Lepers A, Shaw L, Schneckenburger P, Cacan R, Verbert A, Schauer R
Laboratoire de Chimie Biologique, Centre National de Recherche Scientifique no. 111, Université de Sciences et Techniques de Lille Flandres-Artois, Villeneuve d'Ascq, France.
Eur J Biochem. 1990 Nov 13;193(3):715-23. doi: 10.1111/j.1432-1033.1990.tb19391.x.
The relative contribution of N-glycoloyl-beta-D-neuraminic acid (Neu5Gc) to total sialic acids expressed in mouse and rat liver glycoconjugates was found to be 95% and 11%, respectively. This considerable difference in sialic acid composition made these two tissues suitable models for a comparative investigation into the regulation of Neu5Gc biosynthesis and utilization. An examination of the CMP-glycoside specificity of Golgi-associated sialyltransferases using CMP-N-acetyl-beta-D-neuraminic acid (CMP-Neu5Ac) and CMP-Neu5Gc revealed no significant tissue-dependent differences. The Golgi membrane CMP-sialic acid transport system from rat liver did, however, exhibit a slightly higher internalisation rate for CMP-Neu5Ac, though no preferential affinity for this sugar nucleotide over CMP-Neu5Gc was observed. In experiments, where Golgi membrane preparations were incubated with an equimolar mixture of labelled CMP-Neu5Ac and CMP-Neu5Gc, no significant tissue-dependent differences in [14C]sialic acid composition were observed, either in the luminal soluble sialic acid fraction or in the precipitable sialic acid fraction, results which are consistent with the above observations. From this experiment, evidence was also obtained for the presence of a Golgi-lumen-associated CMP--sialic acid hydrolase which exhibited no apparent specificity for either CMP-Neu5Ac or CMP-Neu5Gc. The specific activity of the CMP-Neu5Ac hydroxylase, the enzyme responsible for the biosynthesis of Neu5Gc, was found to be 28-fold greater in high-speed supernatants of mouse liver than of rat liver. No hydroxylase activity was detected in the Golgi membrane preparations. It is therefore proposed that the cytoplasmic ratio of CMP-Neu5Ac and CMP-Neu5Gc produced by the hydroxylase, remains largely unmodified after CMP-glycoside uptake into the Golgi apparatus and transfer on to growing glycoconjugate glycan chains. The close relationship between the total sialic acid composition and the sialic acid pattern in the CMP-glycoside pools of the tissues lends considerable weight to this hypothesis.
已发现，N-羟乙酰神经氨酸（Neu5Gc）在小鼠和大鼠肝脏糖缀合物中表达的总唾液酸中的相对贡献分别为95%和11%。唾液酸组成上的这一显著差异使这两种组织成为比较研究Neu5Gc生物合成和利用调控的合适模型。使用CMP-N-乙酰神经氨酸（CMP-Neu5Ac）和CMP-Neu5Gc对高尔基体相关唾液酸转移酶的CMP-糖苷特异性进行检测，未发现明显的组织依赖性差异。然而，大鼠肝脏的高尔基体膜CMP-唾液酸转运系统对CMP-Neu5Ac的内化速率略高，不过未观察到该糖核苷酸对CMP-Neu5Gc有优先亲和力。在实验中，将高尔基体膜制剂与标记的CMP-Neu5Ac和CMP-Neu5Gc等摩尔混合物一起孵育，在管腔可溶性唾液酸部分或可沉淀唾液酸部分均未观察到明显的组织依赖性[14C]唾液酸组成差异，这些结果与上述观察结果一致。从该实验中还获得了证据，表明存在一种与高尔基体腔相关的CMP-唾液酸水解酶，该酶对CMP-Neu5Ac或CMP-Neu5Gc均无明显特异性。负责Neu5Gc生物合成的CMP-Neu5Ac羟化酶的比活性在小鼠肝脏的高速上清液中比在大鼠肝脏中高28倍。在高尔基体膜制剂中未检测到羟化酶活性。因此，有人提出，羟化酶产生的CMP-Neu5Ac和CMP-Neu5Gc的细胞质比率在CMP-糖苷被摄取到高尔基体并转移到生长中的糖缀合物聚糖链后基本未改变。组织中CMP-糖苷池的总唾液酸组成与唾液酸模式之间的密切关系为这一假设提供了相当有力的支持。
