Abstract
Nitrobenzodiazepine (NBDZ) is a sedative-hypnotic drug used in the treatment of anxiety and sleep problems. Overdose of NBDZ may cause severe neurological effects, especially for people in drug abuse or addiction. Unfortunately, the clinical intervention for NBDZ poisoning exists some disadvantages such as high prescribing dosage and withdraw symptom. The purpose of this study is to discover and develop new candidate that may provide alternative remedy for NBDZ poisoning. First, we investigate NBDZ nitroreduction in rat enteric contents and characterize the role of enterobacterial nitroreductase in the reductive pathway. In the jejunal microflora, flunitrazepam (FZ), an NBDZ, was demonstrated to be significantly reduced to its urinary metabolite 7-aminoflunitrazepam (7AFZ) under anaerobic condition. Escherichia coli type I nitroreductase NfsB (EC 1.5.1.34) was found in rat jejunal microflora and demonstrated participating in FZ nitroreduction. Second, we investigate FZ nitroreduction by various mutants of NfsB designed from the solved crystal structure and characterize the in vivo potency. Conformational changes occurred in the active site of NfsB-N71S/F124W in contrast to the wild-type enzyme, including the flipping of Trp124 and Phe70 side chains as well as the extended hydrogen bond network between Ser71 and FMN. Thus, the active site pocket of NfsB-N71S/F124W was significantly larger than that of the wild-type, which may be correlated with the increased 7AFZ production under both aerobic and anaerobic conditions. Mouse anti-hypnosis study showed that compared to the vehicle group, 50% decrease of sleeping time was observed in the NfsB-N71S/F124W group with over 50% of enzymes still remained in the mice sera after 24 hours. Third, we investigate the protein structures of human and enterobacteria flavin-containing reductases based on the solved NfsB structure. These reductases share conserved catalytic site in the flavin domains that may provide a similar environment for the reduction of NBDZ in the body. Taken together, we demonstrate that for the first time NfsB-N71S/F124W may be used as an effective therapeutic agent for FZ-induced hypnosis and provide the molecular basis for rational design of NfsB and the like in the future.