224 PROTOPLASMIC ACTION AND NERVOUS ACTION 



catalyses are referable to the formation of intermediate 

 compounds, and the same is undoubtedly true of many 

 enz3ane-reactions. The specificity of enzymes and other 

 facts in their behavior indicate that chemical union 

 often occurs between the enz3rme and the substrate 

 molecules, and that it is the combination thus formed 

 which breaks down rapidly, yielding the products of 

 hydrolysis and the free enzyme, which then repeats the 

 chemical cycle of combination and hydrolysis with fresh 

 molecules of substrate. 



Apparently many cases of heterogeneous or contact 

 catalysis are referable to simple increase of concentration 

 due to adsorption. But in the case of metals and other 

 conducting substances the possibility of an additional 

 factor, the formation of local electrical circuits between 

 different portions of the active surface, is also to be 

 considered. In either case the essential condition is 

 some form of surface-action. 



Enzymes are colloidal in their condition, indiffusible, 

 precipitable, readily adsorbed by indifferent adsorbents, 

 and, according to Bayhss, their mode of action is also a 

 surface-action. The clearest proof of this is that emulsin, 

 lipase, urease, and trypsin exert their action in alcohoHc 

 media of such a strength that the enzyme is insoluble 

 and can be filtered off.^ BayHss regards the adsorption 

 of the substrate on the enzyme phase as the first step 

 in the process; the chemical reaction then follows. In 

 some cases the adsorption- compound of enzyme with 

 substrate is separable; e.g., starch- amylase, fibrin- 

 pepsin, and trypsin with caseinogen.^ A close union or 



^ Cf. Bayliss, Principles of General Physiology, p. 325. 

 ' Bayliss, op. cit., p. 326. 



