Summary: The rumen entodiniomorphid ciliate protozoon was shown, by biochemical and electron microscopic techniques, to possess hydrogenosomes. After differential centrifugation of whole cell homogenates the hydrogenosomal marker enzymes pyruvate: ferredoxin oxidoreductase and hydrogenase were recovered predominantly (61% and 70% of activity respectively) in the large granular fractions that were sedimented by centrifugation for 10 g-min (fraction P1) and 10 g-min (fraction P2). These subcellular fractions contained membrane-bound organelles that were approximately 0.4-0.6 μm in diameter and which had a mean equilibrium density of 1.22-1.24 g ml after isopycnic centrifugation in sucrose gradients. Malate dehydrogenase (decarboxylating) activity, however, was predominantly non-sedimentable after centrifugation for 6 x 10 g-min. Numerous hydrogenosome-like organelles were present in the ectoplasm and endoplasm of the cell. Hydrogenase activity was demonstrated and localized in the protozoan cell using a novel staining procedure with distyryl nitroblue tetrazolium chloride (DSNBT).


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