INHIBITION OF SUCCINATE DEHYDROGENASE 17 



only at the antimycin-sensitive factor. It is also possible that ubiquinone 

 and cytochrome b are on a linear pathway rather than on alternate path- 

 ways. In nonphosphorylating systems, the role of cytochrome b is debat- 

 able. The sites of oxidation of p-phenylenediamine and ascorbate are distal 

 to ASF. It should also be pointed out that succinate oxidase from other 

 sources may have different ratios of components and somewhat different 

 pathways. Thus the oxidation of succinate and NADH by the electron 

 transport system of Azotobacter is not sensitive to antimycin (Bruemmer et 

 al., 1957). Two recent observations have extended our knowledge of suc- 

 cinate oxidase and related pathways. Azzone and Ernster (1961) have 

 demonstrated an ATP requirement for mitochondrial phosphorylating 

 succinate oxidation and propose reactions such as the following: 



Succinate + A -l ATP -> fumarate + AH ~ P + ADP 

 AH ~ P + B + ADP -> A + BH2 + ATP 



Succinate + B -> BHj + fumarate 



A is possibly a flavoprotein, a quinone, or some other component of succinate 

 dehydrogenase, and B may be cytochrome b. In nonphosphorylating systems 

 the electrons may reach B more directly. An alternate pathway for AH -^ P 

 is the reduction of NAD: 



AH ~ P + NAD -> A + NADH + P^ 



Succinate has been shown to reduce NAD in mitochondria by Chance and 

 Hollunger (1961 a, b, c) and this is dependent on ATP. Since cndogen^asly 

 formed succinate is more effective than added succinate, it is possible that 

 this reaction may involve succinyl-CoA. The reduction of NAD passes 

 through antimycin-sensitive and Amytal-sensitive links and seems to involve 

 a third flavoprotein component. These findings are not only important with 

 respect to the behavior of succinate oxidase, but also have possible bearing 

 on the responses to malonate in mitochondrial and cellular systems. 



The succinate oxidase particles have been fragmented in various ways to 

 yield smaller particles or soluble preparations with different compositions 

 and properties. One such preparation is the succinate dehydrogenase complex 

 (SDC) from heart mitochondria or ETP, which perhaps represents that frac- 

 tion of the complex up to cytochrome c, since it oxidizes both succinate and 

 NADH and possesses an antimycin-sensitive step. Of more interest for 

 malonate inhibition are the several forms of soluble succinate dehydrogenase 

 that have been prepared. The purest contain no heme or lipid; four atoms 

 of tightly bound nonheme iron and one flavin occur in a molecule (assuming 

 a molecular weight of around 200,000). The flavin is apparently not ribofla- 

 vin but occurs in a dinucleotide form covalently attached to peptide chains 

 of the apoenzyme (Kearney, 1960). There is spectral evidence that both 



