Effect of sydnone SYD-1, a mesoionic compound, on energy-linked functions of rat liver mitochondria

An important antitumour effect of SYD-1 (3-[4-chloro-3-nitrophenyl]-1,2,3-oxadiazolium-5-olate) has been shown. We now report the effects of this mesoionic compound on mitochondrial metabolism. SYD-1 (1.5 μmol mg−1 protein) dose-dependently inhibited the respiratory rate by 65% and 40% in state 3 using sodium glutamate and succinate, respectively, as substrates. Phosphorylation efficiency was depressed by SYD-1, as evidenced by stimulation of the state 4 respiratory rate, which was more accentuated with glutamate (∼180%) than with succinate (∼40%), with 1.5 μmol mg−1 protein of SYD-1. As a consequence of the effects on states 3 and 4, the RCC and ADP/O ratios were lowered by SYD-1 using both substrates, although this effect was stronger with glutamate. The formation of membrane electrical potential was inhibited by ∼50% (1.5 μmol SYD-1 mg−1 protein). SYD-1 interfered with the permeability of the inner mitochondrial membrane, as demonstrated by assays of mitochondrial swelling in the presence of sodium acetate and valinomycin +K+. SYD-1 (1.5 μmol mg−1 protein) inhibited glutamate completely and succinate energized-mitochondrial swelling by 80% in preparations containing sodium acetate. The swelling of de-energized mitochondria induced by K+ and valinomycin was inhibited by 20% at all concentrations of SYD-1. An analysis of the segments of the respiratory chain suggested that the SYD-1 inhibition site goes beyond the complex I and includes complexes III and IV. Glutamate dehydrogenase was inhibited by 20% with SYD-1 (1.5 μmol mg−1 protein). The hydrolytic activity of complex F1Fo ATPase in intact mitochondria was greatly increased (∼450%) in the presence of SYD-1. Our results show that SYD-1 depresses the efficiency of electron transport and oxidative phosphorylation, suggesting that these effects may be involved in its antitumoural effect.