200 BACTERIOLOGICAL CHEMISTRY 



under the influence of yet another enzyme, oxidase : — 



OH { \—0 



+ Go oxidase 2 | + 2H2O 



OH " > K }—o 



(catechol) (catechol peroxidase) 



It is to this action that the spontaneous browning of 

 apples or potatoes in air is due. The peroxides so formed 

 are then activated by peroxidase to regenerate the original 

 catechol, with oxidation of the oxidisable substance : — 



-0 ( \OVL 



I + AH2 peroxidase + A 



-0 > \/^^ 



If the oxidisable substance, AH2, is a compound which 

 is coloured in the oxidised form it can be used as a test 

 for the presence of peroxidase. The most common of 

 these substances are a-naphthol (giving a lavender colour), 

 guaiacol (red), benzidine (blue), ^^-phenylenediamine 

 (greenish), dimethyl ^^-phenylenediamine (purple to black) 

 and indophenol (purple). 



The oxidation enzymes, catalase, oxidase and peroxi- 

 dase, are all inhibited by the action of cyanide, sulphide 

 and carbon monoxide. They all contain iron in the form 

 of hsem (an iron compound of protoporphyrin) which is 

 the prosthetic group carried by different specific proteins 

 to give the complete enzyme . The breakdown of hydrogen 

 peroxide by catalase is accompanied by a reduction 

 and re -oxidation of the ferric iron in the enzyme : — 



4Fe+++ + 2H2O2 > 4Fe++ + 4H+ + 20o 



4Fe++ -f 4H+ + O2 > 4Fe+++ + 2H2O 



The oxidation enzymes play a further important part 

 in the respiration of bacteria in that they are involved 

 in the action of the respiratory pigments. Perhaps the 

 best loiown of these is the cytochrome complex which 

 consists of three iron-containing components, a, h and c, 



