Holmes E H
Pacific Northwest Research Foundation, Seattle, WA 98122.
Glycobiology. 1993 Feb;3(1):77-81. doi: 10.1093/glycob/3.1.77.
Biosynthesis of the Lex series of carbohydrate antigens proceeds by fucose transfer in alpha 1-->3-linkage to the pen-ultimate GlcNAc residue of a neolacto-series oligosaccharide acceptor, a reaction catalysed by multiple enzymes expressed in human tissues. Particularly broad acceptor specificity, including the ability to catalyse fucose transfer to both lacto- and neolacto-series acceptors as well as the precursor Lc3 structure (where Lc3, lactotriaosylceramide, is GlcNAc beta 1-->3Gal beta 1-->4Glc beta 1-->1Cer), exists for one human fucosyltransferase form, the Lewis alpha 1-->3/4fucosyltransferase (FucT-III). To determine if fucose transfer to Lc3 may represent an alternate early step in Le(x) or Le(a) antigen biosynthesis with this enzyme, the chemical structure of the fucosylated Lc3 reaction product formed by the Lewis alpha 1-->3/4fucosyltransferase from Colo 205 cells has been defined. Transfer of [14C]fucose to Lc3 yielded a labelled product migrating as a tetrasaccharide on thin layer chromatography plates. This product remained an acceptor for both beta 1-->3- and beta 1-->4-galactosyl transfer on the terminal GlcNAc residue. The product was degraded to a fucosylated trisaccharide derivative by bovine kidney beta-N-acetylglucosaminidase. Fast atom bombardment mass spectrometry and methylation analysis confirmed that the product was composed exclusively of the following structure containing a fucose linked to the 3-position of the internal Glc residue: [formula; see text] Such a structure does not represent an intermediate in Le(x) or Le(a) antigen biosynthesis. Thus, the evidence suggests that Le(x) or Le(a) antigen synthesis results exclusively from fucosylation of complete core chains.
Lex系列碳水化合物抗原的生物合成是通过岩藻糖以α1→3连接的方式转移到新乳糖系列寡糖受体的倒数第二个GlcNAc残基上进行的，该反应由人体组织中表达的多种酶催化。一种人岩藻糖基转移酶形式，即Lewisα1→3/4岩藻糖基转移酶（FucT-III），具有特别广泛的受体特异性，包括催化岩藻糖转移到乳糖系列和新乳糖系列受体以及前体Lc3结构（其中Lc3，乳糖三糖神经酰胺，是GlcNAcβ1→3Galβ1→4Glcβ1→1Cer）的能力。为了确定用这种酶将岩藻糖转移到Lc3是否可能代表Le(x)或Le(a)抗原生物合成中的另一个早期步骤，已经确定了由来自Colo 205细胞的Lewisα1→3/4岩藻糖基转移酶形成的岩藻糖基化Lc3反应产物的化学结构。将[14C]岩藻糖转移到Lc3上，得到一种在薄层色谱板上迁移为四糖的标记产物。该产物在末端GlcNAc残基上仍然是β1→3-和β1→4-半乳糖基转移的受体。该产物被牛肾β-N-乙酰氨基葡萄糖苷酶降解为岩藻糖基化的三糖衍生物。快原子轰击质谱和甲基化分析证实，该产物仅由以下结构组成，其中岩藻糖连接到内部Glc残基的3位：[化学式；见正文]这样的结构并不代表Le(x)或Le(a)抗原生物合成中的中间体。因此，证据表明Le(x)或Le(a)抗原合成完全是由完整核心链的岩藻糖基化产生的。
