786 



15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



lowers the optimal temperature, it is believed to combine predominantly 

 with the denatured form of the enzyme or to favor denaturation. Inhibitors 

 are classified into two general types on this basis. Type I inhibitors combine 

 with both E^ and E^^ while type II inhibitors combine only with E^^. Such 

 types of inhibition and the interpretation of temperature optima for pure 

 enzymes have been discussed earlier in this chapter. It was pointed out 

 there that even for these relatively simple systems it is impossible to con- 

 clude from a shift in the T^^^ that only the E^ ^ E^^ equilibrium has been 

 affected. It is much less likely that in complex cellular metabolism a simple 

 inactivation of an enzyme is determining the Tg,,,. It was also made clear 

 that it is not as easy as some imagine to designate which form of the 

 enzyme combines with the inhibitor from temperature data alone. Many 

 things might happen within a cell as the temperature rises which would 

 decrease the rate of some metabolic process, and in such cases the T^^^ 

 would not be directly related to the denaturation of enzymes, and the 

 shifts in the T^p^ brought about by inhibitors would not be interpretable 

 as effects on denaturation. 



The bioluminescence in Achromobacter is inhibited strongly by HgClg 

 and the T^p; is shifted 1° to 2° lower (Houck, 1942). The inhibition also be- 

 comes greater as the temperature increases (Fig. 15-9). These results. 



10 

 0.8 

 0.6 

 04 

 02 



,t 







00025mM 



0° 5« 

 T — 



10° 15° 20° 25° 30° 35° 40' 



Fig. 15-9. Effects of temperature on the inhibition 



of bioluminescence in Achromobacter by HgClj. The 



curves are calculated from activity curves presented 



by Houck (1942) and are approximate. 



coupled with the known denaturing action of HgClg, were taken to indicate 

 that the denaturation of some enzyme involved in the luminescence was 

 favored by the HgClg. Before this explanation could be accepted, it would 

 have to be shown that the simple inhibition of the enzyme (that is, the 

 reaction of the HgCL with the sulfhydryl groups of the enzyme) does not 

 increase with rising temperature to the degree observed. It is often difficult 



