108 AROMATIC COMPOUNDS AND [CH. 



Expt. 113. Demonstration of the presence of a peroxidase. Repeat the above 

 experiment, using scales from an Onion bulb. Add a few drops of hydrogen peroxide 

 solution. Green pea tissue may also be used. No blue colour is produced until after 

 the addition of hydrogen peroxide. 



If a systematic examination is made of genera from various natural 

 orders with the guaiacum test, it will be found that the oxidase reaction 

 is shown by all, or nearly all, genera of some orders, for instance most Um- 

 belliferae, Labiatae, Boraginaceae and many Compositae. In other orders, 

 i.e. Cruciferae and Crassulaceae, the peroxidase reaction is predominant. 

 Moreover, the oxidase-containing genera show another phenomenon, 

 as follows. If the tissue extracts are left in air, they become dark- 

 coloured, generally brown or reddish-brown. If the plants themselves 

 are bruised or injured, they turn brown. The same effect may be pro- 

 duced very rapidly by the action of chloroform vapour. These phenomena, 

 the darkening of extracts and the discoloration, both on injury and in 

 chloroform, will be found to be absent from plants giving the peroxidase 

 reaction only. 



Expt. 114. To show the distribution of oxidases and peroxidases in various plants, 

 and the correlation between the presence of oxidase and browning on injury or in 

 chloroform vapour. Take a selection of the plants given below, and in each case grind 

 up a portion of the plant in a mortar with a little water and filter. Divide the 

 nitrate into two parts in small porcelain dishes. Allow one part to stand in air, and 

 note the darkening in colour in cases where an oxidase is present. To the other add 

 a few drops of guaiacum. To extracts containing a peroxidase only, after 5-10 

 minutes, add in addition a few drops of hydrogen peroxide. Further, small pieces 

 of the plants to be tested should be placed in a corked flask containing a few drops 

 of chloroform, and the development of browning noted in the case of plants contain- 

 ing an oxidase. For demonstration of oxidases the following plants may be used : 

 Christmas Rose (Helleborus niger}. Dandelion (Taraxacum officinale), Forget-me-not 

 (Myosotis\ Hawthorn (Crataegus) and White Dead Nettle (Lamium album}. For 

 peroxidases : Arabis, Aubrietia, Buttercup (Ranunculus acris\ Pea (Pisum sativum), 

 Stock (Matthiola\ Wallflower (Cheiranthus Cheiri) and Violet ( Viola}. 



Various hypotheses have been suggested in explanation of the re- 

 actions of oxidizing enzymes. The one most generally accepted (Chodat, 

 1) is that the oxidase consists of two components, a peroxide and a 

 peroxidase, The peroxide may be either hydrogen peroxide or some 

 organic peroxide. The peroxidase acts upon the peroxide, depriving it 

 of an atom of oxygen which is transferred, in an " active " condition, to 

 some substance (acceptor) which becomes oxidized. Guaiacum is the 

 acceptor in the above experiments. The reaction may be approximately 



expressed thus : 



2AO + O 2 = 2AO 2 (peroxide). 

 AO 2 -I- peroxidase = AO + O. 



