Mode of action of the antibiotic X-537A on mitochondrial glutamate oxidation

At 0.05 to 0.01 μM concentrations the monocarboxylic acid antibiotic X-537A inhibits ADP or 2,4-dinitrophenol-activated oxidation of glutamate but has no appreciable effect on state 4 respiration. ATP synthetase activity is also inhibited. There is no efflux of Mg²⁺ or Ca²⁺ from the mitochondria under these conditions. Dissociation of membrane bound Mg²⁺ induced by X-537A is reversed and prevented by Mg²⁺ + ATP but inhibitory effects of the antibiotic are not. Half maximal effects of X-537A occur when the ratio of X-537A to mitochondrial non-diffusible Mg²⁺ is $\text{1}\text{800}$ to $\text{1}\text{400}$. It is proposed that this small fraction of membrane associated Mg²⁺ may be at the catalytic site of energy transfer and irreversible inhibition by X-537A is due to hydrophobic complexation of Mg²⁺ in situ.