Sialic acids acceptors

S = protein or lipid core -[X] = oligosaccharide chain as sialic acid acceptor (taken, and modified, from Ref. 33).]... 

The sialyltransferase assay is usually carried out with radioactive CMP-Neu5Ac and the sialic acid acceptor, followed by identification of the radioactive product. As this product is often membrane-bound and the radioactive cosubstrate is expensive, a non-radioactive assay was developed [625], in which CMP released from CMP-Neu5Ac by the transfer of Neu5Ac is determined with HPLC (see section 5.3.2). Since CMP-Neu5Ac can be analyzed in the same run, this test is also suited for the activity of the CMP-Neu5Ac synthase described above. 

Serrano AA, Schenkman S, Yoshida N et al. The lipid saucture of the glycosylphosphatidylinositol-anchored mucin-like sialic acid acceptors of Trypanosoma cruzi changes during parasite differentiation from epimastigotes to infective metacyclic trypomastigote forms. J Biol Chem 1995 270 27244-27253. 

A particulate sialyltransf erase preparation from rat mammary gland and soluble preparations from goat colostrum and pork liver were assayed in the presence of a variety of possible sialic acid acceptors. The conditions of assay were saturating for both acceptor and donor and the activities therefore represent relative Vm values. Activities are corrected for the small amount of endogenous acceptor activity usually present. 

Figure 6.9 Broad acceptor substrate tolerance of sialic acid aldolase in synthesis of nonnatural disaccharides...

N-Acetvlneuraminic Acid Aldolase. A new procedure has also been developed for the synthesis of 9-0-acetyl-N-acetylneuraminic acid using the aldolase catalyzed reaction methodology. This compound is an unusual sialic acid found in a number of tumor cells and influenza virus C glycoproteins (4 ). The aldol acceptor, 6-0-acetyl-D-mannosamine was prepared in 70% isolated yield from isopropenyl acetate and N-acetyl-D-mannosamine catalyzed by protease N from Bacillus subtilis (from Amano). The 6-0-acetyl hexose was previously prepared by a complicated chemical procedure (42.) The target molecule was obtained in 90% yield via the condensation of the 6-0-acetyl sugar and pyruvate catalyzed by NANA aldolase (Figure 6). With similar procedures applied to KDO, 2-deoxy-NANA and 2-deoxy-2-fluoro-NANA were prepared from NANA. 

Several CMP conjugates made available by this procedure could be transferred effectively to N-acetyl-lactosamine as an acceptor by using the a-2,6-sialyltrans-ferase from rat liver (Fig. 2.2.5.4). In this manner, several trisaccharides 74-77 could be generated carrying natural as well as non-natural sialic acids in good yields [47], limited so far only by the high cost of the commercial glycosyltransferase. 

The lactonized phenyl 2-thioglycosides of dimeric (26) [41] and trimeric (27) [42] sialic acids, which were prepared from the corresponding oligosialic acids according to the procedure employed for the monosialyl derivatives, were each coupled with the suitably protected sugar acceptors (14, 16, 17, and 19) by use of NIS-TfOH as a promoter in acetonitrile, as just described for the monosialyl glycoside synthesis (Fig. 3). 

Recently, the enzyme from Aureobacterium barkerei, strain KDO 372, has been partially purified and used in the preparative synthesis of KDO. The pyruvate attacks from the re face of aldehyde, creating a new R chiral center at C-4 thus, the facial selectivity of KDO aldolase is complementary to the one of sialic acid aldolase. Moreover as the enzyme accepts some flexibility toward the acceptor, several other analogues with 6-, 7-, or 9-carbon atoms could be prepared, such as l-KDN and d-DAH [50]. 

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