[Kinetic and structural characteristics of succinate dehydrogenase components reacting with natural and artificial electron acceptors].

A new catalitic activity of soluble succinate dehydrogenase, i.e. the reduction of low (20-200 muM) concentration of ferricyanide in the presence of succinate is described. The apparent Km value for the acceptor is about 200 muM. The turnover numbers of the enzyme measured in this reaction, with PMS as an electron acceptor and in the system reconstituted from soluble enzyme and alkali-treated submitochondrial particles (succinate oxidase) are found to be almost the same. The new succinate. ferricyanide reductase activity is very sensitive to oxygen, high (3 mM) ferricyanide concentration and mercaptide-forming agents. When the enzyme is stored under aerobic conditions the loss of this activity occurs according to the first-order kinetics with the same rate constants as the reconstitutive activity decreases. The rate constants both for ferricyanide reductase and reconstitution decay do not depend on pH within the range of 6,5--7,5 (k = 8.10(-2) min-1) and increase dramatically at pH 8,5 (K = 4.10(-1) MIN-1). When these two activities are lost after oxygen exposure the PMS-reductase fall down to about 50% of its original activity. The new ferricyanide reductase is found only in the soluble preparation of the enzyme succinate: cytochrome c reductase, succinate dehydrogenase of submitochondrial particles and reconstituted succinate oxidase do not interact with low concentrations of ferricyanide. The treatment of the enzyme after inactivation by oxygen exposure with sulfide ion--iron--mercaptoethanol mixture followed by Sephadex filtration completely restores the original reconstitutive, ferricyanide and PMS reductase activities. The hypothesis is suggested that succinate dehydrogenase contains at least two red-ox centers reacting with electron acceptors. The first one is located in hydrophylic environment (mitochondrial matrix) being accessible for high concentrations of ferricyanide. The second one (iron--sulfur complex, Hipip-type) is responsible for ferricyanide reductase activity described, being located intramembraneously and involved in the electron transfer between dehydrogenase and the rest of the respiratory chain.