Bioscience and Biotechnology Center, Nagoya University, Nagoya 464-8611, Japan.
Glycobiology. 2010 Jun;20(6):752-62. doi: 10.1093/glycob/cwq030. Epub 2010 Mar 1.
N-Glycolylneuraminic acid (Neu5Gc) is the second most populous sialic acid (Sia). The only known biosynthetic pathway of Neu5Gc is the hydroxylation of cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), catalyzed by CMP-Neu5Ac hydroxylase (CMAH). Neu5Gc is abundantly found in mammals except for human, in which CMAH is inactivated due to mutation in the CMAH gene. Evidence has accumulated to show occurrence of Neu5Gc-containing glycoconjugates in sera of cancer patients, human cancerous tissues and cultured human cell lines. Recently, occurrence of natural antibodies against Neu5Gc was shown in healthy humans and is a serious problem for clinical xenotransplantation and stem cell therapies. Studying human occurrence of Neu5Gc is of importance and interest in a broad area of medical sciences. In this study, using a fluorometric high performance liquid chromatography method, we performed quantitative analyses of Sias both inside and in the external environment of the cell and found that (i) incorporation of Neu5Gc was most prominent in soluble glycoproteins found both in the extracellular space and inside the cell as the major Sia compounds. (ii) Of the total Neu5Gc in the Sia compounds that the cells synthesized, 90% was found in the secreted sialoglycoproteins, whereas for Neu5Ac, 70% was found in the secreted sialoglycoproteins. (iii) The Neu5Gc ratio was higher in the secreted sialoglycoproteins (as high as 40% of total Sias) than in intracellular sialoglycoproteins. (iv) The majority of the secreted sialoglycoproteins was anchored on the culture dishes and solubilized by brief trypsin treatment. Based on these findings, a new idea on the mechanism of accumulation of Neu5Gc in cancer cells was proposed.
N-羟乙酰神经氨酸（Neu5Gc）是第二丰富的唾液酸（Sia）。Neu5Gc 的唯一已知生物合成途径是胞苷-5'-单磷酸-N-乙酰神经氨酸（CMP-Neu5Ac）的羟化，由 CMP-Neu5Ac 羟化酶（CMAH）催化。Neu5Gc 在哺乳动物中广泛存在，除了人类，人类由于 CMAH 基因中的突变而失活。有证据表明，Neu5Gc 含有糖缀合物存在于癌症患者、人类癌组织和培养的人类细胞系的血清中。最近，在健康人中发现了针对 Neu5Gc 的天然抗体，这对临床异种移植和干细胞治疗是一个严重的问题。研究人类 Neu5Gc 的发生在广泛的医学科学领域具有重要意义和兴趣。在这项研究中，我们使用荧光高效液相色谱法对内、外环境中的 Sia 进行定量分析，发现：（i）Neu5Gc 的掺入在细胞外空间和细胞内发现的可溶性糖蛋白中最为突出，是主要的 Sia 化合物。（ii）细胞合成的 Sia 化合物中总 Neu5Gc 的 90%存在于分泌的唾液糖蛋白中，而 Neu5Ac 的 70%存在于分泌的唾液糖蛋白中。（iii）Neu5Gc 比在细胞内唾液糖蛋白中的比值更高（高达总 Sia 的 40%）。（iv）大多数分泌的唾液糖蛋白附着在培养皿上，并通过短暂的胰蛋白酶处理而溶解。基于这些发现，提出了一个关于 Neu5Gc 在癌细胞中积累的机制的新想法。
