAc, but not UDP-GalNAc, suggesting that the enzyme essentially functions in the transfer of N-acetylglucosamine. Indeed, UDP-GlcNAc:Gal beta 1-->4GlcNAc/Glc beta-1,3-N-acetylglucosaminyltransferase activity is known to occur in human plasma. The ratio of N-acetylgalactosamine to N-acetylglucosamine transferred to lactose with O or B plasma was 1:20 when assayed separately at 1 microM of donor, but increased to 1.2:1 at 5 mM. The nearly identical ratio was obtained with the partially purified enzyme preparation.