392 ENZYMES 



OXIDASES. 



The oxidases are enzymes which have the power of oxi- 

 dizing various aromatic compounds and chromogens, which 

 action is frequently indicated by a change in colour. This 

 change in colour in vegetable tissues on exposure to air is an 

 everyday phenomenon ; the exposed surfaces of a bitten apple, 

 especially cider varieties, will rapidly turn brown ; similarly 

 the fruit-body of Boletus quickly assumes a Prussian-blue colour 

 on being brokea The darkening in the colour of raw rubber 

 is also due to an oxidase which is associated with the protein 

 of the coagulated latex. * 



These changes are often of considerable economic import- 

 ance ; thus the discoloration of sap wood markedly depreciates 

 the value of the timber,t while the lacquer industry of China 

 and Japan has been built up on the facts relating to the action 

 of the oxidase, laccase, on the expressed sap of species of 

 Rhus. (See below.) 



Oxidases are very widely distributed in the vegetable 

 kingdom ; in the higher pla/its they may occur in any organ 

 stem, root, leaf, laticiferous tissue, petals, and fruits. 



Several oxidases have been distinguished, e.g. laccase, 

 which has already been mentioned ; tyrosinase, which oxidizes 

 tyrosine into homogentisic acid (p. 361); olease, from olives, 

 which can oxidize fats into simpler fatty acids, % arid others 

 which oxidize sugars into carbon dioxide and water. 



The action of oxidases may be illustrated by a brief refer- 

 ence to laccase, an enzyme which was first investigated by 

 Yoshida.ll The latex of many species of Rhus rapidly turns 

 brown and finally black on exposure to the atmosphere ; if 

 the juice be evenly spread out, the final product is black and 

 shiny. The extract of the plant contains urushic acid (laccol) 

 which is oxidized into oxyurushic acid : 



CuHuO, + O = C 14 H 18 3 



The action takes place best at 20 C. in the presence of mois- 

 ture and oxygen ; at higher temperatures it is destroyed, at 



*Spence : " Biochem. Journ.," 1908, 3, 165, 351. 



t Bailey : " Bot. Gaz.," 1910, 50, 142. 



JTolomei: "Chem. CentrbL," 1896, i, 879. 



See Palladin : " Zeit. physiol. Chem.," 1906, 47, 407. 



UYoshida: " J. Chem. Soc., Lond.," 1883, 43, 472. 



