Specific alpha-galactosidase inhibitors, N-methylcalystegines--structure/activity relationships of calystegines from...

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 A7 and B5, in addition to the previously known calystegines A3, A5, A6, B1, B2, B3, B4, C1, C2 and N1, were isolated and determined as 1alpha,2beta,4alpha-trihydroxy-nortropane and 1alpha,2alpha,4alpha,7alpha-tetrahydroxy-nort ropane, 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 B2 and C1, and their N-methyl groups were found to be axially oriented from NOE experiments. 1Beta-amino-3beta,4beta,5alpha-trihydroxycyclohepta ne 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 B2 is a potent competitive inhibitor of almond beta-glucosidase (Ki = 1.9 microM) and coffee bean alpha-galactosidase (Ki = 0.86 microM), while N-methylcalystegine B2 was a more potent competitive inhibitor of the latter enzyme (Ki = 0.47 microM) 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 alpha-galactosidase and inhibits rat liver lysosomal alpha-galactosidase with a Ki of 1.8 microM, 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 B1 and C1, enhances the inhibitory potential towards beta-glucosidase and beta-galactosidase but markedly lowers or abolishes inhibition towards alpha-galactosidase. Hence, the N-methylation of calystegine C1 did not enhance its inhibition of alpha-galactosidase. The chemical N-methylation of calystegines A3 and B4 markedly enhanced inhibition of coffee bean alpha-galactosidase, with Ki values of 5.2 microM and 36 microM, respectively, but almost eliminated their inhibitory potential towards beta-glucosidase and trehalase, respectively. Thus, methylation of the nitrogen atom significantly altered the specificity of the inhibitors.