vii] OXIDIZING ENZYMES 113 



The browning of plants on injury, as already mentioned, is very 

 wide-spread : more rarely, the same type of reaction may lead to the 

 formation of variously coloured products, other aromatic substances in 

 addition to the peroxide being in all. probability involved. The following 

 examples may be quoted. In some of the Higher Fungi, Boletus luridus 

 and B. cyanescens, when the pileus is broken across or injured, the tissue 

 rapidly turns a deep Prussian blue on exposure to the air. Also in the 

 Fungus, Russula nigricans, the colour changes from pink or reddish to 

 black. The latex of the tree, Rhus vernicifera, when exposed to air, 

 turns black and constitutes- the product, Japanese lacquer. 



Analogous pigments formed after death have been observed in 

 Schenckia blumenaviana (Rubiaceae) where the whole plant turns bright 

 red in chloroform vapour, as well as in species of Jacobinia and Aloe. 

 Blue pigments are also formed after death in the Teasel (Dipsacus), in 

 the flowers of an Orchid (Phajus), in the Dog's Mercury (Mercurialis 

 perennis), annual Mercury (M. annua) and in the well-known indigo- 

 producing plants, the Woad (Isatis tinctoria) and the Indigo (Indigofera) 

 (Walther, 28) (see p. 152). It has not been shown, however, that oxidizing 

 enzymes are present in all the above cases. 



There is a well-known oxidizing enzyme, tyrosinase, which is some- 

 what specific in its action in that it will only oxidize tyrosine and a few 

 other substances, such as js-cresol. It occurs in the tubers of the Dahlia 

 (Dahlia variabilis) and Potato (Solarium tuberosum), in grains of barley 

 (Hordeum) and wheat (Triticum) and in Papaver orientale. In the lower 

 plants it is found among the Basidiomycetes. 



When tyrosinase acts upon tyrosine, a yellowish-pink colour is first 

 produced which rapidly darkens to red, brown and finally black. A 

 solution of p-cresol 



can be used as a delicate test for the enzyme. If the enzyme is present, 

 a yellowish- or orange-red colour is formed. 



Tyrosinase of the Potato tuber can be precipitated from a water 

 extract with absolute alcohol : or if the potato tissue is extracted with 

 cold 96 /o alcohol, the enzyme is precipitated and remains in the 

 o. 8 



