280 A SYMPOSIUM ON RESPIRATORY ENZYMES 



THE CITRIC ACID CYCLE IN TISSUE METABOLISM 



FREDRICK J. STARE 



Washington University School of Medicine, St. Louis 



This is an appropriate time to mention the reasons why some of us 

 do not believe in the importance, or even the presence, of citric acid 

 as a component of the metaboHc cycle generally termed the "citric 

 acid cycle." The citric acid cycle is based on carefully established 

 experimental facts. Some of these were mentioned this morning by 

 Dr. Evans. We question, however, the application of these facts, 

 particularly the presence of citrate, to a cycle of importance in re- 

 spiring muscle. 



It was only with high concentrations of pyruvate that Krebs was 

 able to demonstrate increases in citrate, and these increases were of 

 a small order, varying from 1 to 15 per cent. In tissue and body 

 fluids, pyruvate occurs in a far lower concentration. 



It was only with high concentrations of pyruvate, citrate, and 

 malonate that an increase in succinate was detected. I know of no 

 evidence, but I believe that a high concentration of glutamate, and 

 other related compounds not included in the citric acid cycle, would 

 also yield increases in succinate in the presence of a high malonate 

 concentration. 



The citric acid cycle assumes that malonate completely prevents 

 the anaerobic reduction of oxalacetate to succinate, but there is no 

 adequate proof for this assumption. 



Citrate is definitely a weaker substance, as compared with the 

 other members of the cycle, in increasing or prolonging the oxygen 

 consumption of respiring muscle or in eflFecting pyruvate removal 

 by respiring muscle. In fact, citrate frequently inhibits such effects. 



Malonate in a concentration of 0.005 M which is 5 to 6 times less 

 than the concentration used by Krebs, inhibits oxygen uptake and 

 pyruvate utilization on an average of 70 to 75 per cent (pigeon breast 

 muscle). It completely and always inhibits any catalytic effect that 

 citric acid may show. Yet any other member of the cycle when 

 added in an equal concentration of 0.005 M will completely over- 

 come the malonate inhibition of oxygen uptake and of pyruvate 

 removal. If the latter two depend in any way on a mechanism in- 

 volving citric acid, the malonate should stop It because citric acid 

 activity is always inhibited by malonate. 



Recently, in Dr. Barron's laboratory, Lipton, Goldinger, and I 

 have studied pyruvate and citrate utilization in respiring pigeon 

 muscle tissue. According to the citric acid cycle, each molecule of 



