REACTION SITE OF 6-BENZOYLAMINOPURINE INHIBITION IN PLANT MITOCHONDRIAL ELECTRON-TRANSPORT PATHWAY

In order to determine the site of action of 6-benzoylaminopurine (RD1), which inhibits the succinate dehydrogenase mitochondrial complex (complex II) specifically, its effects have been investigated and compared to the effects of the well-known inhibitor of that complex, thenoyltrifluoroacetone (TTFA). The two compounds were less active on state 4 than on state 3 rate of succinate oxidation by potato tuber (Solanum tuberosum) mitochondria. TTFA (below 800 muM) exhibited even a stimulatory effect on the state 4. RD1 and TTFA had the same effect on the succinate-cytochrome c reductase and succinate-oxidase activities. With respect to succinate-Q/DCPIP reductase activity, TTFA was slightly more efficient than RDI. On the succinate-DCPIP reductase activity, RDI had a rather weak effect and the effect of TTFA was variable. The two compounds inhibited (35%) PMS/cytochrome c reductase activity, whereas only a very slight inhibition occurred with PMS/DCPIP reductase activity. A differential effect was observed on the succinate-FeCN reductase activity, RDI being a less potent inhibitor than TTFA. For these different activities with artificial acceptors, the apparent affinity constants (K'i) of the two compounds were high and of the same order: between 100 and 200 muM for the sites of high affinity. RD1, but not TTFA, was competitive with respect to PMS. Both compounds exhibited no competitive behaviour on succinate oxidase, succinate-PMS/DCPIP reductase and succinate-Q/DCPIP reductase activities. From these comparative results it is concluded that RDI may act at the first two Fe-S centres (S1, S2), inhibiting the semi-quinone production, while TTFA would act at the S3 centre.