THE CHEMOTROPHIC ASSIMILATION OF CARBON 65 



pyruvic acid appeairs to be an irreversible process and it has 

 been found that, whereas the oxidation of glucose, glycerol 

 and pyruvate, by thiamine-deficient Prototheca is markedly 

 stimulated by the addition of thiamine (which is necessary 

 for the carboxylation and decarboxylation of pyruvic acid, 

 see p. 9), that of acetate is stimulated to a much smaller 

 extent. This shows that pyruvate is not an intermediate in 

 the oxidation of acetate as it is in the cases of glucose and 

 glycerol.^* Another possible pathway, involving glycollic 

 acid, CHgOH.COOH, as an intermediate, seems unlikely 

 because glycoUate has a catalytic effect on the respiration 

 of Prototheca whereas acetate does not.^* In yeast, acetate 

 has been found to undergo condensation with oxaloacetate 

 to form citric acid: 



CH3.COOH CH2.COOH 



+ I 



CO.COOH — >HO.C.COOH . . (15) 



I I 



CH2.COOH CH2.COOH 



Acetic acid seems not to take part directly in this reaction 

 but must first be converted to 'activated acetate', probably 

 an acetylcoenzyme A.^^ It has not been established that 

 acetate is oxidized by yeast through the tricarboxylic acid 

 cycle but that this may take place in Scenedesmus is sug- 

 gested by the appearance of radioactivity in citric acid and 

 other tricarboxylic acid cycle intermediates following supply 

 of acetate labelled with radioactive carbon. ^^ This mech- 

 anism can, however, only account for the oxidation of 

 acetate since the operation of the cycle will result in the 

 liberation as carbon dioxide of the two carbon atoms of 

 acetate and the regeneration of the original oxaloacetate 

 (Fig. 2). Assimilation to more complex substances must 

 follow other pathways. In certain species and under certain 

 conditions at least, e.g. in Scenedesmus in the light, acetate 

 is most readily assimilated to fats,^^ a synthesis that most 

 probably takes place directly from 'activated acetate' by 

 successive condensations and reductions. ^^' ^^ Carbohydrate 

 could be derived from acetate if this were first transformed 



