Pathways of acetate and propionate production in adult Fasciola hepatica

In adult F. hepatica pyruvate is decarboxylated via pyruvate dehydrogenase to acetyl-CoA; acetyl-CoA is then cleaved to acetate via three possible mechanisms (1) carnitine dependent hydrolysis, (2) CoA transferase, (3) reversal of a GTP dependent acyl-CoA synthetase. Of these three systems, CoA transferase has by far the greatest activity. Propionate production by F. hepatica is similar to the mammalian system, succinate being metabolized via succinic thiokinase, methylmalonyl-CoA isomerase, methyl-malonyl-CoA racemase and propionyl-CoA carboxylase to propionyl-CoA. Propionyl-CoA is then cleaved to propionate by the same three pathways as acetyl-CoA. No ATP or GTP production could be demonstrated when acetyl- or propionyl-CoA were incubated with homogenates of F. hepatica. This indicates that carnitine dependent hydrolysis or CoA transferase are the major pathways of acetyl- or propionyl-CoA breakdown. The CoA transferase reaction would result in the conservation of the bond energy although there is no net ATP synthesis.