Sialyltransferase activity

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... 

CMP, CDP, CTP, and synthetic derivatives of these nueleotides have been found to inhibit sialyltransferase activity.301" 02 Interest in such inhibitors is increasing, as they may be expected to serve as anticancer agents.269 901,303 Therefore, regulation of Golgi sialyltransferase activity appears possible by nucleotides as products of sialyl- and other glycosyl-transferase activities.1" 2 Interestingly, Epstein-Barr virus infection of human B, lymphoblastoid cell-lines leads to a diminution of sialyltransferase activity.304... 

Glycolipid Sialyltransferase Activities in Mouse and Human Tumor Cells... 

A HeLa cells cultured in medium containing 5 mM sodium butyrate and 50 ixCi/mL N-[acetyl-sH]-D-mannosamine after 24 hr, one set of cells harvested and medium replaced with fresh medium with and without 0.5 pg/mL cycloheximide on the other two sets, which were harvested after another 19 hr cells then analyzed for radioactive GM3. B Essentially same as A except the cells were cultured in unlabeled medium and assayed for sialyltransferase activity. C same as B except the cells were assayed for GM3-... 

Table I. The Effect of Various Compounds Implicated in the 2-Stage Theory of Carcinogenesis on CMP-NeuAc lactosylceramide Sialyltransferase Activity in Human Epithelial Carcinoma with (KB) Cells...

CONDITION SIALYLTRANSFERASE ACTIVITY (pmoles/mg protein/hr) % CONTROL... 

Table III. Comparison of Serum-induced ODC Activity with CMP-NeuAc lactosylceramide Sialyltransferase Activity in Various Cell Lines...

Non-bracketed numbers ODC activity nmoles/mg protein/hr Bracketed numbers Sialyltransferase Activity pmoles/mg protein/hr... 

Figure 3. (a) Distribution of endogenous sialyltransferase activities on continuous... 

We have no information on the relative contribution of the various cell types in the observed sialyltransferase activities. Comparable enzyme activities have been reported in the neuronal and glial cell preparations (19). More definitive work will necessarily involve autoradiographic studies as initiated by some investigators (49). 

Pretreatment of enzyme preparations with trypsin diminished all four sialyltransferase activities (16). The sensitivity to... 

Figure 4. Effects of nonionic detergents on sialyltransferase activities. X. Endogenous glycoproteins A, exogenous DS-fetuin O, endogenous glycolipids O, exogenous GM, (16).

The basis for the multiplicity of the sialyltransferase activities remains to be elucidated. We plan to purify these enzyme species to homogeneity, using isoelectric focusing columns of smaller pH ranges in conjunction with affinity chromatography which has been successfully used to purify the soluble sialyl-transferases from bovine colostrum (57). Possibility exists that the heterogeneity of sialyltransferase activities as observed is due to differences in polypeptide sequences, carbohydrate content, or non-covalent interactions with other membrane components, and these possibilities can be clarified only with highly purified enzyme preparations. 

A continuous sucrose gradient of the total homogenate from young rat brain and electron microscopic examination of these fractions found most of the sialyltransferase activities to be localized in smooth microsomal membrane and Golgi complex derivatives and not associated with synaptosomes. 

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