Biosynthesis of the Blood Group P Antigen-Like GalNAc.BETA.1.RAR.3Gal.BETA.1.RAR.4GlcNAc/Glc Structure: A Novel N-Acetylgalactosaminyltransferase in Human Blood Plasma.

Human blood group O plasma was found to contain an N-acetylgalactosaminyltransferase which catalyzes the transfer of N-acetylgalactosamine from UDP-GalNAc to Galβ1→4Glc, Galβ1→4GlcNAc, asialo-α₁, -acid glycoprotein, and Galβ1→4GlcNAcβ1→3Galβ1→4Glc-ceramide, but not to Galβ1→3GlcNAc. The enzyme required Mn²⁺ for its activity and showed a pH optimum at 7.0. The reaction products were readily hydrolyzed by β-N-acetylhexosaminidase and released N-acetylgalactosamine. Apparent K_m values for UDP-GalNAc, Mn²⁺, lactose, N-acetyllactosamine, and terminal N-acetyllactosaminyl residues of asialo-α₁-acid glycoprotein were 0.64, 0.28, 69, 20, and 1.5mM, respectively. Studies on acceptor substrate competition indicated that all the acceptor substrates mentioned above compete for one enzyme, whereas the enzyme can be distinguished from an NeuAcα2→3Galβ-1, 4-N-acetylgalactosaminyltransferase, which also occurs in human plasma. The methylation study of the product formed by the transfer of N-acetylgalactosamine to lactose revealed that N-acetylgalactosamine had been transferred to the carbon-3 position of the β-galactosyl residue. Although the GalNAcβ1→3Gal structure is known to have the blood group P antigen activity, human plasma showed no detectable activity of Galα1→4Gal β-1, 3-N-acetylgalactosaminyltransferase, which is involved in the synthesis of the major P antigen-active glycolipid, GalNAcβ1→3Galα1→4Galβ1→4Glc-ceramide. Hence, the GalNAcβ1→3Galβ1→4GlcNAc/Glc structure is synthesized by the novel Galβ1→4GlcNAc/Glcβ-1, 3-N-acetylgalactosaminyltransferase.