Specific α-galactosidase inhibitors, N-methylcalysegines. Structure/activity relationships of calystegins 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 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 1α,2β,4α-trihydroxy-nortropane and 1α,2α,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 B₂ and C₁, and their N-methyl groups were found to be axially oriented from NOE experiments. 1β-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 B₂ is a potent competitive inhibitor of almond β-glucosidase (K(i) = 1.9 μM) and coffee bean a galactosidase (K(i) = 0.86 μM), while N-methylcalystegine B₂ 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 a-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 B₁ and C₁, 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 C₁ did not enhance its inhibition of α-galactosidase. The chemical N-methylation of calystegines A₃ and B₄ 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.