REACTIONS OF FIXATIVES WITH PROTEINS. 2 55 



would become very acidic; conversely, if it were to block all 

 -COOH groups, the protein would become very basic. These 

 changes would control the reactions with dyes, for the coloured 

 ions of the latter associate with the acidic and basic groups of 

 proteins (see p. 192). It is rather a strange fact that most authors 

 who have considered mercuric chloride in this connexion have 

 taken it for granted that this salt simply blocks -COOH groups 

 and thus makes the protein more basic (that is, more attractive to 

 'acid' dyes (p. 167)).^^^' *^^' ^64 it is stated, however, by the Ameri- 

 can histochemist Gomori ^^^ that mercuric chloride reduces the 

 attraction of proteins for acid dyes, by blocking -NH2 groups. 

 As we have seen, the reaction of this fixative with proteins is in 

 fact very complicated. 



Mercuric chloride is pre-eminent among fixatives for leaving 

 tissues in a condition conducive to brilliant dyeing. It is relevant to 

 consider here the characters of a fixative that will achieve this end. 

 It must not fix proteins in such a tight gel that dyes cannot pene- 

 trate, but must somehow make them porous. It must knock DNA 

 off from combination with protein and precipitate it in a form in 

 which it will associate readily with basic dyes. It must leave the 

 protoplasmic proteins in a state in which they will accept acid 

 dyes, so that these parts may be given a colour that will contrast 

 with that given to the DNA. It is evident that mercuric chloride 

 achieves these ends, in the circumstances of ordinary fixation; 

 but we await a full explanation of its superiority over other fixatives 

 in these respects. 



Most fixatives leave proteins in a digestible state; denaturation, 

 as we have seen (p. 48), promotes digestibility. In experiments 

 of this sort the excess of fixative must be very carefully washed 

 out: for instance, by washing in running water for 20 days. 

 When this has been done, proteins fixed by additive fixatives 

 generally remain digestible. It has been shown, however, 

 that mercuric chloride slows down slightly the rate of digestion 

 of the proteins of blood plasma by pepsin and trypsin.^^® This 

 may be due to a change in the protein, but it is also possible 

 that some of the bound mercury transfers itself to the enzyme 

 and fixes it. 



Picric acid is an alkaloidal reagent: that is to say, it precipitates 

 alkaloidal salts from solution in water. This is a usual property of 

 large, complex anions. Free amino-acids combine with the anion 



