294 SCIENCE PROGRESS 



Effect of Temperature 



Increase of temperature is well known to have a consider- 

 able effect on the rate of ordinary chemical reactions, the usual 

 factor being between 2 and 4 for io° : for example, the velocity 

 at 40 is from 2 to 4 times that at 30 . In the case of enzymes 

 the corresponding factor has been determined for emulsin by 

 Tammann to be 7*14 between 60 and 70 , for trypsin by myself 

 to be 5*3 between 20 and 30 , and for the catalase of blood by 

 Senter to be 1*5 between o and io° C. But whereas the curve 

 expressing this fact becomes steeper as the temperature rises in 

 the case of ordinary chemical reactions, it is found that enzymes 

 show a phenomenon known as the " optimum " temperature — 

 viz. a particular temperature at which during a given time a 

 greater amount of substrate is acted upon than at any tempera- 

 ture either above or below this. To understand the meaning 

 of the " optimum " it is sufficient to bear in mind two facts — 

 viz. that enzymes are rapidly destroyed at high temperatures, 

 probably because of their colloidal character ; and that, in order 

 to determine the degree of their activity, it is necessary to allow 

 the action to proceed for a considerable time at the raised 

 temperature. This being so, we are unable to observe directly 

 the rate of change at the beginning of the action at any new 

 temperature, and at the higher temperatures, before sufficient 

 change has occurred to enable measurements to be made, a 

 considerable part of the enzyme has been destroyed. This is 

 shown very distinctly by the observations of Frost Blackman 

 on the respiration of plants. If curves are drawn expressing 

 the evolution of carbon dioxide at various temperatures and at 

 various times after the commencement of exposure to these 

 temperatures, and these curves are continued so as to show the 

 rate at the first moment of action of the particular temperature, 

 it is found that the points so fixed lie on a curve exactly like 

 that of Van 't Hoff for ordinary chemical reactions, so that the 

 higher the temperature the greater the initial rate of change, 

 and the optimum temperature is merely an expression of the 

 fact that at a certain temperature the increased velocity due to 

 this raised temperature is more than sufficient to counteract the 

 rapid destruction of the enzyme. It follows also from these 

 experiments that the apparent optimum temperature will vary 

 considerably, according to the time which has elapsed between 



