Acetate:succinate CoA-transferase in the anaerobic mitochondria of Fasciola hepatica

Fasciola hepatica contains anaerobically functioning mitochondria that produce acetate and propionate, the main endproducts excreted by this parasite. The final reactions in the pathways leading to these endproducts are performed by acetate:succinate CoA-transferase (ASCT) and propionate:succinate CoA-transferase (PSCT), respectively. The enzymes catalysing these essential reactions in anaerobic mitochondria are still not characterized, nor are the corresponding genes identified. Here we describe the identification of the gene that codes for the F. hepatica ASCT. The F. hepatica gene was heterologously expressed and studies on the corresponding enzyme activity showed that the enzyme is indeed a transferase and uses a ping-pong bi-bi reaction mechanism, like most other known CoA-transferases. This F. hepatica CoA-transferase was shown to be a true transferase and not a hydrolase, as it needs an acceptor for optimal activity. Our studies demonstrated that the F. hepatica ASCT can use other CoA-acceptors than succinate, such as propionate, acetate and butyrate, and is in fact a short-chain acyl-CoA-transferase. We further showed that this F. hepatica CoA-transferase can also catalyze the PSCT reaction, which is responsible for the production of propionate. Analysis of the amino acid sequence of F. hepatica clearly indicated the presence of a mitochondrial targeting sequence, and in CHO cells the enzyme is indeed present in the mitochondrial fraction. F. hepatica ASCT is the first ASCT identified in anaerobic mitochondria. It is homologous to the hydrogenosomal ASCT we earlier identified in Trichomonas vaginalis, but not to the ASCT present in the aerobic mitochondria of Trypanosoma brucei.