820 7. MERCURIALS 



inhibition of the Eg will increase the steady-state level of B or its rate of 

 formation. If the formation of B is being measured and an enzyme destroy- 

 ing it is present, inhibition of this enzyme will appear to stimulate E^. 

 Likewise in a divergent chain, inhibition of one branch may increase the 

 rate of the other branch; whether stimulation will be observed will depend 

 on what is measured. In the incorporation of nucleotides into amino acid 

 transfer UNA, the presence of any enzyme attacking the nucleotides will 

 reduce the incorporation, and the inhibition of such an enzyme will ap- 

 pear to stimulate the incorporation. Stimulation was indeed observed with 

 p-MB by Starr and Goldthwait (1963), who thought that this might be 

 due to a contaminating phosphodiesterase, but Anthony et al. (1963) ob- 

 tained evidence that such an enzyme did not occur at significant levels in 

 their preparation. 



The stimulation of many enzymes by mercurials is strongly pH-depend- 

 ent. The activating effect of Hg++ on glycerate-2,3-diphosphatase becomes 

 progressively less as the pH is increased beyond 7 (Rapoport et al., 1955), 

 and this is true also for ascites cell ribonuclease, although here the activa- 

 tion disappears as the pH is decreased to 5 (Colter et al., 1961). In the latter 

 case, a 50% stimulation changes to a 42% inhibition as the pH rises from 8 

 to 8.5. Liver mitochondrial ATPase stimulation by p-MB is optimal around 

 a pH of 9 and falls off rapidly on both sides (Myers and Slater, 1957 b), 

 and the optimal pH for activation of myosin ATPase is 7.8, little stimula- 

 tion being observed at pH 5.7 or 10 (Tonomura and Furuya, 1960). Such pH 

 effects may be important in working out the mechanisms of the stimulation 

 but have not so far been studied in enough detail to contribute evidence. 

 Temperature can apparently also play a role, since p-MB stimulates myosin 

 ATPase at 25° but only inhibits at 0^ (Fig. 7-30) (Gilmour and Griffiths, 

 1957). It is also evident in this figure that the DNP-activated ATPase is 

 not further stimulated by p-MB. The EDTA-activated ATPase is likewise 

 not stimulated by p-MB, whereas in the presence of Ca++ the stimulation 

 is marked (Fig. 7-23). Finally, the effect of mercurial concentration is oc- 

 casionally very striking, as in the case of myosin ATPase (Fig. 7-23), max- 

 imal stimulation by p-MB being observed when about half the SH groups 

 are reacted. Another example of variation of stimulation with mercurial 

 concentration is shown in Fig. III-l-l for ribonuclease, although here the 

 form of the curve may depend primarily on the nature and amount of 

 the natural inhibitor, as well as the susceptibility of the enzyme itself. 



The problem of enzyme stimulation by mercurials, or other inhibitors, 

 is a very interesting one, and probably quite important in understanding 

 not only the mechanisms whereby the mercurials can affect enzymes but 

 some of the many instances of stimulation of metabolism, which will be 

 mentioned later in this chapter. 



