OXIDASES OR OXIDISING ENZYMES. 263 



also assists to oxidise the chromogens of certain fungi, 

 especially those which yield a blue or a red colouring 

 matter. The laccase exists in the juice of the fungi side 

 by side with the chromogen, but when the juice is boiled 

 before exposure to the air has taken place the laccase is 

 destroyed, and the chromogen in consequence remains un- 

 changed. 



An enzyme similar in many respects to laccase has been 

 described by Pieri and Portier as existing in the gills, palpi 

 and blood of certain molluscs (14). 



TYROSINASE. 



In other fungi there are different chromogens which do 

 not turn blue on exposure to air, but become red and finally 

 black. Of these, Russula nigricans Bull, is perhaps the 

 most conspicuous example. The substance which gives 

 rise to the black colour is almost insoluble in alcohol, but 

 after the fungus has been boiled with this reagent it can 

 be extracted from the residue by subsequent extraction 

 with boiling water. When such an extract is treated 

 with a little fresh cold water extract of the fungus, or a 

 piece of the tissue is added to it, the liquid turns red and 

 after a time black. If the chromogen is extracted from 

 the fungus by boiling water and rapidly pressed, and the 

 exuded liquor filtered and concentrated to a small bulk, 

 it deposits colourless needle-shaped crystals, usually col- 

 lected together into spheres. They are not soluble in 

 alcohol nor readily in cold water, but they dissolve freely 

 in hot water. They have been identified by Bertrand 

 with tyrosin. 



Bertrand has observed the same general course of 

 behaviour in the expressed juice of the roots of the beet, 

 the tuberous roots of the dahlia, and the tubers of the 

 potato. In these cases also he has identified tyrosin in the 

 tissues. 



The similarity of behaviour to that observed in the cases 

 of Boletus, Lactarius, etc., points to a similar cause of the 

 change of colour. Laccase, however, has no power to set 



