Regioselective enzymatic glycosylation of natural polyhydroxylated compounds: Galactosylation and glucosylation of protopanaxatriol ginsenosides

Ginsenoside Rg(1) (1), the most representative Ginsenoside from Panax Ginse ng C. A. Meyer belonging to the protopanaxatriol family, has been galactosy lated by action of the beta-(1,4)-galactosyltransferase (GalT) from bovine colostrum, using UDP-galactose as an activated sugar donor. The enzyme show ed the well-known specificity for the formation of a beta -linkage with the C-4 OH of the glucose acceptor, but it was not able to discriminate betwee n the two glucose moieties of 1, giving a mixture of mono-and digalactosyla ted derivatives. Other natural Rg(1)-analogues such as F1, Rh-1, Re, as wel l as the synthetic derivative 6'-O-acetyl-Rg(1) have been also galactosylat ed, giving monolactosyl derivatives. GalT was also able to accept UDP-gluco se as an activated sugar donor, giving rise to cellobiosyl derivatives of R g(1).