EFFECTS ON AMINO ACID AND PROTEIN METABOLISM 157 



metabolic systems. Most enzymes are probably in a state of simultaneous 

 formation and degradation, so that an inhibitor of synthesis would induce 

 a steady fall of the enzyme level in the cells. This could apply, of course, 

 to all inhibitors of protein synthesis. 



Effects on Urea Formation 



The terminal product of much protein and amino acid metabolism is urea 

 and it has been found that under certain circumstances malonate inhibits 

 the formation of urea quite potently. The inhibition has been mentioned in 

 connection with its antagonism by fumarate (page 116). The most important 

 reactions of the urea cycle comj^rise the following, assuming that glutamate 

 is the immediate amino-group donor: 



(1) Glutamate + oxalacetate ->• aspartate + a-ketoglutarate (transaminase) 



(2) Aspartate + citrulline -j- ATP ->■ argininosuccinate + ADP + P, 



(argininosuccinate synthetase) 



(3) Argininosuccinate -> arginine + fumarate {argininosuccinase) 



(4) Fumarate -> oxalacetate (fumarase and lyialate dehijdrogenase) 



(5) Arginine -> ornithine + urea (arginase) 



(6) Ornithine + NH3 + CO2 ~> citrulline {citrulline synthetase) 



Glutamate + NH3 + CO2 + ATP -► a-ketoglutarate + urea + ADP + P, 



This urea cycle thus makes contact with the tricarboxylate cycle at several 

 points. The a-ketoglutarate formed in the over all reaction can be oxidized 

 through succinate to oxalacetate or can be transaminated to regenerate glu- 

 tamate. The operation of the urea cycle thus requires sources for oxalace- 

 tate and ATP, both of which may be blocked by malonate. 



Cohen and Hayano (1946) found that 5.7 raM malonate inhibits the con- 

 version of citrulline to arginine 90% in liver homogenates when glutamate 

 is the amino donor. The mechanism of the inhibition was not apparent 

 at that time. These results were confirmed by Fahrlander et al., (1947) 

 and, in addition, they showed that fumarate or malate, can counteact the 

 inhibition, indicating a block of succinate oxidation. They interpreted 

 the mechanism as a depletion of ATP and a consequent inhibition of reac- 

 tion (2). Subsequently, they showed that low malonate concentrations (1-2 

 ToM) inhibit urea formation as much as 75% and felt this was evidence for 

 a specific action on succinate dehydrogenase (Fahrlander et al., 1948). 

 The ATP level in the homogenates drops from 130 to 49.6 in the presence 

 of 2.5 yrM malonate and fumarate restores the ATP level to normal. It 

 was believed that glutamate not only furnishes the amino group but 

 also cycle substrates from which the energy is derived; a block by malonate 

 at the succinate level would reduce the amount of ATP formed. Krebs and 



