Inhibition of CMP-N-acetylneuraminic acid:lactosylceramide sialyltransferase by nucleotides, nucleotide sugars and nucleotide dialdehydes

Human dendritic cells contain cell surface sialyltransferase activity

Human monocyte-derived dendritic cells (mo-DCs) express highly sialylated structures, with recognized but poorly understood function in maturation, immunogenicity and endocytosis capacity. We have previously shown that mo-DCs surface sialylation is changeable upon different stimuli, which led us to hypothesise the existence of cell surface (non-intracellular) sialyltransferases, rapidly restoring or altering mo-DC surface sialylation, thus modulating specific functions. Here, we demonstrate that, in the presence of exogenous CMP-Neu5Ac, mo-DCs incorporate considerable amounts of sialic acids into cell surface, predominantly when mo-DCs were previously desialylated or matured. This is a genuine sialyltransferase activity, confirmed by specific inhibition assays, which is not influenced by secreted enzymes. Functionally, the ecto-sialyltransferase activity causes a significant down-regulation of mo-DCs endocytic capacity, without affecting the maturation state. These findings suggest that ecto-sialyltransferases participate in a dynamic control of mo-DC sialylation, with functional repercussions. This activity is possibly related with specific physiological and pathological conditions, as inflammation and infection, contributing to protection and homeostasis regulation.

Trans-sialidase activity of Photobacterium damsela alpha2,6-sialyltransferase and its application in the synthesis of sialosides

Trans-sialidases catalyze the transfer of a sialic acid from one sialoside to an acceptor to form a new sialoside. alpha2,3-Trans-sialidase activity was initially discovered in the parasitic protozoan Trypanosoma cruzi, and more recently was found in a multifunctional Pasteurella multocida sialyltransferase PmST1. alpha2,8-Trans-sialidase activity was also described for a multifunctional Campylobacter jejuni sialyltransferase CstII. We report here the discovery of the alpha2,6-trans-sialidase activity of a previously reported recombinant truncated bacterial alpha2,6-sialyltransferase from Photobacterium damsela (Delta15Pd2,6ST). This is the first time that the alpha2,6-trans-sialidase activity has ever been identified. Kinetic studies indicate that Delta15Pd2,6ST-catalyzed trans-sialidase reaction follows a ping-pong bi-bi reaction mechanism. Cytidine 5'-monophosphate, the product of sialyltransferase reactions, is not required by the trans-sialidase activity of the enzyme but enhances the trans-sialidase activity modestly as a non-essential activator. Using chemically synthesized Neu5AcalphapNP and LacbetaMU, alpha2,6-linked sialoside Neu5Acalpha2,6LacbetaMU has been obtained in one-step in high yield using the trans-sialidase activity of Delta15Pd2,6ST. In addition to the alpha2,6-trans-sialidase activity, Delta15Pd2,6ST also has alpha2,6-sialidase activity. The multifunctionality is thus a common feature of many bacterial sialyltransferases.

Recent development in the design of sialyltransferase inhibitors

Sialylation at the non-reducing end of glycoconjugates is an important biological process in cellular recognitions, tumor metastases, and immune responses, which are mediated by a family of enzymes known as sialyltransferases. Inhibition of sialyltransferases may prove useful in elucidating the biological functions of sialylation and may have therapeutic applications. This review summarizes the recent development in this field with particular focus on the strategies used for the design of carbohydrate mimetics and the structure-activity relationships of substrate-based sialyltransferase inhibitors.

Soyasaponin I, a potent and specific sialyltransferase inhibitor

A growing number of reports demonstrate that hypersialylation, which is observed in certain pathological processes, such as oncogenic transformation, tumor metastasis, and invasion, is associated with enhanced sialyltransferase (ST) activity. There is therefore a need for the development of ST inhibitors to modulate ST activity and thus alleviate the disease processes caused by STs. In the present study, soyasaponin I had been discovered to be a potent and specific ST inhibitor by screening strategy from 7500 samples including micribial extracts and natural products. Kinetic analysis shows that it is a CMP-Neu5Ac competitive inhibitor with for ST3Gal I with an inhibition constant (K(i)) of 2.1 microM. In addition, it is only active against ST, but not against the other tested glycosyltransferases and glycosidases. Our study is the first report to discover ST inhibitor by screening method and also to provide the new chemical structure information that should be useful in the development of other novel ST inhibitors.

Functional organization of the Golgi apparatus in glycosphingolipid biosynthesis. Lactosylceramide and subsequent glycosphingolipids are formed in the lumen of the late Golgi

Biosynthesis of plasma membrane sphingolipids involves the coordinate action of enzymes localized to individual compartments of the biosynthetic secretory pathway of proteins. These stations include the endoplasmic reticulum and the Golgi apparatus. Although a precise localization of all the enzymes that synthesize glycosphingolipids has not been achieved to date, it is assumed that the sequence of events in glycosphingolipid biosynthesis resembles that in glycoprotein biosynthesis, i.e. that early reactions occur in early stations (endoplasmic reticulum and cis/medial Golgi) of the pathway, and late reactions occur in late stations (trans Golgi/trans Golgi network). Using truncated analogues of ceramide and glucosylceramide that allow measurement of enzyme activities in intact membrane fractions, we have reinvestigated the localization of individual enzymes involved in glycosphingolipid biosynthesis and for the first time studied the localization of lactosylceramide synthase after partial separation of Golgi membranes as previously described (Trinchera, M., and Ghidoni, R. (1989) J. Biol. Chem. 264, 15766-15769). Here, we show that the reactions involved in higher glycosphingolipid biosynthesis, including lactosylceramide synthesis, all reside in the lumen of the late Golgi compartments from rat liver.

Studies on the inhibition of sialyl- and galactosyltransferases

The inhibition of the alpha-2,6-sialyltransferase from rat liver, the alpha-2,3-sialyltransferase from porcine submandibular gland and of the galactosyltransferase from human milk were studied using monosaccharide-, nucleoside- and nucleotide-derivatives of their naturally occurring donor substrates cytidine 5'-monophosphate-N-acetylneuraminic acid and uridine 5'-diphosphate-galactose, respectively. Only the corresponding nucleosides/nucleotides showed inhibitory activity. Periodate oxidation of CMP or CMP-Neu5Ac and of UMP or UDP-Gal led to reduced inhibitory efficiency with the respective transferase. The type and reversibility of the inhibition of some of these compounds, as well as the corresponding Ki values were determined.