LEIGH E. CHADWICK 



47 



The temperature dependence of the step, ES — ^ ES*, may therefore be 

 evaluated by computing Vmax in the usual way from the pS-activity curves 

 obtained at different temperatures. If the plot of log Vmax against 1/Tabs 

 is linear, one may then estimate the apparent energy of activation, aE*, 

 from the Arrhenius equation; and the true energy of activation, AH*, as 

 AE* — RT. The standard free energy and entropy changes, AF* and AS*, 

 can also be calculated if the molar concentration of enzyme is known, but 

 this information is not usually available for ChE preparations. 



A prime obstacle to the application of this method of analysis is the 

 fact that most sets of data for ChE's do not give straight lines in the 

 Arrhenius and similar formulations. Our results with homogenates of house 

 fly heads, which contain a specific ChE, are reasonably typical (fig. 1). 



Fig. 1. Activity of fly head 

 ChE as a function of tempera- 

 ture. Vertical bars show the 

 I'ange of variation for ±3 S.E. 

 The .straight tla.shed line was 

 calculated by the method of 

 least scjuares to fit the data at 

 30°C and below. Data from 

 (10). 



140 



100 



60 



20 



> 



>' 



',-+--. 



JC 



>' 



) 



TEMPERATURE IN DEGREES C 



_J I I L 



10 



20 



30 



40 



50 



Here the activity measured was directly iM'oportional to temperature, up 

 to 30°C (10). Within the errors of measurement, similar relationships of 

 nearly the same slope have been found with horse serum (26, 21, 15), for 

 human serum and erythrocytes, respectively (40) , with heads of the orien- 

 tal fruit fly (38), with nerve cords of the cockroach (37), and with the 

 distal portion of the lobster nerve cord (7). For the proximal portion of 

 the lobster cord, the slope was somewhat steeper. The measurements of 

 Tahmisian (44) with the ChE of developing grasshopper eggs do not de- 

 fine the shape of the curve below 35°C, but Carlson (9), using methyl 

 butyrate as substrate with a very similar preparation, found a nearly 

 direct ratio between activity and temperature over the range from about 

 zero to 45°C. A relationship of this form is evidently characteristic of 

 many crude ChE preparations, and shows that the apparent energy of 

 activation, whatever significance may be attached to it, is often not con- 

 stant but temperature-dependent. Thus, the indication is that the true 

 temperature relationships of the reaction are being obscured by compli- 



