Nucleoside N-ribohydrolases from protozoan parasites are targets for inhibi
tor design in these purine-auxotrophic organisms. Purine-specific and purin
e/pyrimidine-nonspecific nucleoside hydrolases have been reported. Iminorib
itols that are 1-substituted with meta- and para-derivatized phenyl groups
[(1S)-substituted 1,4-dideoxy-1,4-imino-D-ribitols] are powerful inhibitors
for the nonspecific nucleoside N-ribohydrolases, but are weak inhibitors f
or purine-specific isozymes [Parkin, D. W., Limberg, G., Tyler, P. C., Furn
eaux, R. H., Chen, X.-Y., and Schramm, V. L. (1997) Biochemisty 36, 3528-35
34]. Binding of these inhibitors to nonspecific nucleoside hydrolase occurs
primarily via interaction with the iminoribitol, a ribooxocarbenium ion an
alogue of the transition state. Weaker interactions arise from hydrophobic
interactions between the phenyl group and the purine/pyrimidine site. In co
ntrast, the purine-specific enzymes obtain equal catalytic potential from l
eaving group activation and ribooxocarbenium ion formation. Knowledge of th
e reaction mechanisms and transition states for these enzymes has guided th
e design of isozyme-specific transition state analogue inhibitors. New synt
hetic efforts have produced novel inhibitors that incorporate features of t
he leaving group hydrogen-bonding sites while retaining the iminoribitol gr
oup. These compounds provide the first transition state analogue inhibitors
for purine-specific nucleoside hydrolase. The most inhibitory I-substitute
d iminoribitol heterocycle is a sub-nanomolar inhibitor for the purine-spec
ific nucleoside hydrolase from Trypanosoma brucei brucei. Novel nanomolar i
nhibitors are also described for the nonspecific nucleoside hydrolase from
Crithidia fasciculata. The compounds reported here are the most powerful im
inoribitol inhibitors yet described for the nucleoside hydrolases.