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

Ginsenoside Rg1 (1), the most representative Ginsenoside from Panax Ginseng C.A. Meyer belonging to the protopanaxatriol family, has been galactosylated by action of the beta-(1,4)-galactosyltransferase (GalT) from bovine colostrum, using UDP-galactose as an activated sugar donor. The enzyme showed 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 between the two glucose moieties of 1, giving a mixture of mono- and digalactosylated derivatives. Other natural Rg1-analogues such as F1, Rh1, Re, as well as the synthetic derivative 6'-O-acetyl-Rg1 have been also galactosylated, giving monolactosyl derivatives. GalT was also able to accept UDP-glucose as an activated sugar donor, giving rise to cellobiosyl derivatives of Rg1.