236 1. MALONATE 



Malonic Esters 



Malonic monoethyl and diethyl esters have been found to have some in- 

 teresting actions. They can increase the survival period of mice infected 

 with mycobacteria (Davies et al., 1956), are occasionally carcinostatic (Freed- 

 lander et al., 1956), can inhibit the breakdown of hexobarbital in liver homo- 

 genates and prolong the narcotic action (Kramer and Arrigoni-Martelli, 

 1960), and inhibit sporulation of Bacillus cereus (Nakata and Halvorson, 

 1960). However, the relation of these effects to succinate dehydrogenase 

 inhibition is obscure. In most cases, malonic esters have been used to cir- 

 cumvent the permeability barriers to malonate, inasmuch as the esters 

 should penetrate into cells readily. It has often been assumed that hydrolysis 

 to malonate occurs within the cells. This hydrolysis must be enzymatic 

 because the esters are quite stable. A lipase from pig liver hydrolyzes one 

 ethyl group from malonic diethyl ester but does not remove the other 

 ethyl group, the product being malonic monoethyl ester (Christman and 

 Lewis, 1921). I have been able to find no direct evidence for the hydrolysis 

 to malonate. Malonic esters are neutral at physiological pH's, since they 

 are very weak acids with pK„ values around 15.75 (Rumpf et al., 1955) 

 and ionize very slowly with a rate constant of 1.8 X 10~^ min~^ (Pearson 

 and Mills, 1950). 



The effects of the malonic esters on metabolism will now be discussed in 

 order to determine if there is any indirect evidence for the intracellular 

 hydrolysis to malonate and the inhibition of succinate dehydrogenase. 

 When injected into fluoroacetate-poisoned rats, malonate and malonic di- 

 ethyl ester have approximately the same effects on the accumulation of 

 citrate in the heart and kidneys (Fawaz and Fawaz, 1954). Furthermore, in 

 kidney slices, the diethyl ester inhibits succinate oxidation 92% at 20 mM, 

 and malonate at the same concentrazion inhibits 75%. Less inhibition by 

 the ester compared to malonate is observed in heart slices. Evidence for 

 hydrolysis by both tissues was adduced from the decreases in the pH ob- 

 served. The respiration of the fungus Zygorrhynchus moelleri is not inhibited 

 readily by malonate although the succinate dehydrogenase from this orga- 

 nisms is quite sensitive, indicating a failure to penetrate (Moses, 1955). 

 Malonic diethyl ester was tested and found to inhibit the oxidation of both 

 glucose and acetate, but at high concentrations (see tabulation). It was felt 



