UNCOUPLING ACTIVITIES OF MONENSIN IN ISOLATED-MITOCHONDRIA, CHLOROPLASTS AND CELLS

At 1 mu M, the carboxylic ionophore monensin is an effective inhibitor of the light-dependent O-2 evolution in spinach class A chloroplasts. This effect is associated with an uncoupling activity which is obtained at the same concentration in isolated spinach thylakoids. This uncoupling effect is dependent on the presence of Na+ (1 mM) or K+ in the medium. The D,, value is little affected when changing the concentration of thylakoid membranes from 9 to 109 mu g chlorophyll ml(-1) medium. In isolated potato mitochondria, in a medium containing Na+ or K+, the uncoupling activity of monensin was very low, the full uncoupling effect needing 1 mM to occur. However, in cultured Acer cambium cells, 10 mu M induced a full uncoupling effect after a 30 min incubation time. At 10 mu M, monensin killed the cultured cells and induced a necrosis effect, after deposition on broad bean leaves. All these effects show that monensin cannot be considered as a selective inhibitor of transfer in Golgi vesicles. The biochemical mode of monensin uncoupling action appears to be complex, and seems not to need the presence of a free carboxyl, as its effects can be compared to those of valinomycin.