Seppala R, Tietze F, Krasnewich D, Weiss P, Ashwell G, Barsh G, Thomas G H, Packman S, Gahl W A
Section on Human Biochemical Genetics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
J Biol Chem. 1991 Apr 25;266(12):7456-61.
Sialuria is a rare inborn error of metabolism caused by excessive synthesis of sialic acid (N-acetylneuraminic acid, NeuAc). Fibroblasts cultured from the three known cases of sialuria contained 70-200-fold increases in soluble sialic acid, but normal concentrations of bound sialic acid. The sialic acid appeared in the cytosolic fraction of the cells on differential centrifugation, and was susceptible to borohydride reduction, suggesting that accumulated sialic acid was in the form of NeuAc and not CMP-NeuAc. In biochemical studies, CMP-NeuAc (50 microM) inhibited the UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase of normal fibroblasts by 84-100%, but inhibited the epimerase from sialuria cells by only 19-31%. Feeding sialuria cells up to 5 mM D-glucosamine for 72 h increased free sialic acid content 20-60%, but normal cells were unaffected by this treatment. Cytidine feeding (5 mM, 72 h) reduced the NeuAc content of sialuria cells, initially 112, 104, and 266 nmol/mg protein, by 63-71 nmol/mg protein; CMP-NeuAc concentrations, initially 4, 2, and 5 nmol/mg protein, increased by 14-33 nmol/mg protein. Consequently, the total cellular content of soluble sialic acid (NeuAc + CMP-NeuAc) was lowered 14-46% by cytidine feeding. The inheritance pattern of sialuria has not been determined. However, cells from both parents of one sialuria patient contained normal concentrations of free sialic acid, and the parental epimerase activity also responded normally to CMP-NeuAc. We conclude that the basic biochemical defect in all known cases of sialuria is a failure of CMP-NeuAc to feedback-inhibit UDP-GlcNAc 2-epimerase and cytidine feeding can lower the intracellular soluble sialic acid concentration of sialuria cells.
唾液酸尿症是一种罕见的先天性代谢紊乱疾病，由唾液酸（N-乙酰神经氨酸，NeuAc）过度合成引起。从已知的三例唾液酸尿症患者培养的成纤维细胞中，可溶性唾液酸含量增加了70 - 200倍，但结合唾液酸浓度正常。经差速离心后，唾液酸出现在细胞的胞质部分，且易被硼氢化钠还原，这表明积累的唾液酸是以NeuAc的形式存在，而非CMP-NeuAc。在生化研究中，CMP-NeuAc（50 microM）可使正常成纤维细胞的UDP-N-乙酰葡糖胺（UDP-GlcNAc）2-表异构酶活性抑制84 - 100%，但对唾液酸尿症细胞的该表异构酶仅抑制19 - 31%。给唾液酸尿症细胞喂食高达5 mM的D-葡萄糖胺72小时，可使游离唾液酸含量增加20 - 60%，但正常细胞不受此处理影响。喂食胞苷（5 mM，72小时）可使唾液酸尿症细胞的NeuAc含量（最初分别为112、104和266 nmol/mg蛋白质）降低63 - 71 nmol/mg蛋白质；CMP-NeuAc浓度（最初分别为4、2和5 nmol/mg蛋白质）增加14 - 33 nmol/mg蛋白质。因此，喂食胞苷可使细胞内可溶性唾液酸（NeuAc + CMP-NeuAc）的总含量降低14 - 46%。唾液酸尿症的遗传模式尚未确定。然而，一名唾液酸尿症患者父母的细胞中游离唾液酸浓度正常，且亲本的表异构酶活性对CMP-NeuAc的反应也正常。我们得出结论，所有已知唾液酸尿症病例的基本生化缺陷是CMP-NeuAc无法反馈抑制UDP-GlcNAc 2-表异构酶，且喂食胞苷可降低唾液酸尿症细胞内的可溶性唾液酸浓度。
