CHEMICAL PROPERTIES 6 



and expressed for this inhibition of succinate oxidation by malonate in the 

 work of Quastel and Wooldridge, although competition specifically was not 

 mentioned. Cook (1930), also at Cambridge, however, stated that a "com- 

 petitive" mechanism had been established, presumably referring to the 

 work of Quastel inasmuch as Cook performed no experiments indicating 

 a competitive relationship. The competitive nature of the malonate inhibi- 

 tion has been substantiated many times and placed on a quantitative basis, 

 so that malonate has come to be recognized as the classical example of 

 inhibition by a pvirely competitive mechanism. The development and ap- 

 plications of this concept will be discussed in more detail in Chapter 2. 

 For 20 years malonate was the only available specific inhibitor of succinate 

 dehydrogenase, and later of the tricarboxylic acid cycle, and actually played 

 an important role in the elucidation of the cycle sequence. Other cycle inhib- 

 itors have been described recently, but no other inhibitors of the succinate 

 oxidation step as specific and useful as malonate have been found. 



CHEMICAL PROPERTIES 



Malonic acid and its salts when obtained commercially are often not 

 sufficiently pure for accurate work and it has been the practice in our lab- 

 oratory to recrystallize all material. Malonic acid may be recrystallized 

 from ethyl acetate and benzene (Adell, 1940), or ether and benzene con- 

 taining 5% light petroleum (Vogel, 1929), or simply from a hot concentrated 

 benzene solution by cooling to 50-10°. The sodium and potassium salts may 

 be dissolved in small amounts of warm water and precipitated by the ad- 

 dition of ethanol, as is commonly done with other dicarboxylate salts 

 (Potter and Schneider, 1942), or, with somewhat less yield, may be crystal- 

 lized by cooling hot concentrated aqueous solutions. In all cases we have 

 decolorized with activated charcoal in the solutions before recrystallization 

 and have washed the products with ether preparatory to drying. It should 

 be emphasized that the choice of the sodium or the potassium salt will 

 depend on whether the preparation to be tested is cellular or subcellular. 



Stability 



Malonic acid and its salts are quite stable and chemically unreactive. 

 Decarboxylation to acetate proceeds very slowly under ordinary conditions. 

 Aqueous solutions of sodium malonate heated to 125° for 48 hr show no 

 perceptible decomposition (Fairclough, 1938), and the half-life of sodium 

 hydrogen malonate in solutions 5-50 mM is at 80° around 40 days, cal- 

 culated from the rate constant for decarboxylation (Hall, 1949). The free 

 energy change for the reaction 



Malonate -> acetate + CO, 



