6 



SCIENCE. 



[N. S. Vol. XX. No. 496. 



duplicated in the vegetable kingdom and 

 also in the laboratory by the aid of the 

 simplest of inorganic reagents. A far more 

 difficult problem for a long time was to ac- 

 count in any way for the changes taking 

 place in the digestion of proteins. The 

 presence of a proteolytic enzyme in the 

 gastric juice was recognized definitely by 

 Briicke over forty years ago, and about the 

 same time a substance called trypsin was 

 found in the extract of the pancreas. 

 These substances acting on proteins under 

 certain conditions convert them into a series 

 of intermediate and end products about 

 which an enormous literature has been de- 

 veloped. In the course of the long discus- 

 sion it was discovered that many of the 

 products which are formed by the enzymes 

 may be obtained by the action of weak 

 acids or alkalies, or water even at an ele- 

 vated temperature, on the original proteins, 

 and finally it was shown that an increase of 

 weight follows in these cases as in the case 

 of the addition of water to starch. All this 

 evidently places the phenomena of protein 

 digestion in the group of hydrolytic reac- 

 tions, along with the much simpler starch 

 and fat reactions. The digestion processes 

 are, therefore, chemical, and the only thing 

 about them which remains mysterious is 

 the fact that from one set of body cells a 

 ferment working in acid medium is pro- 

 duced, while from a second set of cells a 

 somewhat similar ferment working in an 

 alkaline liquid is secured. Furthermore, 

 all these changes seem to belong to the 

 great group of catalytic reactions, of which 

 more will be said presently. 



The general character of these operations 

 was pretty distinctly fixed years ago and 

 their importance clearly recognized. The 

 chemical nature of the several enzymes 

 themselves, however, is not known; the 

 commercial products called pepsin, dias- 

 tase, etc., are merely crude mixtures of 

 which the active substances make up but a 



small part. The investigation of the prop- 

 erties of these enzymes opened the way for 

 the study of other reactions peculiar to the 

 animal organism, which are likewise un- 

 doubtedly of enzymic origin. In fact, the 

 view is gradually gaining ground that by 

 far the largest number of the body func- 

 tions involve in some way the action of 

 enzymes. The digestion phenomena are 

 among the simplest and most readily ob- 

 served, but patient investigation has 

 brought to light other reactions as truly 

 enzymic as these. In the liver alone there 

 are no less than ten well-defined processes 

 in progress, in the initiation of which en- 

 zymes are concerned. For the mainte- 

 nance of the wellbeing of the body the 

 proper performance of these processes is 

 as essential as is digestion itself. In a 

 general way most of these processes have 

 been known or suspected for years, but 

 they were supposed to depend on some pe- 

 culiar vital action of the liver cells them- 

 selves. The situation here is analogous to 

 that regarding the mode of action of the 

 yeast cell, but most investigators now con- 

 sider the enzymic or chemical theory as 

 well established. The liver may, indeed, 

 be compared to a laboratory in which im- 

 portant syntheses and decompositions are 

 constantly taking place. Some of these are 

 of such a character that they may be easily 

 duplicated in vitro, while others appear to 

 be practically beyond artificial control. 

 What is true of the liver is true of other 

 organs where matter undergoes change. In 

 the blood the presence of several of these 

 ferment agents has been shown. 



These various observations have had an 

 important bearing on a discussion which 

 has been of. long duration. Since the days 

 of Lavoisier physiologists have been trying 

 to define the means by which the oxygen 

 taken in by the lungs effects the oxidation 

 of the food stuffs. Sugars and starches 

 consumed yield finally water and carbon 



