38 



Mr. H. Onslow. 



substances which might serve the purpose equally well, such as tryptophane, 

 histidine, or the decomposition product of keratin found by Gortner * which, 

 though free from tyrosine, yielded Millon's reaction. 



II. The Mechanism of the Action of Tyrosinase in Coloured Animals. 



The work described in this paper was undertaken with a view to throwing 

 more light upon the mechanism of the action of tyrosinase, and in it I hope to 

 present further evidence in favour of the current hypothesis that pigmentation 

 is the outcome of the action of an oxidase upon a colourless chromogen, and 

 also to describe the distribution of a peroxidase in the skins of coloured 

 rabbits. Further, I hope to show that a certain dominant white colour pattern 

 in rabbits is due to the presence of an anti-tyrosinase or inhibitor in the skin 

 of these animals, and that the recessive white variety is due to the lack of 

 one or both of the oxidase and chromogen constituents of the pigment- 

 producing system. 



1. Miss Durham's Experiments. 



At the outset of this work I was led to repeat some experiments of 

 Miss Durham's,f carried out in 1904 for the purpose of showing the presence 

 of a tyrosinase in the skins of young black rats, rabbits and guinea-pigs. An 

 aqueous extract was made from the skins, and solid tyrosine as well as ferrous 

 sulphate (as an activator) was added to the resulting reddish fluid. It was 

 then incubated at 37° G. for some days, in the course of which dark 

 precipitates were produced. When the skins of red guinea-pigs were 

 used the fluid became yellow, and an orange precipitate was thrown 

 down. In the absence of tyrosine or ferrous sulphate no coloration was 

 produced, nor was any precipitate found. The boiled extract proved inactive. 



In criticism of these experiments it seems in the first place unlikely that a 

 black precipitate would be the first product formed. Previous investigators 

 have found that the ferment fluid first becomes red or black on the surface 

 where it is in contact with- the air, and a precipitate only forms at the end of 

 the reaction or on acidification. Moreover, it is not stated whether the 

 reaction of the fluid extract was acid or alkaline. Presumably it would be 

 acid owing to the formation of lactic acid, but it is well known that tyrosinase 

 is most active in alkaline solutions, and that it is greatly inhibited by the 

 presence of even a minute amount of acid. Further, it has been observed 

 that the final colour produced by the action of tyrosinase upon tyrosine is 

 invariably black. A reaction, therefore, which finally resulted in the pro- 



* Gortner, 'Jour. Biol. Chemistry,' vol. 9, p. 355 (1911;. 

 t Miss Durham, ' Proc. Roy. Soc.,' vol. 74, p. 310 (1904). 



