INHIBITION OF ENZYMES 



769 



tives of enzymes than such complexes, which is the reason they are discussed 

 briefly at this point. Warburg and Christian suggested that the isolation of 

 mercuri-enzymes might be generally useful, but this technique either was 

 not used or was unsuccessful until Kimmel and Smith (1954) reported the 

 crystallization of mercuri-papain. Krebs (1930) had shown that papain is 

 very sensitive to Hg++, 50% inhibition requiring only 0.005 milf , and this 



-H,hQ-C 



S-H,hQ 



S-H,^^S 



p-MPS 



Fig. 7-10. The side chains introduced 

 onto proteins by various mercurials. 

 The S — Hg — R bonds are not actu- 

 ally linear but are shown in this way 

 for convenience. 



indicated that a tight complex is formed and might be susceptible to crystal- 

 lization. Twice recrystallized papain (1.5-2%) was reacted with 1 mM Hg++ 

 in 70% ethanol in the cold; within 24 hr a precipitate formed and in 3-4 

 days 90% of the activity was in crystalline form. These crystals are long 

 rectangular plates, often large enough to be visible to the eye, and are 

 soluble in water. The properties of mercuri-papain have been reviewed by 

 Kimmel and Smith (1957) and we shall discuss only those aspects relevant 

 to enzyme inhibition. 



Mercuri-papain contains 0.49% Hg and has a minimal molecular weight 

 of 41,400; this corresponds to 1 Hg atom per molecule of mercuri-papain. 

 Since the molecular weight of reduced papain is around 20,500, mercuri- 

 papain must be a 1 : 2 complex or dimer to be represented by E — S — Hg — 



