50 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



The specific ChE of fly heads is more sensitive since some irreversible 

 loss of activity occurs with short exposures to temperatures no higlier than 

 35°C (fig. 4). However, maximal activity is again found at about 40°C, 

 in the usual 20- to 30-minute measurement (fig. 1). Denaturation requires 

 some 44 Cal/mole and is biphasic, with each phase following first-order 

 kinetics as with the serum enzyme. But with fly head ChE there is no 

 convincing evidence of any reversible inactivation by heat, for the un- 

 denatured fraction maintains a direct proportionality between activity 

 and temperature up to 50°C or higher (10) . 



The response of serum ChE's at temperatures above 35°C agrees, at 



Fig. 4. Denaturation of fly 

 head ChE by heat. Open circles 

 I computed from data of fig. 1, on 



/ assumption that the activity of 



undenatured enzyme is directh' 

 proportional to temperature 

 throughout the range of meas- 

 urement. Solid circles deter- 

 mined by incubating tissue 

 samples at the indicated tem- 

 peratures for 20 minutes before 

 returning them to 25°C for ad- 

 dition of substrate and assay. 

 The curve shown was fitted to 



the data by the method of 



'° " *° *' " least squares, in the Arrhenius 



transformation, and corresponds to an apparent energy of activation of 44,550 cal/mole. 

 Data from (10). 



least qualitatively, with numerous examples reviewed by Johnson et al. 

 (25), in which the apparent energy of activation changes with tempera- 

 ture as the result of a shifting equilibrium between active and reversibly 

 denatured forms of the enzyme. Alternatively, one might invoke the sug- 

 gestion of Kavanau ( 27 ) , that some enzymes develop an optimal configura- 

 tion for combining with substrate only over a rather narrow intermediate 

 range of temperatures. However, neither model accounts satisfactorily for 

 the observations with fly head ChE; for here there seems to be no reversible 

 inactivation, while it is hardly reasonable to suppose the enzyme is still 

 unfolding toward an optimal configuration at 50°C, a temperature at which 

 permanent inactivation occurs in a few minutes and one which is several 

 degrees above the thermal death point for the species. 



Nevertheless, the theory of absolute reaction rates is sufficiently com- 

 prehensive to permit alternative explanations (25). Even if one discounts 



9 - - ' ' TEMPERATURE IN DEGREES C 



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