96 1. MALONATE 



recoverable as succinate. The highest conversion efficiency was observed by 

 Krebs and Eggleston (1940) in pigeon muscle brei, where 75-85% of the 

 pyruvate utilized in the presence of fumarate and 12.5 mM malonate went 

 to succinate. Complete conversion to succinate would not, of course, be ex- 

 pected unless one could inhibit succinate oxidation completely and specifi- 

 cally, which in most instances cannot be done. 



Factors Determining Succinate Accumulation 



The effects of inhibition on the concentrations of intermediates in multi- 

 enzyme systems have been treated in Chapter 1-7. Some of the most impor- 

 tant factors involved in malonate inhibition will be summarized. 



(I) Degree of inhibition of succinate oxidase: hence, the concentration 

 of malonate, the affinity of the enzyme for malonate, and the ability 

 of malonate to penetrate if the preparations are cellular. 



(II) Rate of formation of snccinate: this will depend primarily on the availa- 

 bility of cycle substrates and their concentrations. 



(III) Action of malonate on enzymes other than succinate dehydrogenase: 

 such actions may slow down the formation of succinate, as discussed 

 above. 



(IV) Other pathways of succinate metabolism: several reactions of succinyl- 

 CoA or succinate are known and these would tend to prevent accumula- 

 tion. 



(V) Diffusion of succinate from cells: when only cells or tissues are analyzed 

 for succinate, intracellular accumulation will be reduced by the loss of 

 succinate into the medium or the blood. 



(VI) Time after addition of malonate: although this has never been studied, 

 it is probable that the succinate concentration will follow characteristic 

 time courses in each case, in some cases perhaps decreasing after a peak 

 level has been reached. 



Another factor, about which nothing is known, is the possible effect of 

 rising succinate concentration on the reactions forming succinate. The 

 oxidation of a-ketoglutarate forms succinyl-CoA, and succinate can arise in 

 at least three different ways from succinyl-CoA. 



P-enzymes 



Q -Ketoglutarate — >- sue cinyl - Co A *- sue einate 



transaeylases 



