Effect of chlorophenols on isolated plant mitochondria activities: a QSAR study

Twenty-three chlorinated monophenols were investigated for their uncoupling and inhibitory properties on plant mitochondria. All the studied compounds had uncoupling properties but their activity was submitted to quantitative changes from 1- to 400-fold, according to the substitution. They also had inhibitory properties on the electron transfer at a level located upstream to the quinone pool. Three inhibitory types could be recognized: (a) chlorophenols inhibiting similarly the oxidation of every type of substrate (NADH, succinate, alpha-ketoglutarate, at pH 7.2; malate, at pH 7.5), (b) chlorophenols inhibiting first complex I, and (c) chlorophenols inhibiting first complex II. In this last case, the presence of substituents at the 2 and 6 positions increases selectively the inhibition of succinate oxidation. A quantitative structure activity relationship (QSAR) study was undertaken and showed that a good correlation appeared between the steric parameter, 1 chi v, and the inhibitory properties with NADH as substrate. This result could be explained by the binding of the chlorinated phenols with NADH dehydrogenase whose position, on the external surface of the mitochondrial membrane, is favorable to good accessibility of the xenobiotic. Equations for inhibitory properties with other substrates were clearly different. The relation between the uncoupling activity and the studied parameters was more complex but a good correlation occurred with the steric parameter (sigma D, A) and the electronic parameter sigma. Neither log P nor sigma alone gave equations of good quality. These results suggest a competition between the chlorophenol mobility in the inner membrane, necessary for uncoupling, and binding with different proteins in the same membrane on which the inhibitory activity is dependent.