ADDENDUM 



While this edition was in the press, Ochoa and co-workers 

 have published a series of papers [J. biol. Chem., 1950, 187, 

 849 et seq.) dealing with the fixation of carbon dioxide by 

 animal and bacterial cells. They have shown that COg will 

 combine with pyruvic acid in the presence of reduced coenzyme 

 I (in bacteria) to form malic acid directly, the enzyme con- 

 cerned being known at present as the "malic enzyme": — 



CO2 + CH3 . CO . COOH + CoE . H2 



= HOOC.CH2.CHOH.COOH + CoE. 



In the schemes put forward on pp. 135 and 137, the first stage 

 in CO2 fixation is shown as a carboxylation of pyruvic acid by 

 reversal of oxalacetic decarboxylase. It has not been possible 

 to demonstrate convincingly that the bacterial oxalacetic 

 decarboxylase is reversible and the first stage in the fixation 

 process should be represented as a direct formation of malic 

 acid by the "mafic enzyme" without the intermediate forma- 

 tion of oxalacetic acid. 



In the "citric acid cycle" outlined in Fig. 12, citric acid is 

 shown as a side-product not involved in the reactions of the 

 actual cycle. Evidence has accumulated during the past 

 year that citric acid itself is involved in the cycle. It is 

 formed by the condensation of acetyl-phosphate and oxalacetic 

 acid and then gives rise to m-aconitic acid and the other 

 substrates shown in the cycle. It is probable, in the light of 

 the findings concerning the "malic enzyme," that the 

 oxalacetic acid is formed via malic acid and not directly from 

 CO2 and pyruvic acid as shown in Fig. 12. 



