Metabolic Energy Generation In Hydrogenosomes Of The Anaerobic Fungus Neocallimastix - Evidence For A Functional-relationship With Mitochondria

Anaerobic eukaryotes are often devoid of mitochondria but contain special organelles separated from the cytosol by a single (in fungi) or a double (in protozoa) membrane. Hydrogenosomes from the anaerobic fungus Neocallimastix sp. L2 are thought to catalyse the enzymic steps in the ATP-yielding metabolism of malate into acetate, H-2 and CO2. Isolated hydrogenosomes contain a Mg2+- or Mn2+-dependent ATPase activity. This activity is involved in the maintenance of a pH gradient across the hydrogenosomal membrane, which renders these organelles alkaline inside. ATPase activity and DELYTApH generation is sensitive to diethylstilboestrol but not to other known ATPase inhibitors. Typical inhibitors of the mitochondrial ADP/ATP translocase, bongkrekic acid and carboxyatractylate reduced the ATPase activity, suggesting the presence of a nucleotide transporter. Under anaerobic conditions hydrogenosomes produced H-2 and acetate from malate. This process was found to be dependent on the external supply of ATP or ADP and succinate, and was blocked by protonophores, diethylstilboestrol, and the inhibitors bongkrekic acid and carboxyatractylate. These results demonstrate that hydrogenosomes of Neocallimastix sp. L2 perform the essential functions required for the generation of metabolic energy from malate. It is suggested that hydrogenosomes are functionally related to mitochondria but lack an outer membrane.