HOMONOJIRIMYCIN ISOMERS AND N-ALKYLATED HOMONOJIRIMYCINS - STRUCTURALAND CONFORMATIONAL BASIS OF INHIBITION OF GLYCOSIDASES

A series of natural epimers of alpha-homonojirimycin and its N-alkylat ed derivatives have been prepared to investigate the contribution of t he different_chiral centers and conformation of the specificity and po tency of inhibition of glycosidases. These epimers and N-alkylated der ivatives are alpha-homonojirimycin (1), beta-homonojirimycin (2), alph a-homomannojirimycin (3), beta-homoman-nojirimycin (4), alpha-3,4-di-e pi-homonojirimycin (5), beta-4,5-di-epi-homonojirimycin (6), N-methyl- alpha-homonojirimycin (7), and N-butyl-alpha-homonojirimycin (8). Comp ound 1 was a potent inhibitor of a range of alpha-glucosidases with IC 50 values of 1 to 0.01 mu M. Compounds 2, 3, and 4 were surprisingly i nactive as inhibitors of beta-glucosidase and alpha- and beta-mannosid ases but were moderately good as inhibitors of rice and some mammalian alpha-glucosidases. Compound 4 was active in the micromolar range tow ard all alpha-glucosidases tested. Furthermore, compound 4, which supe rimposes well on beta-L-fucose, was a 10-fold more effective inhibitor of alpha-L-fucosidase than 1-deoxymannojirimycin (12) and 3, with a K -i value of 0.45 mu M. Only compounds 5 and 6 showed inhibitory activi ty toward alpha- and beta-galactosidases (6 with an IC50 value of 6.4 mu M against alpha-galactosidase). The high-resolution structure of 1 has been determined by X-ray diffraction and showed a chair conformati on with the C1 OH (corresponding to the C6 OH in 1-deoxynojirimycin) p redominantly equatorial to the piperidine ring in the crystal structur e. This preferred (C1 OH equatorial) conformation was also corroborate d by H-1 NMR coupling constants. The coupling constants for 7 suggest the axial orientation of the C1 OH, while in 8 the C1 OH axial conform ation was not observed. The C1 OH axial conformation appears to be res ponsible for more potent inhibition toward processing alpha-glucosidas e I than alpha-glucosidase II. It has been assumed that the anti-HIV a ctivity of alkaloidal glycosidase inhibitors results from the inhibiti on of processing alpha-glucosidase I, but 1, 7, and 8 were inactive ag ainst HIV-1 replication at 500 mu g/mL as measured by inhibition of vi rus-induced cytopathogenicity in MT-4 cells. In contrast, the EC50 val ue for N-butyl-1-deoxynojirimycin (11), which also inhibits processing cr-glucosidase I, was 37 mu g/mL. Compound 7 has been shown to be a b etter inhibitor of alpha-glucosidase I than 1 and 8 both in vitro and in the cell culture system. These data imply that inhibition of HIV by glycosidase inhibitors can be due to factors other than simply inhibi tion of processing alpha-glucosidase I.