Design, synthesis, and evaluation of 9-D-ribityl-1,3,7-trihydro-2,6,5-purinetrione, a potent inhibitor of riboflavin synthase and lumazine synthase

Reduction of 5-nitro-6-D-ribitylaminouracil (9) afforded 5-amino-6-D-ribity laminouracil (1), which reacted with ethyl chloroformate to yield 5-ethylca rbamoyl-6-D-ribitylaminouracil (12), The latter compound was cyclized to 9- D-ribityl-1,3,7-trihydropurine-2,6,8-trione (13), which was found to be a r elatively potent inhibitor of both Escherichia coli riboflavin synthase (K- i 0.61 muM)and Bacillus subtilis lumazine synthase (K-i 46 muM). Molecular modeling of the lumazine synthase-inhibitor complex indicated the possibili ty for hydrogen bonding between the Lys135 epsilon -amino group of the enzy me and both the 8-keto group and the 4'-hydroxyl group of the ligand. A bis ubstrate analogue of the riboflavin synthase-catalyzed reaction, 1,4-bis[1- (9-D-ribityl-1,3,7-trihydropurine-2,6,8-trionyl)]butane (18), was also synt hesized using a similar route and was found to be inactive as an inhibitor of both riboflavin synthase and lumazine synthase.