300 BACTERTOLOaiCAL riHEMISTRY 



COOH 



CH, 



CH3COOH 



I I + 



HO.C.COOH > H.rOOH + CO > COOH CO^ 



I 11 + 



CH, CHo.COOH CH, > CH, > COOH 



I "^ III 



COOH COOH COOH COOH 



(acetonedicarboxylic (maloiiic acid) 

 acid 



The evidence for these reactions is somewhat meagre ; 

 small amounts of saccharic, acetonedicarboxylic and 

 malonic acids were isolated, but p y-diketo-adipic acid 

 could not be detected as an intermediate nor could added 

 Py-diketo-adipic acid be fermented to citric acid. 



Citric acid can be formed from glycerol and pentoses 

 as well as from glucose, fructose and sucrose. This sug- 

 gests that the carlwn source is broken down to a common 

 intermediate, which is then built up into a reserve carbo- 

 hydrate characteristic of the particular fungus, and that 

 this in its turn breaks down to give a hexose which is the 

 immediate precursor of the citric acid. The same explana- 

 tion may be offered of the formation of gluconic acid from 

 the pentose, arabinose, of kojic acid from pentoses, of 

 mannitol from glycerol and pentoses, and of poly- 

 saccharides composed of hexose units from glycerol and 

 pentoses . 



Assuming the production of a common hexose pre- 

 cursor the theories of Butkewitsch, of Gudlet and of Ciusa 

 and Briill become possible. Butkewitsch suggested that 

 glucose was oxidised to glucuronic acid which underwent 

 intramolecular aldol condensation to give a f ive-membered 

 ring compound which was subsequently split and the 

 terminal C atoms oxidised with formation of citric 

 acid : — ■ 



