46 1. MALONATE 



tors convert a less active form of the enzyme to a more active form, and 

 that this transformation may involve a localized change in the protein 



Activator 



(mJf) (mM) 



structure because of the high energy of activation. Whatever the explanation 

 it is important to remember that malonate can exert an activating effect, 

 as weU as inhibiting, in media low in phosphate. 



Effects of Various Factors on the Inhibition of Succinate Dehydrogenase 



Very few illuminating studies on the modification of malonate inhibition 

 are available. The effects of temperature were mentioned in Volume I, where 

 the following thermodynamic parameters for the inhibition of beef heart 

 succinate dehydrogenase at 38° were calculated: AF = — 6.26 kcal/mole, 

 AH = — 5.48 kcal/mole, and AS = 2.6 cal/mole/degree. The K^^ for malonate 

 was found to be 0.025 mM at 20o-23o and 0.041 mM at 38o (Kearney, 1957). 

 The effects of osmolarity on the inhibition are surprisingly large (Honda and 

 Muenster, 1961). Lupine mitochondria were prepared in media of different 

 sucrose concentrations and assayed in media of two osmolarities (Table 

 1-9). These results were obtamed on mitochondria and it is possible that 

 the effects are not directly on succinate oxidase but on the permeability 

 or structural properties of the mitochondria. In this connection it has been 

 pointed out by Singer and Lusty (1960) that iV-methylphenazine measures 

 the full activity of succinate dehydrogenase in mitochondrial fragments, 

 but in intact mitochondria it measures only a fraction of the activity. 

 Various ways of damaging the mitochondria lead to increased succinate 

 dehydrogenase activity (such as increase in Ca++ concentration). This was 

 interpreted in terms of permeability barriers to A^-methylphenazine (and 

 also FMNHj), limiting the rate in intact mitochondria. It could also be 

 explained on the basis of structural changes in the enzyme complexes. In 

 any case, these observations demonstrate the importance of the mitochon- 

 drial state in the functioning of succinate oxidase, and it would not be 

 surprising if malonate inhibition were similarly sensitive. The inhibition 

 of succinate oxidation by malonate in rat heart mitochondria in KCl me- 

 dium is not altered by either halving or doubling the KCl concentration in 

 the assay medium (Montgomery and Webb, 1956 b). However, the effects 



