54 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



hibitory reaction is a chemical reaction and not a simple adsorption". This 

 conclusion is in accord with much other evidence. 



The reversible, competitive inhibition of ACh hydrolysis by choline was 

 studied by Davies (15) with horse serum ChE, at temperatures between 

 15° and 45°C. Davies concluded that the activation energy for formation 

 of the enzyme-inhibitor complex was about —9.8 Cal/mole below 35°C, 

 with an associated entropy change of about —20 E.U., and about —54 Cal/ 

 mole above 35°C, with the corresponding change in entropy, —163 E.U. 

 There appears to be no formal objection to these calculations; however, 

 the values obtained depend upon an estimate of the temperature depend- 

 ence of the equilibrium constant, Ki , for the reaction between enzyme and 

 inhibitor; Ki depends in turn upon the quotient of estimates for the equi- 

 librium constants, Kg and Kg', of the reaction with substrate in the presence 

 and absence of inhibitor. The estimates of Ki are therefore quite sensitive 

 to experimental error in the determination of Kg and Kg'; recalculation of 

 these constants from Davies' data, fitted by the method of least squares to 

 the Lineweaver-Burk formulation, suggests that there is too much uncer- 

 tainty in the computed values of Ki to warrant placing much reliance on 

 figures Davies reports for the various thermodynamic quantities. With 

 about the same degree of experimental variability, Shukuya (40) concluded 

 that there was no significant change with temperature in the value of Kg . 

 In studies with red cell ChE, Butterworth (8), cited by Davies (15), 

 found that Kj for choline inhibition decreased with increasing temperature, 

 in contrast with Davies' own results with the serum enzyme. The safest 

 conclusion at present would seem to be that we do not yet have sufficiently 

 good data for the values of the equilibrium constants at different tempera- 

 tures to permit us to state with confidence what form their temperature 

 dependence may take. This means that we are also unable at this time to 

 reach any quantitative conclusions with regard to the temperature de- 

 pendence of the reaction between ChE and any competitive reversible 

 inhibitors. This limitation does not apply to situations where the inhibition, 

 though competitive, is virtually irreversible, as it is with DFP and many 

 other organic phosphates, such as those studied by Aldridge (3) . 



Davies (15) also attempted to measure the temperature dependence of 

 the reaction between horse serum ChE and eserine, but was unable to ob- 

 tain data that satisfied the criteria for reversible competitive inhibition. 

 Since the competitive nature of this reaction was already well established, 

 he concluded that there must be some undetected defect in the experiments. 



Roan and Maeda (38) report the degree of inhibition of Dacvs ChE with 

 8 inhibitors, principally organic phosphates, at three different tempera- 

 tures; but their data are unfortunately not given in a form suitable for 

 calculation of the reaction rates, and hence cannot be applied to the prob- 

 lem here under consideration. 



