Surface sialyltransferase

Further studies using NeuSAc have been related to the metabolism of extracellular CMP-NeuSAc and the detection of ectosialyltransferase activity. On the basis of a lag period observed for NeuSAc uptake and incorporation into glycoconjugates, and the absence of such a lag for CMP-NeuSAc incorporation, two routes have been proposed. The uptake of NeuSAc and metabolism as described above has been studied in hamster and mouse fibroblasts, and cell surface labelling of glycoprotein and glycolipid demonstrated (Datta 1974). The breakdown of CMP-NeuSAc was shown, and incorporation due to NeuSAc uptake rather than direct CMP-NeuSAc transfer proposed (Hirschberg et al. 1976). The uptake of CMP-NeuSAc into the cells (NIL, BHK and 3T3 fibroblasts) could be ruled out, and the K , for NeuSAc uptake was estimated to be 10 mM. Other experiments with CMP-NeuSAc and intact cell cultures (Painter and White 1976, Cerven 1977, see section III.9) pointed to surface sialyltransferase. Further studies by Fan and Datta (1980) provided evidence that both transfer and transport occur, by localization of acceptors within the cell and on the cell surface (plasma membrane), and direct demonstration of the presence of a plasma membrane sialyltransferase. The sialylation due to NeuSAc uptake occurs (at least initially) with different acceptors in comparison with CMP-NeuSAc plasma membrane sialylation. 

Sialyltransferase activities in normal and virally transformed mouse 3T3 cells have also been measured with specific endogenous acceptors (Bosmann, 1972a). The acceptors were prepared by incubating the intact cells with neuraminidase and then trypsin. The desialylated material released from the cells was used to measure sialyltransferase activity in crude detergent extracts of the cells. Cells transformed by MSV, RSV, or Py had more acceptor and transferase activity than normal 3T3 cells. Surface sialyltransferase activity as measured by the ability of intact cells to transfer sialic acid- C from CMP-NANA- C to undefined surface acceptors was also higher in the transformed lines (Bosmann, 1972 ). 

Sialyltransferase activity has also been described in plasma membranes from different cells, including blood platelets.285 The function of the enzyme on cell surfaces is unknown, as resialylation of membrane components directly on the cell surface is improbable, owing to the presence of CMP-Neu5Ac hydrolase in the plasma membrane, and the lack of CMP-sialic acids in the extracellular space. Furthermore, evidence has never been obtained for a role of cell-surface gly-cosyltransferases in cellular interaction according to a hypothesis of Roseman s.281... 

In the case of rat and human-breast cancer, an increase in serum-sialyltransferase activity is considered to be the consequence of both increased production and release, the latter perhaps through cell-surface shedding of the enzyme from the metastasizing, mammary-tumor cells.290 Accordingly, release of large amounts of sialyltransferase from hepatoma cell-lines derived from patients having hepatocellular carcinoma was observed, in contrast to cell lines derived from nomial human-liver.291 An increased level of sialyltransferase has been observed in regenerating rat-liver.292... 

Sialyltransferases catalyze the transfer of a sialic acid residue from CMP-sialic acid to a galactose, GalNAc, or another sialic acid residue. Sialyltransferases from bacterial and mammalian sources have been extensively studied and used in the enzymatic synthesis of sialosides, sialoglycoconjugates, and enzymatic modiflcation of cell surface. 

More recently, studies on Campylobacter jejuni have demonstrated how specific bacterial carbohydrate antigens can mimic the fine structure of human cell surface glycolipid structures and elicit autoimmune reactivity. Yuki et al.61 first identified that the lipooligosaccharide (LOS) of C. jejuni mimicked human G(M)1 ganglioside and subsequently extended these studies to demonstrate that specific single amino acid variants of the C. jejuni Cst-II sialyltransferase protein are responsible for the synthesis of LOS variants that in turn, mimic different... 

The field of solid phase synthesis has also benefited from enzyme technology. Several reports have described the use of sialyltransferases in the solid-supported synthesis of oligosaccharides that bear sialic acid [49]. Additionally, metabolic pathways have been harnessed by Bertozzi and coworkers to synthesize glycoconjugates on cell surfaces that bear modihed sialic acids [50]. 

Replacement of A-acyl and 0-acyl by A-thioacyl groups in sialo-sugar chains of cell surface glycoproteins can influence certain biological functions mediated by sialic acids, such as cellular adhesion phenomena or virus specificity for host cells. Interestingly, sialyltransferases specific for different... 

Among the most relevant and well-studied parasite glycotopes ate the sialic acid group of sugars and their derivatives. Expression of surface terminal sialic acid and its derivative is due to the interplay between several enzymes namely sialidases, sialyltransferases, O-acetyl transferase and O-acetyl esterase. Therefore, synthetic analogues design to inhibit their expression appears to be an attractive treatment protocol. 

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