Interactions of succinate dehydrogenase with cyanide

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Succinate dehydrogenase (succinate:(acceptor) oxidoreductase, EC 1.3.99.1) solubilized by KCN, differs from other soluble preparations of this enzyme for the following properties. 1. 1. It has 16–20 nmoles of tightly bound cyanide per mg protein. 2. 2. It contains 3.6–4.1 nmoles of peptide-bound flavin, 24–32 natoms of iron and 16–17 nmoles of acid-labile sulphide per mg protein. An Alteration of the iron-sulphur prosthetic group is thus indicated. 3. 3. Density gradient centrifugation shows two active species of mol. wt 180 000 and 140 000. By gel filtration in chaotropic agents, two components of mol. wt 180 000 and 100 000 can be isolated. The molecular weights, by gradient ultra-centrifugation, of the isolated components correspond to the original values obtained for the unfractionated enzyme. It is proposed that the heavier species is a dimer stabilized by cyanide; the ligher one represents the monomeric form of the flavo-protein, which exists in a dynamic monomer dimer equilibrium. The mechanism of action of KCN in the dissociation of succinate dehydrogenase from the membrane was also investigated. 1. 1. CN is shown to be the species acting in the process. The reaction is found to be first order with respect to CN −. 2. 2. KCN induces spectral modifications on soluble succinate dehydrogenase and an anomalous stability to iron: two moles of CN − per gatom iron remain bound to protein after mild acid treatment. 3. 3. It is concluded taht CN − binds to the iron of the membrane-bound succinate dehydrogenase. As a result of a probable conformation change, the resolution of the linkages to the membrane occurs.