292 DYEING 



dyes he tried were capable of precipitating tr}^psin from solution. 

 The precipitate is coloured by the dye. This suggested that 

 neutral red and other azine dyes might be used as indicators of 

 the presence of proteolytic enzymes, an idea taken up with en- 

 thusiasm by Koehring,^^^ who thought there was some necessary 

 connexion between colouring with this dye and the synthesis or 

 degradation of protein. There may indeed be such a connexion in 

 particular cases, but neutral red can of course colour other cellular 

 constituents than proteolytic enzymes. 



The colouring of mitochondria by Janus green B calls for 

 particular mention. This dye (called green from the colour it 

 gives to certain textiles) generally imparts a blue colour to these 

 particular organelles and nothing else in the cell, if used in 

 sufficiently dilute solution; or if the cell as a whole takes it up, the 

 mitochondria retain it when the rest of the cell has become 

 colourless. This very special (though not complete) specificity of 

 the dye was mentioned on p. 281, w^hen the objects that are 

 coloured by vital dyes were being enumerated. 



Attempts have naturally been made to explain this specificity. 

 It is held to be related to the differential reduction of the dye in 

 various parts of the cell. Because Janus green is both an azine and 

 an azo dye, its reduction is complicated. At the first stage the 

 azine-azo structure is retained ; but further reduction splits the azo 

 linkage, and a red azine dye, rather closely related to safranine, 

 results. This has a tendency to give a diffuse, very pale pink 

 colour to the nucleus, and sometimes to the cytoplasm also. Still 

 further reduction produces the leucobase of the azine. Molecular 

 oxygen can oxidize the leucobase to the red azine, but cannot re- 

 assemble the broken azo-link.^^^ 



It has been suggested ^^^ that the dye is first taken up in its blue 

 form by the mitochondria and subsequently reduced by them to the 

 red compound, which escapes and colours the cell diffusely. 

 Lazarow and Cooperstein,^^^ who have made a special study of 

 Janus green, have reached a very different conclusion. They con- 

 sider that the dye is absorbed everywhere in the cell, but soon re- 

 duced except in the mitochondria, where the cytochrome C/cyto- 

 chrome oxidase system maintains it in the oxidized (blue) form. 

 They have shown that outside the cell Janus green B is taken up 

 by diverse proteins and not by mitochondria only, and they con- 

 sider the specific colouring to be due to the vital activities of the cell. 



The vital activity of mitochondria can be exhibited in an even 



