128 AROMATIC COMPOUNDS [ch. 



is tinged with brown and, on extraction with water, the filtrate will give 

 an oxidase reaction with guaiacum. This is to be explained by the fact 

 that the peroxide has been adsorbed by the tissue residue. This pheno- 

 menon is probably the explanation of the preparation of some oxidases 

 called "laccases." Such enzymes have been obtained by the precipita- 

 tion with strong alcohol of the expressed juices (containing peroxide 

 since they were obtained by crushing the tissues) of plants which brown 

 on injury. The enzyme and other organic matter is precipitated and 

 carries with it the peroxide. Such a product will readily oxidize phenols 

 with other groupings, e.g. pyrogallol, quinol, etc. 



Tyrosinase. This enzyme is widely distributed in plants. It occurs 

 in the Banana (Musa sapientum), Wheat {Triticum vulgar e), Beet {Beta 

 vulgaris), Oriental Poppy (Papaver orientale), Lacquer tree (Rhus 

 vernicifera), Potato (Solarium tuberosum) and Dahlia (Dahlia variabilis). 

 It has been demonstrated in about 16 natural orders and 21 genera. 



Tyrosinase oxidizes tyrosine with the evolution of carbon dioxide 

 and ammonia and the production of a pink colour which darkens through 

 red to black. The final black pigments are known as melanins. A solu- 

 tion of ^-cresol 



CH3 



V 



OH 



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 7o alcohol, the enzyme is precipitated and remains in the tissue 

 residue, as does the peroxidase (Expt. 123(A)], but the tyrosine is 

 almost entirely washed away. 



£Jxpt. 125. Demonstration of the presence of tyrosinase in the Potato. Take about 

 half a potato and proceed as in the preparation of peroxidase [see Expt. 123 (A)]. 

 Roughly dry the powder left on the filter and then add about 100 c.c. of water and 

 allow to stand for 15 mins. Filter, and divide the filtrate into four portions a, 6, c 

 and d. Make a suspension of a little tyrosine in water (tyrosine is only slightly 

 soluble in cold water). 



To a add 5 c.c. of tyrosine suspension. 



To h add 5 c.c. of tyrosine suspension and boil. 



To c add some p-cresol. 



To d nothing is added. 



