METABOLIC ENERGY GENERATION IN HYDROGENOSOMES OF THE ANAEROBIC FUNGUSNEOCALLIMASTIX - EVIDENCE FOR A FUNCTIONAL-RELATIONSHIP WITH MITOCHONDRIA

Anaerobic eukaryotes are often devoid of mitochondria but contain spec ial organelles separated from the cytosol by a single (in fungi) or a double (in protozoa) membrane. Hydrogenosomes from the anaerobic fungu s Neocallimastix sp. L2 are thought to catalyse the enzymic steps in t he ATP-yielding metabolism of malate into acetate, H-2 and CO2. Isolat ed hydrogenosomes contain a Mg2+- or Mn2+-dependent ATPase activity. T his activity is involved in the maintenance of a pH gradient across th e hydrogenosomal membrane, which renders these organelles alkaline ins ide. ATPase activity and DELYTApH generation is sensitive to diethylst ilboestrol but not to other known ATPase inhibitors. Typical inhibitor s of the mitochondrial ADP/ATP translocase, bongkrekic acid and carbox yatractylate reduced the ATPase activity, suggesting the presence of a nucleotide transporter. Under anaerobic conditions hydrogenosomes pro duced H-2 and acetate from malate. This process was found to be depend ent on the external supply of ATP or ADP and succinate, and was blocke d by protonophores, diethylstilboestrol, and the inhibitors bongkrekic acid and carboxyatractylate. These results demonstrate that hydrogeno somes of Neocallimastix sp. L2 perform the essential functions require d for the generation of metabolic energy from malate. It is suggested that hydrogenosomes are functionally related to mitochondria but lack an outer membrane.