Metabolism of Vertebrate Amino Sugars with N-Glycolyl Groups ELUCIDATING THE INTRACELLULAR FATE OF THE NON-HUMAN SIALIC ACID N-GLYCOLYLNEURAMINIC ACID
Metabolism of Vertebrate Amino Sugars with N-Glycolyl Groups ELUCIDATING THE INTRACELLULAR FATE OF THE NON-HUMAN SIALIC ACID N-GLYCOLYLNEURAMINIC ACID
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DOI：
10.1074/jbc.m112.363549
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发表时间：
2012-08-17
影响因子：
4.8
通讯作者：
Varki, Ajit
中科院分区：
文献类型：
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作者：
Bergfeld, Anne K.;Pearce, Oliver M. T.;Varki, Ajit
两种主要的哺乳动物唾液酸是N-乙酰神经氨酸和N-羟乙酰神经氨酸（Neu 5Gc）。产生Neu 5Gc的唯一已知生物合成途径是CMP-N-乙酰神经氨酸转化为CMP-Neu 5Gc，其由CMP-Neu 5Ac羟化酶催化。鉴于这种反应的不可逆性，必须有消除或降解Neu 5Gc的途径，这将允许动物细胞调节Neu 5Gc水平以满足其需要。虽然人类由于失活的CMAH基因而不能合成Neu 5Gc，但在抗Neu 5Gc抗体应答面前，来自饮食来源的外源性Neu 5Gc可以代谢性地掺入组织中。然而，Neu 5Gc的代谢周转，这显然是防止人类细胞继续积累这种免疫反应性唾液酸，尚未阐明。在这项研究中，我们表明，随着时间的推移，预加载的Neu 5Gc从人体细胞中消除，我们提出了一个可以想象的Neu 5Gc降解途径的基础上充分研究的N-乙酰己糖胺的代谢。我们证明，小鼠组织胞质提取物窝藏酶机制，依次转化为N-羟乙酰甘露糖胺，N-羟乙酰葡糖胺，N-羟乙酰葡糖胺6-磷酸，其中不可逆的脱N-羟乙酰化后者的结果在无处不在的代谢产物乙醇酸和葡糖胺6-磷酸Neu 5Gc。我们证实了这一发现，证明在体外的重组人酶的活性，并通过研究放射性标记的途径中间体在培养的人细胞的命运，这表明该途径可能发生在体内。最后，我们证明了所提出的降解途径是部分可逆的，表明N-羟乙酰甘露糖胺和N-羟乙酰葡萄糖胺（但不是乙醇酸）可以作为内源性Neu 5Gc生物合成的前体。
The two major mammalian sialic acids are N-acetylneuraminic acid and N-glycolylneuraminic acid (Neu5Gc). The only known biosynthetic pathway generating Neu5Gc is the conversion of CMP-N-acetylneuraminic acid into CMP-Neu5Gc, which is catalyzed by the CMP-Neu5Ac hydroxylase enzyme. Given the irreversible nature of this reaction, there must be pathways for elimination or degradation of Neu5Gc, which would allow animal cells to adjust Neu5Gc levels to their needs. Although humans are incapable of synthesizing Neu5Gc due to an inactivated CMAH gene, exogenous Neu5Gc from dietary sources can be metabolically incorporated into tissues in the face of an anti-Neu5Gc antibody response. However, the metabolic turnover of Neu5Gc, which apparently prevents human cells from continued accumulation of this immunoreactive sialic acid, has not yet been elucidated. In this study, we show that pre-loaded Neu5Gc is eliminated from human cells over time, and we propose a conceivable Neu5Gc-degrading pathway based on the well studied metabolism of N-acetylhexosamines. We demonstrate that murine tissue cytosolic extracts harbor the enzymatic machinery to sequentially convert Neu5Gc into N-glycolylmannosamine, N-glycolylglucosamine, and N-glycolylglucosamine 6-phosphate, whereupon irreversible de-N-glycolylation of the latter results in the ubiquitous metabolites glycolate and glucosamine 6-phosphate. We substantiate this finding by demonstrating activity of recombinant human enzymes in vitro and by studying the fate of radiolabeled pathway intermediates in cultured human cells, suggesting that this pathway likely occurs in vivo. Finally, we demonstrate that the proposed degradative pathway is partially reversible, showing that N-glycolylmannosamine and N-glycolylglucosamine (but not glycolate) can serve as precursors for biosynthesis of endogenous Neu5Gc.