42 INTERMEDIARY METABOLISM AND GROWTH I 



acid cycle intermediate from GO, and pyruvate. The differences between oxalace- 

 tic carboxylase and the malic enzyme are worth noting (Fig. i6) (Ochoa et al., 

 1950; Ochoa and Kaufman, 1951; Veiga-Salles and Ochoa, 1950). Whereas 

 pyruvate is a required substrate for the malic enzyme, phosphoenol pyruvate is the 

 substrate for the oxalacetic carboxylase enzyme. IDP or GDP are required for CO2 

 fixation in the reaction catalyzed by oxalacetic carboxylase but TPNH is required 

 in the malic enzyme reaction. Oxalacetic carboxylase facilitates the synthesis of 



Oxalacetic 

 corboxylose 



Malic 

 enzyme 



OAA + ITP Malcte + TPN* 



:;{oAAr 



PEP + IDP+C02' "^Pyruvate +C02-t-TPNH2 



Fig. 16. Postulated relationship between oxalacetic carboxylase and malic enzyme. 



phosphoenol pyruvate from oxalacetate or pyruvate by a series of reactions that do 

 not require the participation of pyruvate kinase. This may be advantageous to the 

 organism at a time when the requirements of gluconeogenesis are increased : 



malic enzyme 



1) Pyruvate ^ CO2 + TPNH2 ^ > malate + TPN^ 



malic dehydrogenase 



2) Malate + DPN* . > oxalacetate + DPNH2 



OAA carboxylase 



3) Oxalacetate + ITP " ^ IDP + CO2 + phosphoenol pyruvate 



Reactions i to 3 explain the finding that the hexose of glycogen formed by liver 

 slices from pyruvate-Q-'^C is almost equally labelled in carbon atoms i, 2, 5 and 

 6 although acetyl groups formed from pyruvate show no such randomization. 



The enzymes concerned with the fixation of GOj into succinic acid have recently 

 been obtained in soluble form from acetone powders of rat liver mitochondria, 

 (Lardy and Adler, 1956; Friedberg et al., 1956), kidney mitochondria, and bac- 

 teria (Whiteley, 1953)- Although propionyl-GoA is more effective than propionate, 

 ATP is still required in the liver mitochondrial system: 



ATP 



Propionyl-CoA + CO 2 > succinyl-CoA 



Mg^* or Mn** 



Conceivably, isosuccinate rather than succinate is the primary product (Katz 

 and Chaikoff, 1955). In the event that this is the case, the mechanism of isomeri- 

 zation of isosuccinate to succinate remains to be elucidated. 



Biotin has been implicated in the carboxylation reactions noted above. Oxalacetic 

 carboxylase contains bound biotin at a higher concentration than that found in the entire 

 liver (Bandurski and Lipmann, 1956). Malic enzyme activity is decreased in livers from 

 biotin deficient turkeys although highly purified pigeon liver malic enzyme is essentially 

 free from biotin. However, biotin in vitro does not activate the enzyme from biotin deficient 

 turkey livers (Blanchard et at., 1950; Korkes et al., 1950). Malic enzyme is an adaptive 



