A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads (the most studied of the hydrogenosome-containing microorganisms) produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA transferase and succinate thiokinase. Superoxide dismutase, malate dehydrogenase (decarboxylating), ferredoxin, adenylate kinase and NADH:ferredoxin oxido-reductase are also localized in the hydrogenosome. This organelle is thought to have evolved from mitochondria, though in the case of trichomonad hydrogenosomes the question remains open.


Hydrogenosomes were isolated, purified, biochemically characterized and named in the early 1970s by D. G. Lindmark and M. Müller at Rockefeller University. In addition to this seminal study on hydrogenosomes, they also demonstrated, for the first time, the presence of pyruvate:ferredoxin oxido-reductase and hydrogenase in eukaryotes. Further studies were subsequentially conducted on the biochemical cytology and subcellular organization of anaerobic protozoan parasites (Trichomonas vaginalis, Tritrichomonas foetus, Monocercomonas sp., Giardia lamblia, Entamoeba sp., and Hexamita inflata). Using information obtained from hydrogenosomal and biochemical cytology studies these researchers determined the mode of action of metronidazole (Flagyl) in 1976. Metronidazole is today recognized as the gold standard chemotherapeutic agent for the treatment of anaerobic infections caused by prokaryotes (clostridia, Bacteroides, Helicobacter) and eukaryotes (Trichomonas, Tritrichomonas, Giardia, Entamoeba). Metronidazole is taken up by diffusion. Once taken up by anaerobes, it is non-enzymatically reduced by reduced ferredoxin which is produced by the action of pyruvate:ferredoxin oxido-reductase. This reduction cause the production of toxic products (to the anaerobic cell) and allows for selective accumulation in anaerobes.


Hydrogenosomes are approximately 1 micrometre in diameter and are so called because they produce molecular hydrogen. Like mitochondria, they are bound by distinct double membranes and one has an inner membrane with some cristae-like projections. Some hydrogenosomes may have evolved from mitochondria by the concomitant loss of classical mitochondrial features, most notably its genome. A hydrogenosomal genome could not be detected in Neocallimastix, Trichomonas vaginalis and Tritrichomonas foetus. However, a hydrogenosomal genome has been detected in the cockroach ciliate Nyctotherus ovalis in 1998.


The best studied hydrogenosomes are those of the sexually transmitted parasites Trichomonas vaginalis and Tritrichomonas foetus and those from rumen chytrids such as Neocallimastix.

The anaerobic ciliated protozoan Nyctotherus ovalis, found in the hindgut of several species of cockroach, has numerous hydrogenosomes that are intimately associated with endosymbiotic methane-producing archaea, the latter using the hydrogen produced by the hydrogenosomes. The matrix of N. ovalis hydrogenosomes contains ribosome-like particles of the same size as a numerous type of ribosome (70s) of the endosymbiotic methanogenic archaea. This suggested the presence of an organellar genome which was indeed discovered by Akhmanova and later partly sequenced by Boxma.


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