Ferrero-García M A, Trombetta S E, Sánchez D O, Reglero A, Frasch A C, Parodi A J
Departamento de Bioquímica y Biología Molecular, Facultad de Veterinaria, Universidad de León, Spain.
Eur J Biochem. 1993 Apr 15;213(2):765-71. doi: 10.1111/j.1432-1033.1993.tb17818.x.
Addition of sialic acid residues in the human pathogen Trypanosoma cruzi glycoconjugates is mediated by a trans-sialidase and not by a CMP-sialic acid:glycoconjugate sialyltransferase. Incubation of trans-sialidase with N-[galactose-14C]acetyllactosamine and O-linked oligosaccharides, N-linked glycopeptides (both obtained from fetuin) or sialyllactose showed that the last three compounds were donors of sialic acid residues to the first one. Moreover, N- and O-linked oligosaccharides in asialofetuin and asialomucin, respectively, served as acceptors of sialic acid units. Gangliosides GM3, GD1a and GT1b but not GM2, GM1a nor GD1b donated sialic acid units to N-acetyllactos amine when incubated with trans-sialidase. This showed that only sialic acid units bound to terminal galactosyl residues were transferred. GM1a was converted to GD1a, and GD1b to GT1b when incubated with the appropriate donor. The fact that asialo-GM1a was converted to a ganglioside migrating as GD1a on thin-layer chromatography suggested that sialic acid units may be transferred to internal galactosyl residues, although once linked to those residues they can not be further transferred to other glycoconjugates. Sialic acid residues linked alpha 2,3- but not alpha 2,6- or alpha 2,8- were transferred by the trans-sialidase. Methyl beta-galactoside but not methyl alpha-galactoside served as acceptor of sialic acid units, thus suggesting that terminal alpha-linked galactosyl units in T. cruzi and mammalian glycoproteins are not sialylated by the enzyme. As the trans-sialidase employed in these experiments has been shown to be located on the external surface of the parasite and to be shed to the medium, the relatively broad specificity shown by the enzyme with respect to protein- and lipid-linked oligosaccharides strongly suggests that infection by T. cruzi might alter the sialic acid distribution in glycoproteins and glycolipids of the mammalian host.
人类病原体克氏锥虫糖缀合物中唾液酸残基的添加是由转唾液酸酶介导的，而非由CMP - 唾液酸：糖缀合物唾液酸转移酶介导。将转唾液酸酶与N - [半乳糖 - 14C]乙酰乳糖胺、O - 连接寡糖、N - 连接糖肽（均从胎球蛋白获得）或唾液乳糖苷一起孵育，结果显示后三种化合物是唾液酸残基转移至第一种化合物的供体。此外，去唾液酸胎球蛋白中的N - 连接寡糖和去唾液酸粘蛋白中的O - 连接寡糖分别作为唾液酸单元的受体。神经节苷脂GM3、GD1a和GT1b在与转唾液酸酶孵育时会向N - 乙酰乳糖胺捐赠唾液酸单元，但GM2、GM1a和GD1b则不会。这表明只有与末端半乳糖基残基结合的唾液酸单元会被转移。GM1a与合适的供体孵育时会转化为GD1a，GD1b与合适的供体孵育时会转化为GT1b。去唾液酸 - GM1a在薄层色谱上转化为迁移行为与GD1a相同的神经节苷脂这一事实表明，唾液酸单元可能会转移至内部半乳糖基残基，尽管一旦与这些残基连接，它们就不能再进一步转移至其他糖缀合物。转唾液酸酶转移的是α2,3 - 连接而非α2,6 - 或α2,8 - 连接的唾液酸残基。β - 甲基半乳糖苷而非α - 甲基半乳糖苷作为唾液酸单元的受体，因此表明克氏锥虫和哺乳动物糖蛋白中的末端α - 连接半乳糖基单元不会被该酶唾液酸化。由于这些实验中使用的转唾液酸酶已被证明位于寄生虫的外表面并释放到培养基中，该酶对蛋白质和脂质连接寡糖表现出的相对广泛特异性强烈表明，克氏锥虫感染可能会改变哺乳动物宿主糖蛋白和糖脂中唾液酸的分布。
