754 



7. MERCURIALS 



(1941) found it difficult to determine if p-MB reacts with ovalbumin or 

 not, since ferricyanide and nitroprusside cannot be used to detect these 

 SH groups (they do not react with ovalbumin) and iodine oxidizes the 

 group whether free or combined with p-MB. Using an indirect method in- 

 volving the determination of the binding of p-MB to cysteine in the pres- 

 ence of ovalbumin, Anson claimed that either p-MB does not react at all 

 with the ovalbumin SH groups, or, if so, the binding is much less strong 

 than with cysteine. If the ovalbumin is denatured, demonstrable binding 

 of the mercurial occurs. W. L. Hughes (1950) reported that MM reacts 

 with ovalbumin very slowly, the reaction requiring a day to come to equi- 

 librium. However, MacDonnell et al. (1951) obtained a crystalline derivative 

 of ovalbumin treated with 59-MB by adjusting the pH to 4.7 and adding 

 ammonium sulfate to opalescence. Three of the four SH groups of oval- 

 bumin react and the fourth reacts only after denaturation. The complex 

 is very stable and is not dissociated by cysteine or prolonged dialysis, al- 

 though 6-day dialysis against cysteine at pH 7.9 dissociates about 75% of 



EQUIVALENTS 



AMOUNT OF MERCURIAL ADDED 



Fig. 7-3. Theoretical curves for the reac- 

 tion of a mercurial with a protein, such as 

 ovalbumin with three reactive SH groups. 

 See text for explanation. 



the mercurial. In such cases as this, if one plots the amount of mercurial 

 combined, or the disappearance of SH groups from the protein, against the 

 amount of mercurial added, one may obtain different curves (Fig. 7-3). 

 If the reactions with the free SH groups are equivalent, a linear relation- 

 ship to saturation will be obtained (curves A and B), but if interaction be- 

 tween the SH groups occurs (i.e., if the binding of a mercurial reduces the 

 binding of the next) or the SH groups combine with the mercurial with 

 different affinities, a curve concave downward will be obtained (curve C). 

 If no further reaction with protein groups occurs after saturation of the 

 SH groups, the curve will be horizontal (curve A), but if other groups with 



