WHY DOES CLOSTRIDIUM-ACETIREDUCENS NOT USE INTERSPECIES HYDROGEN-TRANSFER FOR GROWTH ON LEUCINE

Clostridium acetireducens is the first reported anaerobic bacterium th at is dependent on acetate as an electron acceptor for growth on branc hed-chain amino acids and alanine. The fermentation pathway of leucine and its deamination product alpha-keroisocaproate were studied in thi s organism. Addition of Methanobacterium formicicum to pure cultures o f C. acetireducens stimulated the degradation of alpha-ketoisocaproate but not the degradation of leucine, indicating that the electrons pro duced during the oxidative deamination of leucine were not transferred to hydrogen, This conclusion is supported by the observed low NAD(P)H ferredoxin reductase activity. Not only acetate but also crotonate pr oved to be an appropriate electron sink for the regeneration of NAD(P) (+) in this bacterium. Interestingly, C. acetireducens was shown to fo rm polyhydroxybutyrate during growth on lecuine plus acetate.