EFFECTS ON TRICARBOXYLIC ACID CYCLE 71 



will always tend to lessen the effect of malonate and under certain circum- 

 stances it might effectively overcome the inhibition. The third principle 

 is that the degree of malonate inhibition will depend on the level of suc- 

 cinate accumulation in the system studied. 



Oxalacetate can often be formed by reactions outside the cycle. Pyruvate 

 and phosphoenolpyruvate can be carboxylated to oxalacetate in the presence 

 of oxalacetate decarboxylases or oxalacetokinase (Bandurski, 1955), and 

 transamination between a-ketoglutarate and aspartate may also give rise 

 to oxalacetate. In such cases the inhibition of pyruvate oxidation in the 

 cycle by malonate will be reduced because the incorporation of pjTuvate 

 will not be dependent only on the regeneration of oxalacetate (Holland and 

 Humphrey, 1953). Such reactions may occur in isolated mitochondria, as 

 well as in cells, since in heart mitochondria, where pyruvate alone is not 

 oxidized at all, the presence of bicarbonate or COg allows a substantial rate 

 of pyruvate oxidation (Montgomery and Webb, 1956 a), presumably 

 through the carboxylation of some of the pyruvate to oxalacetate. The 

 fourth principle is that the degree of malonate inhibition will depend on 

 noncycle sources of oxalacetate. 



Alternate metabolic pathways involving cycle substrates or intermediates 

 may occur in some tissues. There are many opportunities for the metabolism 

 of pyruvate, in addition to its oxidation through the cycle, and in the pres- 

 ence of malonate these pathways may become important. This is particu- 

 larly true in microorganisms but the ability to decarboxylate pyruvate to 

 acetate is common to most species and tissues. Thus, in the presence of 

 high concentrations (50 mJf ) of malonate, pyruvate is quantitatively trans- 

 formed into acetate by rabbit heart mitochondria (Fuld and Paul, 1952). 

 An alternate pathway for succinate that would circumvent a malonate 

 block is the cleavage of succinate (in the presence of NADH, CoA, and 

 ATP) to 2 acetyl-CoA molecules. This succinate-cleaving enzyme was dis- 

 covered in Tetrahymena (Seaman and Naschke, 1955) but it is also active 

 in several rat tissues and in certain bacteria. This reaction will not, of course, 

 restore cycle activity but it can lead to the formation of acetate or other 

 products from acetyl-CoA, as well as reduce the concentration of succinate. 

 Finally, the recently delineated glyoxylate cycle (Kornberg and Krebs, 

 1957) could bypass that region of the cycle containing succinate oxidase, 

 malate being formed from isocitrate through the condensation of glyoxylate 

 and acetyl-CoA, the over-all process being the formation of succinate from 



2 PjTuvate + 3/2 O2 -> succinate + 2 CO2 + HgO 



pyruvate. This shunt would allow a greater utilization of pyruvate and a 

 greater oxygen uptake in the presence of malonate than would be the case 

 with the tricarboxylic acid cycle alone. The glyoxylate cycle has been 

 found in many microorganisms and there is some evidence for its occurrence 



