Hydrogenosome, a Cytoplasmic Organelle of the Anaerobic Flagellate Tritrichomonas foetus, and Its Role in Pyruvate Metabolism

Abstract Homogenates of Tritrichomonas foetus have pyruvate synthase and hydrogenase, but lack pyruvate-formate lyase and formate dehydrogenase. Pyruvate synthase was measured by three anaerobic methods: (a) CO2-pyruvate exchange, which was inhibited by CoA and by several electron acceptors; (b) pyruvate oxidation as measured by the CoA-dependent reduction of methyl viologen, FMN, FAD, riboflavin or clostridial ferredoxin; (c) pyruvate synthesis from acetyl-CoA, CO2, and H2 in the presence of methyl viologen. Hydrogenase as measured by the reduction of methyl viologen, FMN, FAD, riboflavin, or ferredoxin by H2 showed no cofactor requirements. Both enzymes are O2-sensitive and require thiol compounds. As shown by differential sedimentation and isopycnic centrifugation, these activities are in a subcellular particle identified previously as the site of malate and α-glycerophosphate dehydrogenases. Al-though resembling a microbody, this particle lacks catalase and NADH oxidase and is not a peroxisome. It plays a central role in pyruvate degradation, generating electrons leading to molecular hydrogen, by reactions similar to those occurring in clostridia. The term hydrogenosome is suggested to designate this organelle.