162 BREAKDOWN OF NITROGENOUS MATERIAL 



Reductive deamination 



In this case hydrogen is added to the substrate with the 

 production of a saturated fatty acid: 



R . CHNHg . COOH + 2H -> R . CH2 . COOH + NH3. 



This type of deamination has been demonstrated with certain 

 strict aerobes (e.g. Mycoh. phlei) in the case of aspartic acid, 

 which is reduced to succinic acid with the liberation of ammonia : 



HOOC.CH2.CHNH2.COOH + 2H 



> HOOC . CH2 . CH2 . COOH + NH3. 



Desaturation deamination 



In this case NH3 is removed from the amino-acid molecule, 

 leaving an unsaturated fatty acid: 



R.CH2.CHNH2.COOH ^R.CH= CH.COOH + NH3. 



When intact cells of Esch. coli deaminate aspartic acid, the 

 final product is succinic acid, but if the deamination takes 

 place in the presence of certain inhibitors such as toluene, 

 then the end-product is not succinic acid but fumaric acid, 

 and the deamination takes place according to the equation : 



HOOC . CH2 . CHNH2 . COOH 



> HOOC . CH = CH . COOH + NH3. 



In the absence of inhibitors fumaric acid is reduced to succinic 

 acid. The enzyme responsible for the desaturation deamina- 

 tion is called " aspartase," and has been isolated in a cell-free 

 state. The aspartase reaction is reversible so that aspartic 

 acid can be synthesised from ammonia and fumaric acid. 

 Since the reaction is reversible, the end-products of either 

 forward or back reactions form an equilibrium mixture of 

 ammonia, fumaric acid, and aspartic acid. When the intact 

 organism is used as source of the enzyme, the equilibrium 

 mixture is further complicated by the presence of another 

 reversible enzyme, fumarase, which catalyses the hydration 

 of fumaric acid to malic acid. In the intact organism the 

 deamination of aspartic acid may lead to the formation of 



