52 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



EFFECT OF TEMPERATURE ON RATE OF REACTION OF cllE WITH INHIBITORS 



A third phase of this discussion concerns the manner in which tem- 

 perature influences the effect of certain ChE inhibitors. Here the available 

 information is even more spotty than in the areas already considered. Dif- 

 fering types of temperature response have been observed, depending ap- 

 parently on the nature of the inhibitory mechanism as well as on the source 

 of enzyme; and with some inhibitors, the mechanism may vary with the 

 inhibitor concentration. 



Shukuya (41) found that urethane at about 0.2 m concentration inhibited 

 human serum ChE reversibly at temperatures between 20° and 37.5°C, 

 with some reduction in degree of inhibition as temperature increased. With 

 0.8 M urethane, however, inhibition at temperatures of 35°C and above was 

 irreversible, and had a positive temperature coefficient. Here the apparent 

 energy of activation was about 65 Cal/mole, from which fact protein de- 

 naturation was inferred. Similar results were obtained with Na-salicylate 

 (42), with which reversible inhibition, little affected by temperature, was 

 observed at an inhibitor concentration of 0.15 m. With 0.27 m Na-salicylate, 

 enzyme was destroyed, and the process was temperature dependent, with 

 an apparent energy requirement of about 42 Cal/mole. 



Goldstein and Doherty (24) had earlier noticed a difference in the mode 

 of inactivation of ChE by HgCl2 at low and high concentrations. The tem- 

 perature coefficient for inactivation at high concentrations was large. 

 Prostigmine gave no protection, but BAL (dimercaptopropanol) did. How- 

 ever, BAL failed to reverse such inhibition as had already occurred. Al- 

 though HoS alone was not inactivating, HoS in the presence of HgCU 

 enhanced the degree of inhibition; and this increase in inhibition was shown 

 not to be due to formation of HgS. 



In these instances, there is little doubt that irreversible inactivation with 

 high concentrations of inhibitor resulted from denaturation of the enzyme 

 protein, even though the attempt by Goldstein and Doherty (24) to show 

 that disruption of sulfhydryl linkages was the mechanism gave somewhat 

 contradictory results. 



Goldstein and Doherty (24) stated further that the temperature co- 

 efficient for inactivation by HgCU was surprisingly high in comparison with 

 the coefficient for physostigmine determined in their earlier work ; however, 

 I have been unable to find pertinent data in the reference (22) they cite. 

 There is nevertheless little doubt that their statement is correct, for physos- 

 tigmine acts as a competitive inhibitor and low temperature coefficients 

 have been observed with other inhil)itors of this type. 



Thus, for example, Nachmansohn et ol. (32) gave as a rough estimate 

 Qio of about 2 for the inactivation of electric eel ChE by DFP (di-iso- 

 propylfluorophosphate) between temperatures of 7° and 22.5 °C. The same 



