Serum free sialic acid

This Neu5Ac derivative lacking a glycosidic hydroxyl group at C2 has been found as free sialic acid in small quantities in blood serum, saliva and urine of man [5,8,13] and in tissue extracts of starfish [36] (Table 1). It is unknown, whether this compound, which is a potent competitive inhibitor of sialidases (see below), has a biological function. Its formation under physiological conditions has been described in section 2. 

Birds. Early studies showed the presence of sialic acids in the serum of hen (Bohm and Baumeister, 1956) and mucous from pheasant and goose (Wessler and Werner, 1957). Feeney et al. (1960) studied the chemical and the physical characteristics of avian egg white proteins and found sialic acids in the proteins of 25 species belonging to six orders of Aves. There are marked variations in sialic acid content between bird species, but they all appear to contain the same sialic acid, V-acetylneu-raminic acid. Only a trace amount of free sialic acid was detected. 

A group of compounds, called the sialic acids, are N- and 0-acyl derivatives of neuraminic acid, itself a derivative of D-mannosamine (Figure 28). The sialic acids are widely distributed (e.g. in egg yolk, human semen and in blood serum) both free and bound to the polysaccharide part of most, if not all, mucoproteins. They are closely... 

Several laboratories have reported automated procedures for the quantitation of sialic acid in biological fluids, column effluents or for miscellaneous samples. The periodic acid/resorcinol assay has been adapted for use in a Technicon autoanalyser for routine assay of serum sialic acid (Rey et al. 1975). The method is based on the assay developed by Jourdian et al. (1971) and does not involve the organic solvent extraction. As with the original assay, the automated procedure can be adapted to measure free or glycosidically bound sialic acid (Reyc/ al. 1975). Modification of this system allows measurement of erythrocyte membrane sialic acid simultaneously with protein (Gerbaut et al. 1978). 

In investigations by Morell et al. (1968), later by Ash well and Morell (1974), and by others (Pricer and Ashwell, 1971 Morell et al., 1971), it was demonstrated that all serum glycoproteins, with only one known exception, transferrin, require the presence of bound sialic acid for continuous circulation. By the use of glycoproteins labeled with either, or both, and Cu, it was shown that the sialic-acid-free... 

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