A soluble fumarate reductase in Trypanosoma brucei procyclic trypomastigotes

The enzyme NADH‐fumarate reductase associated with the membrane fraction of Trypanosoma brucei procyclic trypomastigotes, can be solubilized by more than 50% when increasing the ionic strength to the equivalent of 150 mM KCI. The apparent KMs for NADH (125 μM) and fumarate (50 μM) remain close to those previously reported for the membrane‐bound form of this enzyme. Other electron acceptors (i.e. oxygen or cytochrome c) appear to accept electrons in the absence of fumarate (KM for cytochrome c= 50 μM). The drug L‐092,201 (Merck, Sharp and Dohme Research Laboratories, Rahway, NJ), an inhibitor of the membrane‐bound fumarate reductase, also blocked the solubilized enzyme. Given the relatively high ionic strength of the intracellular environment we propose that, in vivo, the enzyme fumarate reductase is in the mitochondrial matrix or in the soluble fraction of another intracellular compartment.