INHIBITION OF ENZYMES 815 



This phenomenon of spontaneous recovery must be even more common in 

 celkilar preparations than with pure enzymes, because there is much greater 

 opportunity for redistribution of the mercurial. 



Stimulation of Enzymes by Mercurials 



Mercurials is common with other heavy metals and SH reagents frequent- 

 ly increase enzyme activity, especially at low concentration, the action- 

 concentration curves being biphasic. Polis and Meyerhof (1947) first ob- 

 served the stimulation of Ca++-activated myosin ATPase by PM, a 30-40% 

 elevation of the rate occurring with concentrations between 0.005 and 0.12 

 mM, and this has been confirmed in several more recent reports, the degree 

 of stimulation, however, varying greatly with the experimental conditions. 

 Many different types of enzyme exhibit this phenomenon (Table 7-11) but 

 the mechanisms involved have only rarely been clarified. Let us briefly 

 consider some possible mechanisms and what relevant evidence is available. 



(A) The mercurial inactivates a naturally occurring inhibitor. If an in- 

 hibitor is isolated w^ith the enzyme and is suppressing the activity, and if 

 this inhibitor is an SH protein (as many natural inhibitors seem to be), a 

 mercurial by reacting preferentially with the inhibitor may release the en- 

 zyme from its inhibition. J. S. Roth (1953 a, 1956, 1958) has been a pro- 

 ponent of this theory with respect to the activation of rat liver homogenate 

 ribonuclease by p-MB or PM, and has found a natural inhibitor with which 

 the mercurials react at concentrations having no direct effect on ribonu- 

 clease. The degree of stimulation varies with the tissue from which the ri- 

 bonuclease is obtained — all the way from 0% with pancreas, 57% with 

 brain, 104% with muscle, 284% with liver, to 1500% with ascites carci- 

 noma — and this may be due to the different amounts of inhibitor present 

 (Ellem and Colter, 1961). Indirect evidence often points to such a mechan- 

 ism for other enzymes. PhiUips and Langdon (1962) found that p-MB stim- 

 ulates microsomal NADPHxytochrome c reductase but only inhibits the 

 purified enzyme. Of course, the activation could also be due to some effect 

 of the mercurial on the microsomal structure. It has also been noted occa- 

 sionally that stimulation occurs, and is relatively constant, over a wide 

 range of mercurial concentration, inhibition appearing rather suddenly when 

 this range has been exceeded, and this indicates some component with which 

 the mercurial reacts readily and completely. 



(B) The mercurial reacts with the substrate to labilize it. Ledoux (1953) 

 initially attempted to explain the stimulation of ribonuclease by p-MB as 

 due to a reaction with RNA, this favoring in some manner the enzymic 

 hydrolysis, and detected spectral changes upon mixing RNA and p-MB 

 (see page 741). Although mercurials do complex with nucleic acids, it is 

 doubtful if this is a major factor in the activation. The proper preincubation 



