OXIDATIVE MECHANISMS 43 



In this manner oxidations involving large changes of energy 

 are split up into a number of steps, each involving smaller changes 

 of energy, and each catalysed by a specific enzyme. There are 

 yet more complex respiratory systems involving further carriers 

 in the chain between initial substrate and oxygen, but we need 

 not discuss these here as our knowledge of bacterial oxidation 

 has not yet progressed beyond the stages so far outlined. 



In order of increasing complexity then we have: 



(a) Oxidase systems: substrate type S■^_, Table III. 



In animal tissues we have oxidases attacking D-amino-acids, 

 amines, uric acid, etc., but few oxidases have so far been 

 identified in bacteria. Some organisms oxidise amines to the 

 corresponding aldehydes, but no analysis of the enzyme 

 systems involved has yet been made. Aerobic organisms 

 and those possessing cytochrome (Table IV) possess cytochrome 

 oxidase. An L-amino-acid oxidase has been found in Proteus 

 vulgaris and obtained in a cell-free condition by supersonic 

 disintegration of the cells : it carries out the reaction 



R . CHNH2 . COOH + > R . CO . COOH + NH3 



in which, presumably, the first step is a dehydrogenation to 

 the unstable imino-acid 



R . CHNH2 . COOH - 2H > R . C :NH . COOH. 



The oxidation of their substrates by certain mammalian 

 oxidases gives rise to the production of HgOg, but this L-amino- 

 acid oxidase of Pr. vulgaris is. said not to produce peroxide 

 and no evidence has been presented concerning the nature of 

 the protein, whether it has a flavine prosthetic group or not. 

 Hydrogen peroxide is highly toxic to living cells and many 

 organisms possess an enzyme, catalase^ which destroys peroxide 

 by breaking it down to water with the liberation of oxygen 



Catalase 

 2H2O2 ^2H20 + 02. 



Catalase is a haematin-enzyme and recently it has been shown 



