116 1. MALONATE 



some information on the site and specificity of the malonate inhibition. 

 Quite different results on acetate utilization by yeast were obtained by 

 Stoppani et at. (1958 b), who found that fumarate is completely unable to 

 reverse the inhibition by malonate (see accompanying tabulation). These 



Additions Acetate utilized 



Acetate 30 

 Acetate + malonate 4.4 



Acetate + fumarate 27 

 Acetate + malonate + fumarate 3.8 



data would nuply that the inhibition of acetate utilization by malonate is 

 not mediated through a block of succinate oxidation but by another jnecha- 

 nism. It is difficult to explain this by a failure of fumarate to penetrate 

 into the cells since malonate seems to enter readily. It was suggested that 

 malonate might interfere with acetate activation by depleting the system 

 of coenzyme A, due to the formation of relatively stable malonyl-CoA. 

 Whatever the mechanism, these results point to an action of malonate 

 other than on succinate oxidase. 



An interesting illustration of the useful information that may be ob- 

 tained from the use of fumarate is given in the inhibition of urea formation 

 by malonate. The formation of both arginine and urea from citrulline and 

 glutamate in liver homogenates is potently inhibited by malonate (Cohen 

 and Hayano, 1946; Krebs and Eggleston, 1948). Fumarate is able to counter- 

 act this block completely. These results were difficult to understand ini- 

 tially, but it is now known that transamination must occur between gluta- 

 mate and oxalacetate to form aspartate, which reacts with the citrulline 

 to form arginosuccinate, from which arginine and urea are derived. The 

 effect of malonate is to reduce the supply of oxalacetate for transamination 

 and it is clear why fumarate will abolish this inhibition (Ratner, 1955). 

 Krebs and Eggleston (1948) observed the formation of 3-5 molecules of 

 urea for each molecule of fumarate added. This may be explained by the 

 fact that the formation of arginine from arginosuccinate involves the release 

 of fumarate, which can again go to oxalacetate. 



The demonstration that fumarate will counteract the inhibitory action 

 of malonate on some tissue function is indicative of a primary block of the 

 succinate oxidase, but even complete reversal does not prove a specific ac- 

 tion of malonate. One example would be the inhibition of Br~ uptake in 

 barley roots by malonate (Machlis, 1944). Malonate (10 mM) inhibits the 

 uptake 57% but if fumarate is present the uptake is 35% above the control. 

 On the surface this would imply an effective reversal of the malonate inhi- 



