298 



BACTERIOLOGICAL CHEMISTRY 



obtained in practice. This limitation obviously applies 

 to any scheme which involves decarboxylation. That the 

 same initial steps as occur in alcoholic fermentation 

 leading to the formation of acetaldehyde (see p. 275) 

 are not involved in citric acid production is shown by 

 the fact that formation of the acid is not inhibited by the 

 presence of mono-iodoacetic acid, but actually accelerated. 

 Phosphorylation however, is involved since inorganic phos- 

 phate and glucose disappear from the medium, with forma- 

 tion of organic phosphate esters, as A. niger produces 

 gluconic and citric acids. Inorganic phosphate is liberated 

 again as gluconic and citric acids are further broken 

 down to oxalic acid in older cultures. 



Eaistrick and Clark in 1919 investigated the produc- 

 tion of oxalic acid from a large number of acids and 

 showed that it was not formed from any three-carbon 

 acid, such as propionic, pyruvic or lactic acids, nor from 

 any mono -basic four-carbon acid, like butyric acid, but 

 that the dibasic four -carbon acids, malic or succinic acids, 

 and also acetic acid, gave rise to good yields of oxalic 

 acid. They considered that the mechanism of its forma- 

 tion from glucose was via ay-diketo-adipic acid which 

 split to oxalacetic acid and acetic acid ; the oxalacetic 

 acid split down further to give oxalic and acetic acids, 

 whilst acetic acid was oxidised to oxalic acid, according 

 to the scheme : — 

 CHO 



CHOH 



COOH 



I 

 CHOH 



I ^ 



COOH 



I 

 CO 



CH, 



CO OH 



•I-- + 1 

 I H 

 CH, 



COOH 



CO OH 



•I-- + I 

 CH, H 



COOH 



+ 



COOH 



--> COOH 

 + 

 CH, 



COOH 



COOH 



COOH 



CH3 



(JOOH 



COOH 



(aY-i'ilvct(>- (oxalacetic 

 a(li|iic ucid) and acetic acids) 



COOH 



COOH 



(oxalic acid) 



