Internal 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... 

The internal, sialic acid residue of GMj or GM2 is readily susceptible to sialidase action after enzymic release of the peripheral Gal and GalNAc residues, leading to GM3 having a terminal sialyl residue, and this represents the natural pathway of the catabolism of ganglio-sides.110 354 355... 

Interestingly, polysialic acid is a rather labile substance, which easily degrades spontaneously even under physiological conditions, due to an anomalously high p/f value of the internal sialic acid residues [258], This intramolecular self-cleavage may influence neuronal adhesion, embryogenesis and bacterial virulenee. 

Sugita, M. (1979) Studies on the Glycosphingolipids of the Starfish Asterina pectinifera III Isolation and Structural Studies of Two Novel Gangliosides Containing Internal Sialic Acid Residues , Journal of Biochemistry (Tokyo), 86, 765-72... 

The DMB-labeled sialic acids are applied to the HPLC equipped with a TSK-gel ODS-120T column (250 x 4.6 mm i.d., Tosoh), and a fluorescence detector (FP-2025, JASCO). The column is equilibrated using acetonitrile/methanol/water (9 7 84 by volume) at 26°C. Then 2-20 pL of the supernatants are applied to HPLC analysis for an isocratically flow at 1.0 mL/min and the DMB-labeled sialic acid is detected with a fluorescence detector at an excitation of 373 nm and an emission at 448 mn (Fig. 4). The estimate of the ratios of the quantity of internal sialic acid residues (C9(Sia)) to that of total sialic acid residues (C7(Sia) + C9(Sia)) can then be obtained. 

Sugita, M., 1979, Studies on the glycosphingolipids of the starfish, Astemia pectinifera. III. Isolation and structural studies of two novel gangliosides containing internal sialic acid residues, J. Biochem. 86 765-772. 

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