ACID AND ENZYMATIC-HYDROLYSIS OF THE INTERNAL SIALIC-ACID RESIDUE IN NATIVE AND CHEMICALLY-MODIFIED GANGLIOSIDE GM1

The sialic acid of gangliosides not containing GalNAc (i.e., GM3, GD3) is readily hydrolyzed either enzymatically by sialidases or chemicall y in acid conditions. On the other hand, in gangliosides having the si alic acid on the internal galactose residue linked to GalNAc (i.e., GM 1, GM2) the Neu5Ac is largely resistant to acid or enzymatic hydrolysi s. In the present work GM1(NH4+), GM1(H+), and several de-acetylated d erivatives in the sialic acid and in both sialic acid and N-acetylgala ctosamine moieties were prepared. Studies by counterion exchange with DEAE-Sephadex A-25 and Dowex 50WX8, acid-base titration, and acid or e nzymatic hydrolysis with sialidases were performed on these derivative s. Our results provide cumulative evidence supporting that a hydrogen bonding interaction between the hydrogen atom of un-ionized carboxyl g roup in Neu5Ac and the oxygen atom of the carbonyl group in GalNAc red uces the dissociation of the Neu5Ac carboxyl group and impairs its enz ymatic and acid hydrolysis. In addition, our results suggest that the enzymatic hydrolysis of the ionized form of sialic acid in GM1(Na+) an d GM1(NH4+) is impaired by a second hydrogen bonding interaction betwe en the proton of the acetamide group in GalNAc and the carbonly moiety of the carboxyl group of the Neu5Ac.