Further studies on the stimulating and inhibiting effect of DNP on the oxidation of succinate by rat-liver mitochondria have shown that the activity of succinic dehydrogenase with 2,6-dichlorophenolindophenol as acceptor was not affected when the aerobic oxidation of succinate was almost completely inhibited by DNP. This shows that the inhibition of succinate oxidation by DNP is not due to the inhibition of the dehydrogenase. When the oxidation of succinate was inhibited by DNP, the addition of substrates of the nicotinamide nucleotide-linked dehydrogenases could restore the inhibited respiration to a level significantly higher than the oxidation rates of these substrates themselves. The reversal of the DNP-inhibition by a-ketoglutarate required either inorganic phosphate or arsenate but was prevented by arsenite, an inhibitor of α-ketoglutaric dehydrogenase. It appears that the dehydrogenating process but not the substrate-linked phosphorylation is responsible for the effect of α-ketoglutarate. Similar effect of glutamate was not affected by high concentration of cycloserine, thus excluding the participation of L-aspartate:2-oxoglutarate aminotransferase in this action. The results presented in this communication support the view that a high-energy intermediate is required in the oxidation of succinate. It has been suggested that this high-energy compound could be formed by the reversed reaction of the succinate-linked endergonic reduction of NAD+ in the presence of DNP.
