Nature of the activation of succinate dehydrogenase by various effectors and in hypobaria and hypoxia

1. Hepatic mitochondrial succinate dehydrogenase (succinate:(acceptor)oxidoreductase, EC 1.3.99.1) was activated by preincubation of mitochondria with four diverse classes of compounds, the dicarboxylic acids, nitrophenols, quinols (and ubiquinols) and pyrophosphates. Of the various compounds tested malonate, oxaloacetate and pyrophosphate, well-known competitive inhibitors of the enzyme, and also hydroquinone and ubiquinols were effective even at low concentrations and showed maximal stimulation in 2 min.2. Activation of succinate dehydrogenase by ubiquinol-9 and ubiquinol-10 was comparable to succinate activation in fresh mitochondria, and was much higher in the aged samples.3. Preincubation of mitochondria with succinate, 2,4-dinitrophenol, pyrophosphate and ATP also stimulated the succinate-2,2′,5,5′-tetraphenyl-3,3′-(4,4′-biphenylene) ditetrazolium chloride (NT) reductase activity, whereas malonate, hydroquinone and ubiquinol-9 were ineffective. A differential activation of the flavoprotein by the oxidized and reduced forms of ubiquinone-9 was observed, the former stimulating the reduction of NT and the latter of phenazine methosulphate-2,6-dichlorophenolindophenol.4. Repeated washing of the activated mitochondrial samples with the sucrose homogenizing medium, partially reversed the activation by effectors other than succinate. Further washing of the activated preparations after a second preincubation with succinate reverted the enzyme activity to the basal level in the case of malonate, ATP and pyrophosphate but not that of hydroquinone and ubiquinol-9.5. Increase in the activity of hepatic mitochondrial succinate dehydrogenase, but not of succinate-NT reductase, known to occur in rats exposed to hypobaria was also observed in hypoxia indicating that it is an effect of lowered O₂ tension. The enzyme activity in these “partially activated” preparations was stable to washing with the sucrose homogenizing medium and could be fully activated to the same level as in the controls showing thereby the qualitative nature of the change. On washing these succinate-activated preparations further with the medium, the “hypobaric activation” was not reversed to the basal level, whereas the “hypoxic activation” was reversed. These results suggest that the effectors responsible for the activation of succinate dehydrogenase under hypobaric and hypoxic conditions are probably different; the former may be of the ubiquinol type and the latter of the malonate type.