LEIGH E. CHADWICK 



49 



The thermal dependence of this reaction was investigated for the ChE of 

 human erythrocytes by Shiikuya (40), who found that the equilibrium 

 constant was independent of temperature, so that the standard enthalpy 

 change appeared to be nil. The change in standard free energy was cal- 

 culated as —2.7 Cal/mole at 310. 5°K, and the standard entropy change 

 as +8.8 cal/mole/degree. 



Reaction 5 does not occur with the nonspecific ChE's, to which type the 

 serum enzymes belong in most mammals that have been studied; and its 

 temperature dependence has not been investigated with the specific ChE's 

 of nonmammalian organisms. 



INACTIVATION OF cllE BY HEAT 



At temperatures slightly above the physiological range, evidence of 

 gradual inactivation of ChE is ordinarily seen, and permanent destruction 



Fig. 3. Activity of horse 

 serum ChE as a function of 

 temperature, to show maximum 

 at about 40°C. The dechne at 

 higher temperatures is largely 

 reversible, for short exposures. 

 Data from (21). 





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/ 



/ 



y 



/ 



/ 



/ 



/ 



of enzyme, though varying to some extent with the source and other con- 

 ditions, commonly is extensive wnth exposures of more than a few minutes 

 to temperatures of 55°C or higher (1,4-6, 10, 13, 15, 16, 19, 21, 23, 26, 30, 

 31, 36, 38, 44) . Many of these references give only incidental or qualitative 

 information. 



According to Glick (21) and Davies (15), maximal activity of horse 

 serum ChE occurs at 40°C (fig. 3), although there is little permanent in- 

 activation at temperatures below 50°C. Part of the decline in activity at 

 temperatures above 40°C must therefore be reversible. Irreversible de- 

 naturation requires about 60 Cal/mole (23, 15), and has a biphasic course, 

 being rapid initially and slower thereafter ; both phases follow first-order 

 kinetics (15). Reversible inhibitors, such as prostigmine and methylene 

 blue, afford some protection (23) , but previously denatured serum does not 

 (15) , while destruction is accelerated at extreme values of pH and in solu- 

 tions of low ionic strength (15). The energy requirement for denaturation 

 of the specific ChE of ox erythrocytes is similar, about 61 Cal/mole (13). 



