756 7. MERCURIALS 



that adult hemoglobin contains two SH pairs, sickle cell hemoglobin three 

 SH pairs, and hemoglobin C no pairs, while Huisman obtained quite differ- 

 ent results indicating more reactive SH groups than were found by most 

 previous workers. At pH 7, adult hemoglobin reacts with 3 p-MB molecules 

 and fetal hemoglobin with 2, whereas at pH 4.6 the values are 6 and 4, 

 respectively, the lowering of the pH presumably altering the configuration 

 of the protein. Studies of mercaptalbumin show that Hg++ can induce the 

 formation of dimers of the protein, but Allison and Cecil (1958) believe 

 that only monomers occur in the case of hemoglobin, and found that Hg++ 

 and PM give the same titer, while the results of Cecil and Snow (1962) are 

 more in accord with those of Ingram, 2.2 reactive SH groups of a total of 

 6 in adult hemoglobin being detected, the 3.8 sluggish SH groups reacting 

 differently with Hg++, PM, and p-MPS. These studies of hemoglobin not 

 only show the influence of many factors on the interaction of proteins with 

 mercurials, but point out the difficulties of SH titration of even relatively 

 simple proteins. 



Reaction of hemoglobin with mercurials brings about striking changes in 

 the characteristics of oxygenation: The affinity of hemoglobin for Og may 

 be unaffected or increased, but the interactions between the heme groups 

 are reduced or abolished by all the mercurials (A. F. Riggs, 1952; Wolbach 

 and Riggs, 1955; Riggs and Wolbach, 1956; A. Riggs, 1959). Mersalyl and 

 Hg++ at pH 6.8 increase the affinity for 0^ quite markedly, Pjq decreasing 

 to about one third of normal, but p-MB and MM do not alter the affinity. 

 AU these mercurials lower the interaction constant n from around 2.9 to 1 

 (or near 1), the latter corresponding to complete loss of heme-heme interac- 

 tion. Glutathione completely reverses these effects. It is very interesting 

 that the maximal effects are produced at ratios close to 2 moles of mer- 

 curial to 1 mole of hemoglobin; however, as the amount of mersalyl is in- 

 creased, the changes in heme interaction and Og affinity progressively dis- 

 appear, so that at a ratio of 15-16 moles of mercurial per mole of hemoglobin 

 there is no longer an effect. This curious reversal is unexplained. The ob- 

 servation may have some bearing on the use of mercurials for enzyme inhi- 

 bition, and Riggs and Wolbach (1956) state, "Our observations suggest 

 that the attempt to inhibit an enzyme with only a single high concentration 

 of mercurial may lead to spurious conclusions." I know of no example in 

 which enzyme inhibition is lost at higher mercurial concentrations, but in 

 any case it would presumably be a rare phenomenon. Occasionally one finds 

 stimulation of enzyme activity at low mercurial concentrations and this 

 reverses to inhibition as the concentration is increased, but it is not known 

 if this has any relation to the above reversal. Mercuration of oxyhemoglobin 

 increases the rate constants for the dissociation of Og from three of the 

 hemes, but decreases the rate constant for the dissociation of the last Og, 

 this being in fair accord with the effects on the Og dissociation curve ob- 



