To demonstrate the importance of electron siphoning by the metronidazole reductase system from reduced ferredoxin to the mechanism of action of the drug in Clostridium pasteurianum, the effects of the reduction of metronidazole on the phosphoroclastic reaction were studied. Metronidazole concentrations between 0.5 and 5 mM caused a significant increase in acetyl phosphate production by the phosphoroclastic reaction compared to the control system without metronidazole. When this enzymatic reaction was assayed by standard manometric techniques under nitrogen gas, two simultaneous effects of electron siphoning were demonstrated: (i) the electrons from reduced ferredoxin were initially consumed for the reduction of metronidazole instead of being evolved as H2 via the ferredoxin-linked hydrogenase and (ii) phosphoroclastic activity was stimulated, with augmented production of CO2 and acetyl phosphate. This work further supports the notion of preferential scavenging of electrons away from ferredoxin-linked enzymatic reactions by metronidazole reductase(s) in C. pasteurianum
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