Trifluoperazine inhibition of electron transport and adenosine triphosphatase in plant mitochondria

Trifluoperazine inhibits ADP-stimulated respiration in mung bean ( Phaseolus aureus) mitochondria when either NADH, malate, or succinate serve as substrates (IC 50 values of 56, 59, and 55 μ m, respectively). Succinate:ferricyanide oxidoreductase activity of these mitochondria was inhibited to a similar extent. The oxidation of ascorbate/TMPD was also sensitive to the phenothiazine (IC 50 = 65 μ m). Oxidation of exogenous NADH was inhibited by trifluoperazine even in the presence of excess EGTA [ethylene glycol bis(β-aminoethyl ether)- N,N′-tetraacetic acid] (IC 50 = 60 μ m), indicating an interaction with the electron transport chain rather than with the dehydrogenase itself. In contrast, substrate oxidation in Voodoo lily ( Sauromatum guttatum) mitochondria was relatively insensitive to the phenothiazine. The results suggest the bc 1 complex to be a major site of inhibition. The membrane potential of energized mung bean mitochondria was depressed by micromolar concentrations of trifluoperazine, suggesting an effect on the proton-pumping capability of these mitochondria. Membrane-bound and soluble ATPases were equally sensitive to trifluoperazine (IC 50 of 28 μ m for both), implying the site of inhibition to be on the F 1. Inhibition of the soluble ATPase was not affected by EGTA, CaCl 2, or exogenous calmodulin. Trifluoperazine inhibition of electron transport and phosphorylation in plant mitochondria appears to be due to an interaction with a protein of the organelle that is not calmodulin.