| Popis: |
The sialic acid of gangliosides not containing GalNAc (i.e., GM3, GD3) is readily hydrolyzed either enzymatically by sialidases or chemically in acid conditions. On the other hand, in gangliosides having the sialic acid on the internal galactose residue linked to GalNAc (i.e., GM1, GM2) the Neu5Ac is largely resistant to acid or enzymatic hydrolysis. In the present work GM1 (NH4+), GM1(H+), and several de-acetylated derivatives in the sialic acid and in both sialic acid and N-acetylgalactosamine moieties were prepared. Studies by counterion exchange with DEAE-Sephadex A-25 and Dowex 50WX8, acid-base titration, and acid or enzymatic hydrolysis with sialidases were performed on these derivatives. Our results provide cumulative evidence supporting that a hydrogen bonding interaction between the hydrogen atom of un-ionized carboxyl group in Neu5Ac and the oxygen atom of the carbonyl group in GalNAc reduces the dissociation of the Neu5Ac carboxyl group and impairs its enzymatic and acid hydrolysis. In addition, our results suggest that the enzymatic hydrolysis of the ionized form of sialic acid in GM1(Na+) and GM1(NH4+) is impaired by a second hydrogen bonding interaction between the proton of the acetamide group in GalNAc and the carbonyl moiety of the carboxyl group of the Neu5Ac. |
