Uncoupling Properties of the Herbicide N-n-Pentyl-N-Methyl-N'-(3,4-Dichlorophenyl)-Urea

The herbicide D₅ (N-n-pentyl-N-methyl-N'-(3, 4-dichlorophenyl)-ureacan uncouple oxidative phosphorylation in isolated plant mitochondria.This paper confirms that D₅ is an uncoupler that catalyzes thecollapse of the transmembrane potential gradient by inducinga movement of protons across the membrane. However, D₅ is notitself capable of transporting protons. D₅ gives complete uncouplingat 40 µM, a lower concentration than that required foruncoupling by the n-butyl homologue ‘neburon’. Analysisof the shape of the state 4 stimulation curve suggests thatD₅ might act as a dimer in the membrane. Attempts to demonstrate binding of D₅ to a membrane target gaveambiguous results, binding is not evident at 10 °C and 25°C but might occur at 15 °C and 20 °C. The calculatedherbicide concentration in the membrane (40 µM of which4–0 µM is as the dimer) is high and similar to thatof the major phospholipids. The calculated partition coefficientbetween medium and membrane (3.8 x 103) is in agreement withthe lipophilicity of the substituted urea herbicides. In the presence of a substrate, D₃ blocks both influx and effluxcalcium movement through the mitochondrial membrane but in theabsence of substrate, D₅ induces binding of calcium. Bindingrequires Mg⁺⁺ but not K⁺ or phosphate and leads to a releaseof H⁺. Ruthenium Red causes a partial inhibition of bindingbut no other reagent or ionophore tested had any effect. Sincebinding does not occur in turnip mitochondria which are unableto transport Ca⁺⁺ it is concluded that the effect is not directlylinked to the uncoupling action. The mechanism of action of D₅ is discussed and it is concludedthat D₅ probably acts as a dimer and perturbs membrane structure.The site of action is probably the lipid components of the membrane. Key words: Plant mitochondria, Herbicide, Substituted ureas, Calcium, Uncoupling