BIOSYNTHESIS OF THE BLOOD GROUP P ANTIGEN-LIKE GalNAcβ1→3Galβ1→4GlcNAc/Glc STRUCTURE: KINETIC EVIDENCE FOR THE RESPONSIBILITY OF N-ACETYLGLUCOSAMINYLTRANSFERASE

Abstract

Previously we reported the occurrence of UDP-GalNAc: Galβ1→4GlcNAc/Glc β-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: Galβ1→4GlcNAc/Glc β-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 μM of donor, but increased to 1.2: 1 at 5 mM. The nearly identical ratio was obtained with the partially purified enzyme preparation.