Sialyltransferases sialyltransfer

Figure 6. Effect of CMP-NeuAc concentration (V/S), of pH (V/pH), of enzymatic protein concentration (V/protein), and of incubation time (V/t) on the activity of synaptosomal membrane-bound sialyltransferase. Calf brain cortex. Acceptor substrates for sialyltransf erase (if) lactosylceramide ( ) desialylated fetuin (%) endogenous glycoprotein (endogenous glycolipids.

In the chemo-enzymatic synthesis of LLG-3 performed by Withers et al. [34], the glycoside of 8-0-Me-Neu5Ac with glycolic acid 59 was synthesized from NeuSAc derivative 58 by a chemical method because of the absence of a sialyltransferase capable of sialyltransfer to the hydroxyl group of the glycosyl amide in the penultimate sialic acid residue and an 8-O-methyltransferase for NeuAc (Scheme 11.10). 

Fig. 8. Specificity of sialyltransfer to Galp( 1-3)GalNAc by purified sialyltransferases. Glycosyltransfer to the disaccharide using purified glycosyltransferase enzymes is shown.

Long term hormonal control functions in the expression of enzyme concentrations and de novo acceptor molecule synthesis in the case of sialyltransfer reactions. Examples in development have been cited (sections II.2, II. 3, and III. 10), and the regenerating liver has proved valuable in studies of this kind, e.g. UDP-GlcNAc 2-epimerase (Okubu et al. 1976, Okamoto and Akamatsu 1980). Elevation of enzyme concentration could be shown to be a function of de novo protein synthesis. Study of hormone effects (oestrogen, progesterone) on sialyltransferase activity and glycosidically linked sialic acid levels in the cervical cyclic phenomena have been reported (Moghissi and Syner 1976, Chantler and Debruyn 1977, Hatcher et al. 1977, Nasir-ud-Din et al. 1979, Wolf et al. 1980). Similar effects have been demonstrated in rat endometrium (Nelson et al. 1975). 

The attachment of sialic acids is mediated by the sialyltransferases. Fifteen or more mammalian sialyltransferases are believed to exist to account for the complete complement of sialic acid linkages in mammals. Of these, 13 distinct sialyltransfer-ase species have been cloned [31, 91, 96]. These are summarized in Table 1. Sialic acids and sialyltransferases are not unique to mammals they have also been documented in birds [91], fish [16, 21, 97], amphibians [5, 24, 53], echinoderms [37, 46], and bacterial sources [9, 22, 106]. Sialyltransferases in yeast and in insects [83] have also been reported but their existence remains somewhat controversial. 

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