Enzymatic elaboration of oxime-linked glycoconjugates in solution and on liposomes

Oxime formation is a convenient one-step method for ligating reducing sugars to surfaces, producing a mixture of closed ring alpha- and beta-anomers along with open-chain (E)- and (Z)-isomers. Here we show that despite existing as a mixture of isomers, N-acetylglucosamine (GlcNAc) oximes can still be substrates for beta(1,4)-galactosyltransferase (beta 4GalT1). beta 4GalT1 catalysed the galactosylation of GlcNAc oximes by a galactose donor (UDP-Gal) both in solution and in situ on the surface of liposomes, with conversions up to 60% in solution and ca. 15-20% at the liposome surface. It is proposed that the beta-anomer is consumed preferentially but long reaction times allow this isomer to be replenished by equilibration from the remaining isomers. Adding further enzymes gave more complex oligosaccharides, with a combination of alpha-1,3-fucosyltransferase, beta 4GalT1 and the corresponding sugar donors providing Lewis X coated liposomes. However, sialylation using T. cruzi trans-sialidase and sialyllactose provided only very small amounts of sialyl Lewis X (sLe(x)) capped lipid. These observations show that combining oxime formation with enzymatic elaboration will be a useful method for the high-throughput surface modification of drug delivery vehicles, such as liposomes, with cell-targeting oligosaccharides.

Automated summary

Despite existing as a mixture of isomers, N-acetylglucosamine (GlcNAc) oximes can still be substrates for beta(1,4)-galactosyltransferase (beta 4GalT1). Combining oxime formation with enzymatic elaboration can be a useful method for the high-throughput surface modification of drug delivery vehicles, such as liposomes, with cell-targeting oligosaccharides. The beta-anomer is consumed preferentially but long reaction times allow this isomer to be replenished by equilibration from the remaining isomers.