LEIGH E. CHADWICK 



53 



authors later (33) reaffirmed this value, with observation at 10° and 23°C; 

 but my calculations from their reported results indicate a Qio between 1.22 

 and 1.44, i.e. somewhat less than their own estimate and much like that 

 usually found for hydrolysis of ACh by ChE's. 



iVIore detailed measurements of the effect of temperature on the inhi- 

 bition of ChE by DFP were made by Chadwick and Lovell (10) with fly 

 heads. They found that the specific reaction rate, of the order of 10*^ liters/ 

 mole DFP/rainute at 25°C, was almost directly proportional to tempera- 

 ture between 15° and 35°C. (fig. 5) . The average Qio for the inhibitory 



^ :i 



TEMPERATURE IN DEGREES C 



J5 



Fig. 5. Rate of reaction of fly head ChE with DFP as a funftion of temperature. 

 Open circles: average reaction rates measured 4 minutes after mixmg of enzyme and 

 inhibitor. The straight line shown was fitted to the data by eye. Data from (10). 



reaction was 1.58 as compared with 1.34 for the hydrolysis of ACh by this 

 enzyme over the same temperature range. 



Aldridge (3) studied the inhibition of rabbit erythrocyte ChE by com- 

 pound E-600 (diethyl p-nitrophenyl phosphate, also known as paraoxon). 

 In contrast to our results with DFP, his data, at temperatures of 19° to 

 37°C, are in reasonable agreement with the Arrhenius formulation, from 

 which he calculated an apparent energy of activation of 10.6 Cal/mole. 

 Aldridge also determined the temperature dependence of the nonenzymic 

 hydrolysis of E-600 and its dimethyl analog, and found apparent energies 

 of activation of 18.5 and 16 Cal/mole, respectively. ChE inhibited by the 

 dimethyl compound regains activity relatively rapidly after removal of ex- 

 cess inhibitor; and Aldridge showed that this recovery process is tempera- 

 ture-dependent, with an apparent energy requirement of 14.4 Cal/mole. On 

 the basis of these results he states (p. 446) that "it is clear that the in- 



