Reaction site of carboxanilides and of thenoyltrifluoroacetone in complex II.

Oxathiin carboxanilides are systemic fungicides that inhibit the oxidation of succinate by interrupting electron transport between succinate dehydrogenase [succinate:(acceptor) oxidoreductase, EC 1.3.99.1] and coenzyme Q. Kinetic and electron paramagnetic resonance studies have established that the specific binding site of carboxanilides and of thenoyltrifluoroacetone responsible for the inhibition is the same. Although the binding of carboxanilides to membrane preparations of the dehydrogenase is very tight (Ki = 0.01-0.1 microM), it is noncovalent. Identification of the membrane component(s) to which specific binding occurs has therefore required the introduction of a photoaffinity label onto the carboxanilide molecule. By using [G-3H]3'-azido-5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide, it was found, in accord with earlier data with other carboxanilides, that unresolved complex II specifically binds about 0.6 mol of the inhibitor per mol of succinate dehydrogenase in equilibrium dialysis experiments. The resolved components of the complex, succinate dehydrogenase and the two binding peptides CII-3 and CII-4, failed to bind the inhibitor; however, when these were recombined with reconstitution of coenzyme Q reductase activity, the initial binding titer was restored. Azidocarboxanilide-inhibited complex II was irradiated to generate covalent linkages with the binding site, and the components of the complex were separated on polyacrylamide gel. Most of the specifically bound inhibitor was found in the low molecular weight binding peptides and phospholipids.