A comparative study of the succinic dehydrogenase-cytochrome system in heart muscle and in kidney

The cytochrome system in the cecropia silkworm, with special reference to the properties of a new component

1. In the majority of tissues in the cecropia silkworm, cytochromes b and c are apparently absent, being replaced by a hitherto undescribed component which we have tentatively termed cytochrome x. 2. Spectroscopically, the new cytochrome is characterized in the reduced state by a broad and indivisible absorption band extending from 551 to 562 mmicro. 3. The enzyme can be demonstrated only in the larval stage of the insect and undergoes breakdown prior to pupation. It occurs in highest concentration in the walls of the larval midgut and resists extraction, being apparently bound to subcellular particles. 4. Evidence is presented, based on spectroscopic and manometric studies in the presence of various substrates and inhibitors, that cytochrome x is a single component which mimics certain of the properties of cytochromes b and c and of succinic dehydrogenase. 5. Detailed studies served to differentiate cytochrome x from the cytochromes b(1) and b(2) which it resembles spectroscopically. 6. On the basis of spectroscopic studies at the temperature of liquid air, it is concluded that cytochrome x closely resembles, and may be identical with, the component which Keilin has recently described as cytochrome e.

The prosthetic groups in succinate dehydrogenase. Number and stoichiometry

I. Succinate:Q oxidoreductase (EC 1.3.99.1) as present in beef-heart submitochondrial particles contains equal amounts of FAD, a [2Fe-2S] cluster and a [4Fe-4S] cluster. Both Fe-S clusters are reducible by succinate. 2. A second type of [2Fe-2S] cluster, called center S-2, that has been proposed to be present in purified preparations of succinate dehydrogenase and isolated Complex II (Ohnishi, T., Winter, D.B., Lim, J. and King, T.E. (1973) Biochem. Biophys. Res. Commun. 53, 231--237) is an artifact introduced by the purification procedure. 3. It is suggested that the 70 000 dalton subunit which is known to bind the flavin, accomodates also the [4Fe-4S] cluster whereas the 28 000 dalton subunit contains the [2Fe-2S] cluster.

Reversible activation of succinate dehydrogenase

1. Treatment of particulate respiratory chain preparations in ways expected to raise or lower the concentration of endogenous soluble low-molecular-weight compounds respectively increased and diminished the capacity of succinate dehydrogenase to become activated reversibly and ;spontaneously' when preparations were diluted in tris acetate buffer and incubated at 37 degrees . 2. Addition of critically low concentrations of recognized activators to preparations that failed to undergo reversible ;spontaneous' activation when incubated at 1mg. of protein/ml. conferred on them the capacity to do so. 3. Preparations with a diminished tendency to undergo reversible ;spontaneous' activation had an increased tendency to become irreversibly inactivated on prolonged incubation at 1mg. of protein/ml. in tris acetate. 4. Extraction procedures designed to demonstrate the presence of possible endogenous activators in enzyme preparations failed to reveal a single substance to which such a role could be conclusively attributed. A mixture of compounds was found, however, including certain amino acids that have been shown to act as activators. It is questionable whether these compounds would be present at sufficiently high concentrations to act as activators when enzyme preparations are diluted to 1mg. of protein/ml. 5. Despite the failure to demonstrate conclusively the presence of endogenous activators, the balance of evidence appears to favour the hypothesis that reversible ;spontaneous' activation of these preparations can best be explained by the presence of such substances, and a scheme describing the mechanism of activation and deactivation of succinate dehydrogenase is discussed in relation to these and other observations.