SPECIFIC ALPHA-GALACTOSIDASE INHIBITORS, N-METHYLCALYSTEGINES - STRUCTURE ACTIVITY RELATIONSHIPS OF CALYSTEGINES FROM LYCIUM CHINENSE/

An examination of the roots of Lycium chinense (Solanaceae) has result ed in the discovery of 14 calystegines, a cycloheptane bearing an amin o group and three hydroxyl groups, and two polyhydroxylated piperidine alkaloids. Calystegines A(7) and B-5, in addition to the previously k nown 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 1 alpha,2 beta,4 alpha-trihydroxy- nortropane and 1 alpha,2 alpha,4 alpha,7 alpha-tetrahydroxy-nortropane , respectively. L. chinense also had two polyhydroxytropanes bearing a methyl group on the nitrogen atom, unlike the previously reported nor tropane alkaloids. They were established as N-methylcalystegines B-2 a nd C-1, and their N-methyl groups were found to be axially oriented fr om NOE experiments. 1 beta-Amino-3 beta,4 beta,5 alpha-trihydroxycyclo heptane was also present in L. chinense and may be a biosynthetic prec ursor of the calystegines that occur in this plant. Two polyhydroxypip eridine alkaloids, fagomine and 6-deoxyfagomine, were isolated. Calyst egine B-2 is a potent competitive inhibitor of almond beta-glucosidase (K-1 = 1.9 mu M) and coffee bean alpha-galactosidase (K-i = 0.86 mu M ), while M-methylcalystegine B-2 was a more potent competitive inhibit or of the latter enzyme (K-i = 0.47 mu M) than the parent compound but show ed a marked lack of inhibitory activities towards most other gly cosidases. Since this compound is a very specific inhibitor of alpha-g alactosidase and inhibits rat liver lysosomal alpha-galactosidase with a K-i of 1.8 mu M, it may provide a useful experimental model for the lysosomal storage disorder, Fabry's disease. The addition of a hydrox yl group at C6exo, as in calystegines B-1 and C-1, enhances the inhibi tory potential towards beta-glucosidase and beta-galactosidase but mar kedly lowers or abolishes inhibition towards alpha-galactosidase. Henc e, the N-methylation of calystegine C-1 did not enhance its inhibition of alpha-galactosidase. The chemical N-methylation of calystegines A( 1) and B-4 markedly enhanced inhibition of coffee bean alpha-galactosi dase, with K, values of 5.2 mu M and 36 mu M, respectively, but almost eliminated their inhibitor?! potential towards beta-glucosidase and t rehalase, respectively. Thus, methylation of the nitrogen atom signifi cantly altered the specificity of the inhibitors.