To assess the potential involvement of mitochondria in ABA-related hepatotoxicity, we assessed its effects on the bioenergetics of rat liver mitochondria. The results obtained using mitochondria energized with glutamate + malate (electron donors to complex I), succinate (electron donor to complex II) and TMPD/ascorbate (artificial donor of electrons to complex IV) showed that ABA inhibits state-3 respiration in a concentration-dependent manner at concentrations from 5 to 25 μM. According Cisplatin to Chance
and Williams (1955), state-3 respiration involves mitochondria, ADP and a respiratory substrate, and the speed of ADP phosphorylation is the limiting factor of the process. The inhibition observed in the three experiments may result from the direct action of abamectin on the respiratory chain, or from an inhibitory effect on FoF1-ATPase or ANT. It is possible to distinguish between inhibition of oxidative phosphorylation and inhibition of the electron transport chain by using an uncoupler-stimulated respiration test. If inhibition occurs in electron transport chain, uncoupler-stimulated oxygen consumption will be inhibited. If the tested compound instead acts on the oxidative phosphorylation, it will be innocuous. We conducted such a test using CCCP as an uncoupler and succinate as the substrate. Mitochondrial Doxorubicin oxygen consumption was not inhibited by ABA but was inhibited for KCN
(respiratory chain complex IV inhibitor), indicating that the inhibition of state-3 respiration
by the compound does not occur through direct action on the respiratory Thymidylate synthase chain. The effect is probably due to interaction with FoF1-ATPase and/or the ADP/ATP translocator because it is similar to those of oligomycin, a specific inhibitor of FoF1-ATPase, and carboxyatractyloside, an ANT inhibitor. In addition, mitochondrial oxygen consumption inhibited by 25 μM ABA was further stimulated with 1 μM CCCP, demonstrating that the mitochondrial respiratory chain was not inhibited (data not shown). The complex I (NADH dehydrogenase) is the most vulnerable complex of the electron transport chain. The smaller, simpler complex II contains succinate dehydrogenase, the only enzyme of the Krebs cycle linked to the inner mitochondrial membrane (Boelsterli, 2007). We corroborated our results cited in the item 3.5 that saw no ABA effect on NADH dehydrogenase and succinate dehydrogenase. ABA did not dissipate membrane potential, as do inhibitors of respiratory chain complexes, such as rotenone and uncoupling substances such as CCCP, i.e., those capable of acting on the linkage between ATP synthesis and electron transport. Our results support the hypothesis, proposed earlier, that ABA behaves similarly to oligomycin and/or carboxyatractyloside, indicating that the toxic mechanism of ABA involves direct action on FoF1-ATPase and/or ANT.
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