Dicyclohexylcarbodiimide inhibition of succinate- and ubiquinol-cytochrome c reductase in beef heart mitochondria.

We have found that dicyclohexylcarbodiimide (DCCD) inhibits both the succinate-cytochrome c and the ubiquinol-cytochrome c reductases in cytochrome c-depleted mitochondria. On the other hand the succinate-ubiquinone reductase is not decreased at the same levels of the inhibitor. The inhibition curve of DCCD results sigmoidal for succinate-cytochrome c reductase, whereas it is hyperbolic for the ubiquinol-1-cytochrome c reductase, with also a lower apparent KI. The inhibition appears dependent both on the time of preincubation and on the mitochondrial concentration. The apparent Km for ubiquinol-1 is increased and the maximal velocity of ubiquinol-cytochrome c reductase is decreased by DCCD. The effects do not appear to be caused by unspecific modification of the physicochemical state of the bc1 region of the respiratory chain. The results therefore suggest the presence of a DCCD-sensitive electron transfer step in the redox pathways from ubiquinol to cytochrome c.