SUBSTRATE-SPECIFICITY OF THE TRYPANOSOMA-CRUZI TRANS-SIALIDASE

Trypanosoma cruzi trypomastigotes acquire sialic acid (SA) from host glycoconjugates by means of a plasma membrane-associated trans-sialidase (TS). Here we study the substrate specificity of TS, which differs from all known sialyl-transferases in that it does not require cytidine monophosphate (CMP)-SA as donor. The T.cruzi TS reversibly transfers SA to saccharides with terminal beta-Gal (but not alpha-Gal) residues. Donors are saccharides with SA linked to terminal beta-Gal residues by (alpha2-3), but not (alpha2-6) bonds. The type of beta-linkage of the terminal Gal residue is of minor importance (beta1-4 and beta1-6 are slightly better than beta1-3), whereas chain length and the structure of additional vicinal sugar residues are not relevant. SA on the surface of living trypomastigotes of T.cruzi is transferred back and forth between the parasite surface and acceptor molecules with terminal beta-Gal, either in solution or on the surface of neighbouring mammalian cells. Addition of fucose residue on or close to the terminal galactose impairs TS activity. As a consequence, the enzyme acts poorly on the E-selectin ligand sialyl-Lewis(x) and its precursor Lewis(x), and in vitro adhesion of TS-treated neutrophils to L-cells expressing L-selectin is not affected. Modifications in the structure of the (alpha2-3)-linked N-acetyl-neuraminic acid (Neu5Ac) (deoxy or methoxy) of the donor molecules do not impair transfer if the changes are at C-9, whereas changes at C4, C-7 and C-8 impair the ability to donate the modified SA. Compounds with modified C4 and C-8 inhibit TS at relatively high inhibitor/substrate ratios.