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 chai n of plant mitochondria. In order to identify its site of action, the effects of BOAP on mitochondrial membranes of potato tubers were studi ed using electron paramagnetic resonance (EPR), and compared to those of thenoyltrifluoroacetone (TTFA), an inhibitor of complex II, Under a erobic conditions, BOAP induced no change in the EPR signal of the Fe- S center S-3 of succinate dehydrogenase (SDH), unlike TTFA which decre ased the height and modified the spectrum of the signal, In the presen ce of ferricyanide (FeCN), BOAP weakly enhanced the S-3 signal whereas TTFA changed only its shape but not its height. Under anaerobic condi tions, center S-l was completely reduced in the presence of BOAP, whil e in the presence of TTFA center S-l was less reduced than in the cont rol, and the center S-3 signal was not different from that under aerob ic conditions. Reoxidation of anaerobic preparations was obtained by O -2 or FeCN addition. Oxygen was ineffective as an oxidant in the prese nce of either inhibitor, but a partial reoxidation of center S-3 was o btained 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.