REACTIONS WITH PROTEINS 761 



or their individual reactivity, by effects on the structure of the protein, or 

 on the association of subunits. (2) The pH will determine the over-all charge 

 on the protein; e.g., with increased pH the protein will become more neg- 

 atively charged and possibly repel negatively charged mercurials, such as 

 p-MB, p-PMS, or the higher Ch complexes with Hg++. (3) Rise in pH will 

 increase the OH" concentration and this ion will compete with the SH 

 groups for the mercurial. (4) The pH may determine the degree of hydrogen 

 bonding of SH groups and thus their reactivity with mercurials. It is likely 

 that the dependence on the pH will depend on the mercurial used, but in- 

 sufficient data are available for comparisons. 



A final effect of pH involves dimerization where it occurs. The rate and 

 degree of dimerization in the presence of Hg++ will depend on the total 

 protein charge, being maximal at the isoelectric point, all else being equal. 

 Straessle (1951) reported that the dimerization of mercaptalbumin with a 

 bifunctional mercurial is slower at pH 6 than at 4.75, and Edelhoch et al. 

 (1953) found the rate of dimerization with Hg++ to be increased 60 times 

 when the pH is decreased from 6 to 4.75, and doubled with further de- 

 crease to 4.25. It was calculated that a charge of 9 charge units would ac- 

 count for this, and titration data indicated a change of 10 units over this 

 pH range, so the electrostatic mechanism seems to be correct. 



Effects of Mercurials on Protein Structure and Properties 



The importance of secondary changes in protein structure upon reaction 

 with a mercurial cannot be overemphasized in studies of enzyme inhibition 

 and its reversibility, but unfortunately little exact information is available. 

 Configurational changes have been postulated to explain certain results, 

 such as have already been mentioned in regard to mercaptalbumin (page 

 757) and hemoglobin (page 755), and additional examples wiU be presented 

 in connection with enzyme inhibition, but in most instances the evidence is 

 indirect and tenuous. Nevertheless most investigators agree, I believe, in 

 accepting that such changes occur in certain cases; the problems are the 

 nature of the changes and the mechanisms by which they are induced. 



Higher concentrations of Hg++ and most organic mercurials decrease the 

 solubility of proteins, and may precipitate or coagulate them. This gave 

 rise to the early concept of the mercurials as denaturing agents. However, 

 it would appear that the primary effect is seldom denaturation (in the sense 

 of disruption of the polypeptide chain structure), and that the altered prop- 

 erties of the protein are more directly related to modification of side-chain 

 groups and the introduction of new groups. Prolonged contact of proteins 

 with mercurials occasionally leads to true denaturation as a secondary reac- 

 tion, but complete reversibility can usually be achieved by removing the 

 mercurial; this indicates that if structural changes occur they are probably 

 localized, and that the normal configuration can be restored. Such direct 



