826 7. MERCURIALS 



Low concentrations of reversor in a definite molar ratio to enzyme or 

 mercurial have seldom been used, but in the examples we have the reversal 

 is only partial. Succinate oxidase preparation from pigeon muscle is inhi- 

 bited 88% after 15-min incubation with 0.03 mM Hg++; reversal by di- 

 mercaprol for 30 min depends on the amount of the dithiol added (see ac- 

 companying tabulation) (Barron and Kalnitsky, 1947). Myocardial succin- 



Dimercaprol : Hg++ % Reversal 



3.3 



5 33 



10 63 



ate oxidase inhibited completely by p-MB can be 31% reactivated by glu- 

 tathione at a molar ratio to the mercurial of 1 : 1 and 65% at a ratio of 5 : 1 

 (Slater, 1949). Treatment of 3-phosphoglyceraldehyde dehydrogenase with 

 33-MB leads to changes in the optical rotation; if the exposure to the mer- 

 curial is only 1 min, cysteine at a molar ratio of 6 : 1 reverses these changes 

 around 75%, but the structural changes become progressively more irre- 

 versible (Elodi, 1960). To reverse the inhibition of glutamate dehydrogen- 

 ase by p-MB maximally (80%) it requires around 60 times as much glu- 

 tathione as mercurial* (Olson and Anfinsen, 1953). 



* These experiments were done by varying the p-MB concentration from 0.003 

 to 3.3 mM and keeping the reversor, glutathione, at 10 mM, so that the more normal 

 conclusion is simply that the reversibility is less, the higher the mercurial concentration, 

 a phenomenon commonly observed with other enzymes. The plotting is ambiguous; 

 the final enzyme activity is plotted as % of the uninhibited enzyme, but the results 

 with inhibitor alone are not given, although they can be estimated from another figure, 

 so the degree of reversal is not immediately apparent. For example, when p-MB is 

 0.11 mM the inhibition is given elsewhere as 50%; after 10 mM glutathione (molar 

 ratio 90 : 1) the enzyme activity is given as approaching 80%, which I would call 

 60% reversal on one basis or 30% on another. However, this is stated to be 70% 

 reversal in the text. No incubation times were mentioned so possibly the failure to 

 achieve more reversal is due to an insufficient time with the reversor. Other than the 

 work on reversal, this investigation is an excellent, detailed, and quantitative study 

 on an enzyme, and thus illustrates a rather common phenomenon — the lackadaisical 

 and disoriented approach to inhibition reversal. In 90% of the reports in which re- 

 versal is determined, apparently an arbitrary (but high) concentration of reversor is 

 added, and the enzyme activity is measured after an arbitrary interval, so the conclu- 

 sions should usually be taken as arbitrary. If these remarks, and others scattered 

 throughout the book, can stimulate the performance of more accurate and interpretable 

 reversal experiments, my aim will be achieved. 



