CATALYSIS AND ENZYMES 325 



a variety of enzymes on this hypothesis alone, although one must not be too hasty 

 in making such a statement until more is known as to the nature of adsorption 

 in its manifold aspects. 



With respect to the numerous theories of catalysis that have been suggested, 

 the reader may consult the work of Mellor (1904, chapter x.). 



Before proceeding to a discussion of what we know as to the mode of action 

 of enzymes, a brief description of their physical and chemical properties, as far 

 as they are known, is requisite. 



Physical Properties of Enzymes. They are all in the colloidal state in solution. 

 They do not diffuse through thick parchment paper, but, as samples of this paper 

 vary in the dimensions of their pores, it may be found that an enzyme in a 

 highly dispersed condition may diffuse slowly through some papers. This was 

 the case with the amylase of Fraenkel and Hamburg (1906). 



Their destruction by heat has been referred to above. 



As colloids, they have an electrical charge, varying with the electrolytes 

 present with them. This charge appears to play some part in the mechanism of 

 their action, as will be seen presently. 



There is indirect evidence that many, if not all, are optically active. 



The Chemical Nature of Enzymes. It is obvious that great practical difficulty 

 exists in the investigation of this subject, owing to the minute amounts of these 

 intensely active substances which we have at our disposal. It was thought at 

 one time that they had the composition of proteins, but, as preparations were 

 made of greater purity, it was found that the protein reactions disappeared more 

 and more, although the preparation gained in activity. Moreover, according to 

 Beijerinck, they are incapable of serving as nitrogen food for bacteria or yeast. 

 It is probable that, like inorganic catalysts, they are of very varied chemical 

 nature, but what this is cannot as yet be stated definitely in respect of any one 

 of them. 



It is possible that they are not single chemical individuals, but complex 

 systems, as was suggested by Bertrand some years ago. In an address to the 

 French Association for the Advancement of Science in 1909, the theory is stated 

 as follows : One of the constituents of the system is capable, on its own account, 

 of producing the reaction in question to a slight degree, but requires the presence 

 of another substance, inactive in itself, before its activity becomes appreciable. 

 The former is, according to the case, some such substance as acid, alkali, calcium 

 or manganese salt, etc. The latter is a more complex substance, often similar to 

 egg-white, colloidal in character. This view is similar to that stated by von 

 Wittich (1872, p. 469) as regards pepsin, which is held merely to intensify the 

 action of hydrochloric acid. It is not quite clear, however, whether von Wittich 

 intended to make the general statement that all enzyme actions are of this 

 nature, although it seems implied. This view receives support also from the facts 

 connected with the "artificial laccase " prepared by Dony-Henault (1908, p. 151), 

 in which the active agent is colloidal manganese hydroxide, but protected from 

 aggregation by the presence of a " stable " colloid, gum arabic. 



At this point I feel bound to make a slight protest against Bunge's gibe at physiologists 

 (1907, p. 241), in which he says that "the less a physiologist knows about chemistry, the 

 greater is he inclined to work at the most difficult chemical subjects the proteins and 

 ferments." If the chemistry to which reference is made here is pure statical, structural, 

 organic chemistry, as would appear, it is a remarkable fact that such a mode of attack has 

 taught us practically nothing about the nature of enzymes, and has only led to the multiplica- 

 tion of names, on which Bunge himself justifiably throws contempt as "a drag and a brake to 

 science." It is only since the question has been attacked from the kinetical standpoint of 

 physical and colloidal chemistry that we are beginning to see light. It is, of course, far from 

 my intention to undervalue the work of organic chemistry as one of the helps to the 

 comprehension of our difficult problems, as must be apparent from the previous pages of the 

 present book, and would be of self-evident absurdity ; but, in view of opinions sometimes 

 expressed, it is necessary to point out that there are other bodies of doctrine of equal import- 

 ance in the study of physiology. 



Enzymes Act at their Surfaces. The clearest direct proof of this fact is that 

 emulsin, lipase, urease, and trypsin exert their activity in alcoholic media of such a 

 strength that the enzyme is completely insoluble, and can be filtered off. In such 



