Unusual β-D-xylosides that prime glycosaminoglycans in animal cells

The biosynthesis of glycosaminoglycans (GAG) takes place while the polysaccharide chains are usually attached to a proteoglycan core protein. Cells also will assemble GAG chains on β-D-xylosides containing hydrophobic aglycones. In order to evaluate the relationship of the structure of the sugar to priming activity of the glycoside, we synthesized β-D-xyloside analogs in which the hydroxyls were substituted with hydrogen, fluorine, - O-methyl, amino, -O-isopropyl, and -O-benzyl groups. Epimers at the 2-, 3-, and 4-position of xylose also were made. Their ability to prime GAGs was tested in Chinese hamster ovary cells by measuring ³⁵SO₄ incorporation into polysaccharide chains and by assaying the transfer of galactose to the xylosides by galactosyltransferase I (UDP-D-galactose:xylose β1-4- galactosyltransferase) in vitro. All of the analogs failed to act as primers of GAGs in vivo and as substrates in vitro with the following exceptions. Substitution of 2-OH and 3-OH with -OCH₃ were active at high concentration (1 mM), but the deoxygenated derivatives were inactive. Efficient priming also occurred on a derivative with fluorine instead of the 3-OH group, suggesting that the oxygen atoms at C-2 and C-3 were involved as hydrogen bond acceptors. Methylated and deoxy analogs at C-4 were inactive, due to the loss of the acceptor hydroxyl group. Interestingly, benzyl-β-D-threo- pentopyranos-4-uloside (4-keto derivative) and benzyl-4-methyl-β-D- xyloside, with a methyl group in place of an axial hydrogen at C-4, primed GAG chains. Priming by these unusual xylosides suggests the possibility of designing inhibitors of GAG synthesis based on xyloside analogs with reactive groups in key positions.