Activation of soluble succinate dehydrogenase by reduction.

Since the experiments of KEARNEY 1 it has been known that both particle-bound and soluble succinate dehydrogenase can be activated by its substrate succinate and by competitive inhibitors such as fumarate, malonate and phosphate. Binding at the active centre apparently brings the enzyme in an active conformation. GUTMAN et al. 2) 3 have recently reported that NADH is as effective as succinate in activating succinate oxidation in submitochondrial particles. Activation by N A D H could be inhibited by rhein or piericidin, but not by thenoyltrifluoroacetone. Since activation by NADH was not possible in ubiquinone-depleted particles, they concluded that ubiquinone is necessary for the activation by NADH and that ubiquinol is the direct activator. This paper describes the activation by NADH of solubilized succinate dehydrogenase, made by the procedure of WANG et al. 4 as modified by KEILIN and KING5, and ZEYLEMAKER6. Ubiquinone could not be detected in this preparation by the procedure o f KRÖGER and KLINGENBERG 7 ; 0 . 0 1 m o l e u b i quinone per mole flavin would have been detected. The preparation was contaminated with a soluble NADH dehydrogenase, and 10% of the total of flavin, analysed by the method of KING et al. 8> 9 , CERLETTI et a l . 1 0 and ZEYLEMAKER 6 , c o u l d b e shown to be FMN. Because of this contaminating activity all of the succinate dehydrogenase flavin and iron-sulphur could be slowly reduced by NADH, as measured by spectrophotometry and EPR spectroscopy. This reduction, which was insensitive to rhein and rotenone, presumably takes place by direct reaction between the two flavoproteins.