OXIDASES 505 



by a drop of dilute hydrogen peroxide, gives a deep blue 

 turning to brown. 



4. One per cent a-naphthol in 50 per cent alcohol and a 

 drop of hydrogen peroxide, gives a green colour. 



[b) Peroxidases — 



1. Guaiacum used as above gives a deep blue colour only 

 after the addition of hydrogen peroxide. 



2. Ten per cent aqueous solution of pyrogallol, followed 

 by a drop or two of dilute hydrogen peroxide, gives a reddish- 

 brown colour. 



TYROSINASE. 



This is the name given to another oxidizing enzyme which 

 is distinct from oxidases and peroxidases though it frequently 

 occurs in the same plants with these ; thus it occurs in many 

 Fungi, notably Riissula, and amongst the higher plants it 

 may be found in wheat bran and in the potato, especially in 

 the peripheral layers adjacent to the skin. 



The distinction between tyrosine, on the one hand, and 

 the oxidases and peroxidases, on the other, is that the latter 

 have no action upon monohydric phenols or their derivatives 

 either in presence or absence of hydrogen peroxide, whereas 

 tyrosinase acts upon p-cresol (i.) or tyrosine (11.) as well as 

 upon phenol and aminophenol even in the absence of hydrogen 

 peroxide — - 



HO— < )— CH3 HO— /^ \— CH2CHNH2 . COOH 



I. II. 



producing in the case of p-cveso\ an orange-red colour and 

 with tyrosine a series of colours through yellow, red, and 

 brown to black. On the other hand, tyrosinase has no action 

 upon pyrogallol in the presence of hydrogen peroxide in which 

 respect it differs from peroxidase. 



The mechanism of the reaction which takes place when 

 tyrosinase acts upon tyrosine has been investigated by Raper 

 and his fellow- workers.* 



It appears from their results that there are three stages in 

 the reaction, the first involving the production of the red 



* Raper and others : " Biochem. Journ.," 1923, 17, 454 ; 1924, 18, 84, 

 92 ; 1925, 19, 69 ; 1926, 20, 735 ; 1927, 31, 1370. 



