It is well known that succinate dehydrogenase (SD) is transformed from an inactive to an active state on combination with substrates or competitive inhibitors and that SD becomes deactivated on removing the activator. Activation has a high temperature coefficient and is thought to be due to a conformational change in the enzyme. In the present paper it is shown that in well washed membrane preparations, in which SD occurs largely in the deactivated form, NADH induces activation of SD. The activation progresses as long as NADH is present; once NADH is removed by oxidation, SD reverts rapidly to the deactivated form. NADH-induced activation has the same temperature coefficient as that produced by succinate and the kinetic properties of SD activated by either agent are the same. Activation is not due to a combination of NADH with SD, since rhein and piericidin, inhibitors of NADH oxidation, interfere with the activation of SD by NADH. Extraction of CoQ10 with pentane prevents the activation by NADH nearly completely, while readdition of CoQ10 restores activability by NADH. It is suggested that the agent responsible for this type of activation is reduced CoQ and that NADH serves only to maintain CoQ10 in the reduced state. Since thenoyltrifluoroacetone prevents succinate oxidation but not activation of SD by NADH, it appears that CoQH2 can interact with and induce changes in SD in the absence of electron transport. The possibility that this type of rapid activation-deactivation may be of regulatory significance in the Krebs cycle is discussed.
|Number of pages||8|
|Journal||Biochemical and Biophysical Research Communications|
|State||Published - 19 Mar 1971|