O-acetyl sialic acid

This enzyme [EC 4.1.3.3], also known as A-acetylneu-raminate aldolase, will convert A-acetylneuraminate to A-acetylmannosamine and pyruvate. The enzyme will also act on A-glycoloylneuraminate and on O-acetylated sialic acids, other than O -acetylated derivatives. 

The occurrence of%-O-acetylated sialic acid in colominic acid has also been reported.47 Although the substance has not yet been obtained in pure form, there is some evidence for the existence of... 

The aeylneuraminic acids can be released from their glycosidic linkages either by dilute (aqueous or methanolie) acids or sialidases. Special care must be taken in the isolation of the rather labile, O-acv-lated sialic acids, which arc partially non-susceptible to the action of sialidases. Furthennore, in gangliosides, non-O-acetylated sialic acid residues occur, and these are also more or less resistant towards these enzymes. 

To obtain individual sialic acids in pure, or almost pure, form, the sialic acid mixture obtained from the procedures just described are subjected to partition chromatography on cellulose powder, using, as the solvent,74107 1 2 1 (v/v/v) 1 -butanol-l-propanol-water. The (less hydrophilic) tri- and di-O-acetylated sialic acids are eluted first, followed by mono-O-acetylated sialic acids, NeuoAc, and Neu5Gc, ac-... 

Periodate oxidation of sialic acids had earlier been used for structural determination of O-substituted sialic acids.89 Whereas, for example, one mole of a 4-O-acetylated sialic acid consumes 2 moles of periodate within 10-20 min at 0°, the same amount of the 7-O-acetyl isomer is oxidized by only one mole. As already discussed in connection with the periodic acid-thiobarbituric acid assay, 9-O-substituted sialic acids exhibit a very low rate of oxidation as compared with the unsubstituted sialic acids13 this observation originally led to the erroneous assignment89 of the 9-O-acetyl group to 0-8. All other sialic acids having O-acetyl groups on the side chain are expected to be unaffected by periodate. 

Histochemical demonstration of most of the O-acetylated sialic acids is possible because substituents on the side chain of Neu hinder periodate oxidation of this part of the molecule to an extent dependent on the number and position of the O-acetyl groups already mentioned. Correspondingly, removal of these ester groups by alkaline treatment (0.5% KOH in 70% ethanol91) may increase the staining reaction of a sialic acid. For example, the presence of O-acylated sialic acids has been demonstrated in colonic, epithelial mucin of man and various mammals (summarized in Ref. 91), in healthy and diseased, human small-intestine,188-190 in bovine submandibular gland,182 in mouse and rat erythrocyte-membranes,191 and in human lymphocytes.192... 

The group of 7- and 8-mono-O-acetylated, 7,9- or 8,9-di-O-acetylated, and 7,8,9-tri-O-acetylated sialic acids (the existence of which in bovine, submandibular glands is well established82) is specifically stained in the following way. Tissue slices are treated with 0.5 M periodate for 120 min at 25°, causing complete oxidation of the side chain to the C7 aldehyde of the unsubstitutecl and 9-mono-O-acetylated sialic acids. 

After reduction of the aldehyde with borohydride, the ester groups of the remaining mono-, di-, and tri-O-acetylated sialic acids that are unaffected by these vigorous conditions of oxidation are hydrolyzed by alkali, the products are subjected to mild oxidation with periodate,... 

In contrast to the biosynthesis of Neu5Ae, the pathways leading to the many other sialic acids known have not yet been completely elucidated. Insight has, however, been obtained into the biosynthesis of Neu5Gc and the 4-0- and 9-O-acetylated sialic acids. 

Apart from the 4-O-acetylated sialic acids, another sialidase-resist-ant sialic acid exists in Nature, namely, the internal, Gal-bound, sialic acid residue of GM,. In contrast to this side-positioned sialic acid, the sialyl residues bound to the peripheral Gal of gangliosides, or to both the peripheral and the internal sialic acid residues forming oligo-sialvl chains in several members of the large ganglioside family,la can be readily removed by sialidases, as was tested with viral, bacterial, and mammalian enzymes.35 -35 1... 

H. Ogura, K. Furuhata, S. Sato, and K. Anazawa, Synthesis of 9-O-acyl and 4-O-acetyl sialic acids, Carbohydr. Res. 167 11 (1987). 

One of the main functions of sialic acid O-acetyl groups is their inhibitory effect on the action of both sialidases and sialic acid-specific lyases [5,33]. While a 4-O-acetyl group completely hinders the action of these enzymes (with the exception of a slow release by viral sialidases [252]), such ester groups at the sialic acid side chain appreciably hinder hydrolysis of the glycosidic bond of these sugars and their further breakdown by lyases. The existence of esterases acting on O-acetylated sialic acids prior to sialidase is therefore a prerequisite or at least supports the rapid turnover of O-acetylated sialoglycoconjugates. The possibility of the existence of such enzymes was raised by the observation that... 

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