Studies on the structure of gangliosides I. On the linkage of N-acetyl neuraminic acid in monosialoganglioside

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Purified ganglioside preparations appear to contain at least four different compounds ( Kuhn et al. , 1961 ; Dain et al. , 1962 ; Klenk and Gielen, 1961), which are believed to be due to the occurrence of mono- di- and trisialogangliosides ( Kuhn et al. , 1961 ). These authors have demonstrated that the di- and trisialoganliosides can be degraded to the monosialoganglioside by treatment with the enzyme neuraminidase. This enzyme specifically cleaves terminal N-acetylneuraminic acid (NANA) residues having an α-ketosidic linkage ( Gottschalk, 1960a). It has been observed by several investigators that 35 – 60% of the NANA present in a mixture of gangliosides is resistant to hydrolysis by neuraminidase ( Trams and Lauter, 1962; Svennerholm, 1962; Dain et al. , 1962 ). However, 90% of the total NANA can be liberated by hydrolysis with dilute mineral acids ( Klenk and Gielen, 1960; Trams and Lauter, 1962) and the remainder with more concentrated acid. Therefore, although nearly all of the NANA linkages have similar acid lability, approximately only one-half of the NANA is neuraminidase-labile. The present study was undertaken in order to gain some insight into the nature of the linkage of the neuraminidase-resistant NANA. In contrast to the other NANA residues, it was assumed that the keto group on carbon 2 of the NANA molecule was not ketosidically linked. Borohydride reduction has been used by several investigators to distinguish between ketosidically linked and free NANA ( Roseman, 1962; Warren and Blacklow, 1962). Under these conditions free NANA is reduced and no longer reactive in the chemical determination of Warren (1959) while ketosidically linked NANA is unaffected by borohydride treatment. We wished to learn if the keto group of the NANA residue in monosialoganglioside was reduced by borohydride. Orosomucoid was employed as a control compound because all of the NANA in this material has been demonstrated to be neuraminidase-labile and therefore ketosidically linked ( Gottschalk, 1960b).