OXIDASES 177 



was present in this extract: the soluble tyrosinase could 

 be precipitated with ammonium sulphate from the filtrate 

 left on filtering off the insoluble enzyme. The soluble enzyme 

 was capable of colouring tyrosin dark violet black, with the 

 final formation of a precipitate, within twenty-four hours ; this 

 reaction did not take place if the extract was previously 

 heated to 90° C. The insoluble tyrosinase caused the tyrosin 

 solution to undergo a series of colour changes ranging through 

 pink, rose, violet and blue-black to a deposition of a black 

 pigment-like substance, leaving the supernatant liquid 

 completely decolourised. That a small quantity of the 

 enzyme was able to affect a large quantity of tyrosia was 

 proved by pouring away the colourless supernatant liquid 

 and adding more tjnrosin solution, when the series of colour 

 changes was repeated. This operation of pouring off the 

 colourless solution and adding more tyrosin was done seven 

 times with one specimen of insoluble tyrosinase, weighing 

 approximately O'Ol gram. Tyrosinase, therefore, has the 

 property of continuous activity which is characteristic of 

 enzymes in general. 



The brown colour of tea is due to an oxidase formed in 

 the growing leaf. This has been investigated by Mann, who 

 extracted the enzyme in the following way : — 



Ten grams of fresh, or 6'6 grams of the withered leaf, were 

 ground to pulp, 5 grams of hide powder were added in 

 order to precipitate the tannin, and the mixture ground 

 together with a known quantity of water in which the enzyme 

 is soluble. After standing two hours the mixture was pressed 

 through a cloth and precipitated by alcohol, the amount of 

 enzyme present in the solution could be determined by the 

 intensity of the blue colour produced in a known amount of 

 the solution by guiachum tincture ; a certain proportion of 

 the enzyme gave a blue colour with guiachum resin alone, 

 while another portion required the addition of hydrogen 

 peroxide before the blue colour was obtained. Only those 



