-48 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



eating factors, although we have as yet little notion of what these factors 

 may be. 



In some instances, data that seem to fit the Arrhenius formulation have 

 been secured. Ormerod (34) , studying the hydrolysis of eight derivatives 

 of benzoylcholine by horse serum ChE, obtained apparent energies of ac- 

 tivation ranging, with the different substrates, from 5.1 to 13.3 Cal/mole. 

 Davies (15) found a constant AE* of 4.2 to 4.6 Cal/mole for hydrolysis 

 of ACh by partly purified preparations of this same enzyme ; but with whole 

 or diluted serum he found that the energy requirement decreased with in- 

 crease in temperature (fig. 2) . 



22 



/ 



/ 



/ 



/ 





\ 



\ 



\ 



\ 



\ 



l/Tx 10 ' 



Fig. 2. Activity of horse serum ChE as a function of temperature. Open circles: 

 dialyzed preparation; solid circles, 20% serum, not dialyzed. Data from (15). 



Irrespective of the exact form of the relationship, the increment in ChE 

 activity with rise in temperature is relatively small. The Qio in the physi- 

 ological range is usually of the order of 1.3-1.5 and falls as temperature 

 is increased. Average values of about 1.7 reported by Abdon and Uvniis 

 (2) for the plasma enzyme of two human subjects are exceptionally high. 

 The nature of the activity-temperature relationship will be reconsidered 

 below after the data on heat inactivation of the enzyme have been reviewed. 



As is well known, the pS-activity curve for hydrolysis of ACh by the so- 

 called true or specific ChE's shows a definite maximum, usually at sub- 

 strate concentrations of about 0.01 m. The decline in activity at higher 

 substrate concentrations is attributed to formation of an inactive complex 

 between one molecule of enzyme and two of ACh : 



ES + S . ''' - ESS (5) 



