Thiogalactopyranosides are Resistant to Hydrolysis by α‐Galactosidases

Fluorescently tagged glycosides containing terminal α(1→3) and α(1→4)-linked thio-galactopyranosides have been prepared and tested for resistance to hydrolysis by α-galactosidases. Eight fluorescent glycosides containing either galactose or 5-thio-galactose as the terminal sugar were enzymatically synthesized using galactosyltransferases with lactosyl glycosides as acceptors and UDP-galactose or UDP-5′-thio-galactose, respectively, as donors. The glycosides were incubated with human α-galactosidase A (CAZy family GH27, a retaining glycosidase), Bacteroides fragilis α-1,3-galactosidase (GH110, an inverting glycosidase) or homogenates of MCF-7 human breast cancer cells and NG108-15 rat glioma cells. Substrate hydrolysis was monitored by capillary electrophoresis with fluorescence detection. All compounds containing terminal O-galactose were readily degraded. Their 5-thio-galactose counterparts were resistant to hydrolysis by human α-galactosidase A and the enzymes present in the cell extracts. B. fragilis α-1,3-galactosidase hydrolyzed both thio- and O-galactoside substrates, however, the thio-galactosides were hydrolyzed at only 1-3% of the rate of O-galactosides. The hydrolytic resistance of 5-thio-galactose was also confirmed by an in vivo study using cells in culture. The results suggest that 5-thio-galactosides maybe useful tools for the study of anabolic pathways in cell extracts or in single cells.