Thenoyltrifluoroacetone (TTA) and carboxin inhibit soluble ubiquinone-deficient succinate: ubiquinone reductase according to the mixed type (with respect to added Q2) inhibition. pattern. The Ki values for the inhibitors are mutually dependent, thus indicating the presence of a single binding site for both TTA and carboxin. The enolic form of TTA was shown to be the species interacting with the enzyme. Carboxin prevents the alkali-induced inactivation of the membrane-bound succinate dehydrogenase without having any effect on the reconstitution of succinate: ubiquinone reductase from the soluble dehydrogenase and b-c1 complex. The reduction of the respiratory chain by succinate protects succinate dehydrogenase against inactivation (solubilization) by alkali; under these conditions, carboxin does not affect the inactivation process. The cumulative data suggest that the degree of the mutual mobility of the succinate dehydrogenase smaller subunit and ubiquinone reactivity-conferring protein (QPs) is a prerequisite for the catalytic mechanism of succinate: ubiquinone reductase. A mechanism of the enzyme inhibition by TTA and carboxin is proposed, which consists in non-covalent cross-linking of the subunits by the inhibitors.