Specific α‐Galactosidase Inhibitors, N‐Methylcalystegines Structure/Activity Relationships of Calystegines from Lycium Chinense

An examination of the roots of Lycium chinense (Solanaceae) has resulted in the discovery of 14 calystegines, a cycloheptane bearing an amino group and three hydroxyl groups, and two polyhydroxylated piperidine alkaloids. Calystegines A 7 and B 5 in addition to the previously known calystegines A 3 , A 5 , A 6 , B 1 , B 2 , B 3 , B 4 , C 1 , C 2 , and N 1 , were isolated and determined as 1α,2β,4α‐trihydroxy‐nortropane and 1α,2β,4α,7α‐tetrahydroxy‐nortropane, respectively. L. chinense also had two polyhydroxytropanes bearing a methyl group on the nitrogen atom, unlike the previously reported nortropane alkaloids. They were established as N ‐methylcalystegines B 2 and C 1 , and their N ‐methyl groups were found to be axially oriented from NOE experiments. 1α‐Amino‐3β,4β,5α‐trihydroxycycloheptane was also present in L. chinense and may be a biosynthetic precursor of the calystegines that occur in this plant. Two polyhydroxypiperidine alkaloids, fagomine and 6‐deoxyfagomine, were isolated. Calystegine B 2 is a potent competitive inhibitor of almond β‐glucosidase ( K i = 1.9 μM) and coffee bean α‐galactosidase ( K i = 0.86 μM), while N ‐methylcalystegine B 2 was a more potent competitive inhibitor of the latter enzyme ( K i = 0.47 μM) than the parent compound but showed a marked lack of inhibitory activities towards most other glycosidases. Since this compound is a very specific inhibitor of α‐galactosidase and inhibits rat liver lysosomal α‐galactosidase with a K i of 1.8 μM, it may provide a useful experimental model for the lysosomal storage disorder, Fabry's disease. The addition of a hydroxyl group at C6exo, as in calystegines B 1 and C 1 , enhances the inhibitory potential towards β‐glucosidase and β‐galactosidase but markedly lowers or abolishes inhibition towards α‐galactosidase. Hence, the N‐methylation of calystegine C 1 did not enhance its inhibition of α‐galactosidase. The chemical N‐methylation of calystegines A 3 and B 4 markedly enhanced inhibition of coffee bean α‐galactosidase, with K i values of 5.2 μM and 36 μM, respectively, but almost eliminated their inhibitory potential towards β‐glucosidase and trehalase, respectively. Thus, methylation of the nitrogen atom significantly altered the specificity of the inhibitors.