EPR studies on 6-benzoylaminopurine and thenoyltrifluoroacetone inhibition sites of succinate dehydrogenase in plant mitochondria

6-Benzoylaminopurine (BOAP) inhibits succinate oxidation at the level of complex II (succinate-ubiquinone reductase) in the respiratory chain of plant mitochondria. In order to identify its site of action, the effects of BOAP on mitochondrial membranes of potato tubers were studied using electron paramagnetic resonance (EPR), and compared to those of thenoyltrifluoroacetone (TTFA), an inhibitor of complex II, Under aerobic conditions, BOAP induced no change in the EPR signal of the Fe-S center S-3 of succinate dehydrogenase (SDH), unlike TTFA which decreased the height and modified the spectrum of the signal, In the presence of ferricyanide (FeCN), BOAP weakly enhanced the S-3 signal whereas TTFA changed only its shape but not its height. Under anaerobic conditions, center S-l was completely reduced in the presence of BOAP, while in the presence of TTFA center S-l was less reduced than in the control, and the center S-3 signal was not different from that under aerobic conditions. Reoxidation of anaerobic preparations was obtained by O-2 or FeCN addition. Oxygen was ineffective as an oxidant in the presence of either inhibitor, but a partial reoxidation of center S-3 was obtained with FeCN in the presence of BOAP. These observations point to two different sites for TTFA and BOAP. Both inhibitors act on the O-2 side of center S-3, BOAP interfering with the environment of the Fe-S cluster of S-3, while TTFA would act on the cluster itself.