Takeya A, Hosomi O, Shimoda N, Yazawa S
Department of Legal Medicine, Gunma University School of Medicine.
J Biochem. 1992 Sep;112(3):389-95. doi: 10.1093/oxfordjournals.jbchem.a123910.
Human blood group O plasma was found to contain an N-acetylgalactosaminyltransferase which catalyzes the transfer of N-acetylgalactosamine from UDP-GalNAc to Gal beta 1-->4Glc, Gal beta 1-->4GlcNAc, asialo-alpha 1-acid glycoprotein, and Gal beta 1-->4GlcNAc beta 1-->3Gal beta 1-->4Glc-ceramide, but not to Gal beta 1-->3GlcNAc. The enzyme required Mn2+ for its activity and showed a pH optimum at 7.0. The reaction products were readily hydrolyzed by beta-N-acetylhexosaminidase and released N-acetylgalactosamine. Apparent Km values for UDP-GalNAc, Mn2+, lactose, N-acetyllactosamine, and terminal N-acetyllactosaminyl residues of asialo-alpha 1-acid glycoprotein were 0.64, 0.28, 69, 20, and 1.5 mM, 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 NeuAc alpha 2-->3Gal beta-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 beta-galactosyl residue. Although the GalNAc beta 1-->3Gal structure is known to have the blood group P antigen activity, human plasma showed no detectable activity of Gal alpha 1-->4Gal beta-1,3-N-acetylgalactosaminyltransferase, which is involved in the synthesis of the major P antigen-active glycolipid, GalNAc beta 1-->3Gal alpha 1-->4Gal beta 1-->4Glc-ceramide. Hence, the GalNAc beta 1-->3Gal beta 1-->4GlcNAc/Glc structure is synthesized by the novel Gal beta 1-->4GlcNAc/Glc beta-1,3-N-acetylgalactosaminyltransferase.
发现人O型血血浆中含有一种N - 乙酰半乳糖胺基转移酶，该酶催化N - 乙酰半乳糖胺从UDP - GalNAc转移至Galβ1→4Glc、Galβ1→4GlcNAc、去唾液酸α1 - 酸性糖蛋白以及Galβ1→4GlcNAcβ1→3Galβ1→4Glc - 神经酰胺，但不转移至Galβ1→3GlcNAc。该酶活性需要Mn2 +，最适pH为7.0。反应产物易被β - N - 乙酰己糖胺酶水解并释放出N - 乙酰半乳糖胺。UDP - GalNAc、Mn2 +、乳糖、N - 乙酰乳糖胺以及去唾液酸α1 - 酸性糖蛋白的末端N - 乙酰乳糖胺基残基的表观Km值分别为0.64、0.28、69、20和1.5 mM。对受体底物竞争的研究表明，上述所有受体底物竞争同一种酶，而该酶可与同样存在于人体血浆中的NeuAcα2→3Galβ - 1,4 - N - 乙酰半乳糖胺基转移酶区分开来。对N - 乙酰半乳糖胺转移至乳糖形成的产物进行甲基化研究表明，N - 乙酰半乳糖胺已转移至β - 半乳糖基残基的碳 - 3位。尽管已知GalNAcβ1→3Gal结构具有血型P抗原活性，但人体血浆中未检测到参与主要P抗原活性糖脂GalNAcβ1→3Galα1→4Galβ1→4Glc - 神经酰胺合成的Galα1→4Galβ - 1,3 - N - 乙酰半乳糖胺基转移酶的活性。因此，GalNAcβ1→3Galβ1→4GlcNAc/Glc结构是由新型Galβ1→4GlcNAc/Glcβ - 1,3 - N - 乙酰半乳糖胺基转移酶合成的。
