THE NATURE OF ENZYME ACTION 295 



the commencement of the exposure to the temperature and the 

 period at which the observation is made. 



The fact that during the brief life of an enzyme at a high 

 temperature its activity is so enormously increased makes 

 caution necessary in experimental work. It is sometimes the 

 practice, in order to stop the action of an enzyme at a given 

 moment, to raise the temperature of the reacting mixture as 

 rapidly as possible to ioo° or thereabouts. This cannot be done 

 instantaneously, and during the necessary interval of time the 

 enzyme becomes exceedingly active, so that considerable further 

 change may take place. That such change, in point of fact, 

 does occur, was found by Delezenne in the case of papain, and by 

 myself in the case of trypsin. This method of stopping further 

 action is, therefore, inadmissible in accurate investigations. It 

 is better to freeze solid as rapidly as possible, or, when such 

 admixture is immaterial to subsequent work, to add some sub- 

 stance which stops the action of the enzyme, such as alkali to 

 solutions containing pepsin or invertase. 



From the point of view of the theory of enzyme actions, the 

 large temperature coefficient has some importance. As pointed 

 out by Senter, it shows that the interpretation of the form of 

 the velocity curve as due to diffusion in a heterogeneous system, 

 on the lines of the theory of Nernst, does not agree with the 

 experimental facts. The temperature coefficient of diffusion 

 processes is low, whereas that of enzyme actions is unusually 

 high. 



Accelerators and Retarders 



Of Ehrlich's two classes of bodies acting on the living cell 

 enzymes belong to that one which consists of bodies of high 

 molecular weight, like the foodstuffs and bacterial toxins, which 

 are directly assimilated by the protoplasm and built up into its 

 giant molecule. The other class — to which drugs belong, as 

 well as the various chemical messengers, or hormones, produced 

 by the organism — are of simpler chemical constitution, and act 

 by the physico-chemical characters of their molecule as a whole. 

 The first class of bodies alone gives rise, when injected into the 

 living organism, to the production of antagonistic bodies — anti- 

 toxins or anti-enzymes, etc. Of these latter several are known 

 to be normally present in the blood, such as anti-trypsin and 

 anti-rennet. Others have been produced by hypodermic injec- 



