Enzymatic synthesis of the glycosides of calystegines B1 and B2 and their glycosidase inhibitory activities

Several glycosides of calystegines B1 and B2 were synthesized by use of rice α-glucosidase and the whole cells of Rhodotorula lactosa, and their glycosidase inhibitory activities were investigated. Incubation of a mixture of calystegine B1 and maltose with rice α-glucosidase gave 3-O-α-d-glucopyranosylcalystegine B1 (2, 11.3%). An enzymatic β-transglucosylation reaction of calystegines B1 or B2 with cellobiose using the whole cells of R. lactosa gave 3-O-β-d-glucopyranosylcalystegine B1 (1) (0.9%) or 4-O-β-d-glucopyranosylcalystegine B2 (3, 11.2%), respectively, while a similar β-transgalactosylation of calystegine B2 from lactose gave 4-O-β-d-galactopyranosylcalystegine B2 (4, 10.1%). The glycosylation of calystegines B1 and B2 markedly decreased or abolished their inhibition against β-glucosidase, α- or β-galactosidase. Compound 4 however retained more or less the potency of calystegine B2 against trehalase. Interestingly, compound 1 was a noncompetitive inhibitor of rice α-glucosidase, with a Ki value of 0.9 ± 0.1 μM.