52 FIXATION 



These substances will now be considered in turn. 



The reactions of mercuric chloride and other mercuric 

 salts with proteins have been carefully studied by several 

 authors. ^^^' ^*^' ^*^' ^^^ The metal becomes attached to protein in 

 several different ways at the same time, unless special precautions 

 are taken to isolate the reactions. 



Mercuric salts react with the sulphydryl (-SH) group of the 

 cysteine component of proteins. If the amount of mercury be 

 restricted to what will combine with the sulphur present in the 

 protein, no other reaction will occur; this may therefore be called 

 the primary reaction. It is possible to isolate a fraction of serum 

 albumin that contains only one sulphydryl group in each mole- 

 cule. ^^^ This may be crystallized in the form of a compound with 

 mercury, which contains one atom of mercury to two molecules 

 of the albumin fraction. The evidence suggests strongly that 

 mercury forms a link between cysteine residues. Each of the latter 

 is potentially mercurium captans, a mercury-catcher, ready to 

 form a compound (mercaptide) with the metal. 



"? 



NH NH 



I 

 CH2.S-Hg-S.H2C.CH 



I 



Q=o c=o 



. . ^^ . 



Mercury forming a link between cysteine side-groups in tzvo protein chaitn 



Many such links could be formed between protein chains in 

 which cysteine occurred repeatedly, and many chains could be 

 bound together into a single, polymeric whole. This would tend 

 towards coagulation. 



It is unlikely that mercaptide-formation is the main cause of 

 coagulation. When an excess of mercuric salt is present, as in 

 ordinary fixation, much more of the mercury is taken up in other 

 ways, to which we must now turn our attention. 



The uptake of mercury by proteins is profoundly affected by 

 acidity and alkalinity. The main features of the process are set out 

 schematically in table 3. A scale of pH is not provided, because 

 each protein would require a different one. The lines dividing the 

 degrees of alkalinity and acidity must not be regarded as separating 

 sharply the several reactions, for in fact these overlap, so that more 

 than one reaction may occur at any particular pH. The table calls 

 attention to the reactions that are dominant in certain broad regions 

 of alkalinity and acidity. 



