DETERMINATION OF THE SPECIFICITIES OF RAT-LIVER GAL(BETA-1-4)GLCNAC ALPHA-2,6-SIALYLTRANSFERASE AND GAL(BETA-1-3 4)GLCNAC ALPHA-2,3-SIALYLTRANSFERASE USING SYNTHETIC MODIFIED ACCEPTORS/

Apparent kinetic parameters have been measured for the transfer of N-a cetyl-D-neuraminic acid (Neu5Ac) from CMP-Neu5Ac to analogues of the G al(beta1-4)GlcNAc (type II) and Gal(beta1-3)GlcNAc (type I) substrates by the rat liver Gal(beta1-4)GlcNAc alpha2,6-sialyltransferase and th e Gal(beta1-3/4)GlcNAc alpha2,3-sialyltransferase. In these acceptor a nalogues, the substituents of the pyranose rings were modified, partic ularly by deoxygenation, to identify (i) the key polar groups required for efficient transfer and (ii) the substituents that can be removed or modified. A topography including the 6-hydroxyl of the betaGal and the 2-acetamido of the GlcNAc unit is required for transfer to a termi nal type II disaccharide by the alpha2,6-sialyltransferase. The other hydroxyls can be replaced by hydrogen without a substantial decrease i n activity. The alpha2,3-sialyltransferase requires the 3-, 4-, and 6- hydroxyls of the terminal betaGal and some contribution from the subte rminal sugar. This may explain the cross-reactivity of this enzyme for the type I and type II acceptors. For both enzymes, an influence of t he hydrophobic nature of the aglycone is noticed. The results allow an evaluation of the efficiency of the transfer of Neu5Ac to modified su bstrates.