Addition of
sialic acid residues in the human pathogen Trypanosoma cruzi
glycoconjugates is mediated by a
trans-sialidase and not by a
CMP-sialic acid:
glycoconjugate sialyltransferase. Incubation of
trans-sialidase with N-[
galactose-14C]acetyllactosamine and O-linked
oligosaccharides, N-linked
glycopeptides (both obtained from
fetuin) or
sialyllactose showed that the last three compounds were donors of
sialic acid residues to the first one. Moreover, N- and O-linked
oligosaccharides in
asialofetuin and asialomucin, respectively, served as acceptors of
sialic acid units.
Gangliosides GM3, GD1a and GT1b but not GM2, GM1a nor GD1b donated
sialic acid units to N-acetyllactos
amine when incubated with
trans-sialidase. This showed that only
sialic acid units bound to terminal galactosyl residues were transferred. GM1a was converted to GD1a, and GD1b to GT1b when incubated with the appropriate donor. The fact that asialo-GM1a was converted to a
ganglioside migrating as GD1a on thin-layer chromatography suggested that
sialic acid units may be transferred to internal galactosyl residues, although once linked to those residues they can not be further transferred to other
glycoconjugates.
Sialic acid residues linked alpha 2,3- but not alpha 2,6- or alpha 2,8- were transferred by the
trans-sialidase.
Methyl beta-galactoside but not methyl alpha-galactoside served as acceptor of
sialic acid units, thus suggesting that terminal alpha-linked galactosyl units in T. cruzi and mammalian
glycoproteins are not sialylated by the
enzyme. As the
trans-sialidase employed in these experiments has been shown to be located on the external surface of the parasite and to be shed to the medium, the relatively broad specificity shown by the
enzyme with respect to
protein- and
lipid-linked oligosaccharides strongly suggests that
infection by T. cruzi might alter the
sialic acid distribution in
glycoproteins and
glycolipids of the mammalian host.