124 AROMATIC COMPOUNDS [ch. 



show this phenomenon. Since the oxidase provides an active oxidizing 

 system, it is probable that a general oxidation of aromatic and other 

 substances {in addition to catechol) takes place after death, in many 

 cases leading to the production of dark pigments, e.g. the blackening of 

 lacquer from latex of the Lacquer tree {Rhus verniciferay. In Schenckia 

 hliimenaviana (Rubiaceae), also, the whole plant turns bright red in 

 chloroform vapour, and blue pigments are formed in flowers of an Orchid 

 (Phajus) after death. 



Tests for peroxidases are based on the property of a number of sub- 

 stances (benzidine, a-naphthol, guaiacum, pyrogallol, etc.) of giving highly 

 coloured oxidation products in presence of active oxygen. Hence solutions 

 of the above substances in the presence of hydrogen peroxide provide 

 tests for peroxidases: 



XD' 



HoN<r V-< >NH 



OH NH2 



a-Naphthol Benzidine p-Phenylenediamine 



Expt. 118. Demonstration of the presence of a peroxidase. Pound' up a little 

 Horse-radish root {Cochlearia Armoracia) with water. Filter and, taking a few c.c. 

 each time in a small evaporating dish, make the following tests : 



{a) Add a few drops of a 10 % solution of guaiacum. No colour is developed. 

 Add a few drops of hydrogen peroxide : a deep blue colour appears. 



Guaiacum gum is obtained from two West Indian species of Guaiacum trees, 

 G. offi^cinale and O. sanctum^ partly as a natural exudation and partly by means of 

 incisions. It gives a yellow solution with alcohol which contains guaiaconic acid, and 

 the latter, on oxidation, yields guaiacum blue. As far as possible, inner portions of 

 the resin lumps should be used, as the resin oxidizes in air, and then may give un- 

 reliable results. It is best to make the tincture freshly before use, and, as a precaution, 

 to boil it on a water-bath with a little blood charcoal (preferably Merck's) and filter. 

 Guaiacum gum tends to form peroxides on exposure to air, and these are removed 

 by the above treatment. 



(6) A 1 0/0 solution of a-naphthol in 50 % alcohol, followed by a few drops of 

 hydrogen peroxide. A lilac colour is developed. 



(c) A 1 ^Iq solution of benzidine in 50 % alcohol followed by a few drops of 

 hydrogen peroxide. A blue colour is developed. 



{d) A\ ^/q solution of jo-phenylenediamine hydrochloride in water followed by a 

 few drops of hydrogen peroxide. A greenish colour is developed. 



Repeat the above experiments with an enzyme extract that has been boiled. No 

 colour is given, showing that the enzyme has been destroyed by boiling. Other 



1 The chief constituent of the latex, however, is a catechol derivative. 



