30 PHYSIOLOGY CHAP. 



and of acting as pure chemical agents, independent of the living 

 elements which produced them. The several digestive enzymes 

 of the gastro - intestinal tract in animals were considered as 

 examples of these non-organised ferments. 



The second class comprised the so-called organised ferments, or 

 ferments proper, represented by micro-organisms (fungi, bacteria, 

 etc.), the action of which was then held to be in direct dependence 

 upon the vitality of the latter, and to cease on their death or 

 disorganisation. Saccharomyces cerevisiae, which determines the 

 alcoholic fermentation of glucose (Pasteur), was regarded as the 

 prototype of such organised ferments. 



Now, however, in consequence of Buchner's work (1899), this 

 distinction can no longer be maintained. Buchner has demon- 

 strated experimentally that it is possible to extract from the cells 

 of beer-yeast, when exposed to enormous pressure, a substance rich 

 in protein, which is free from living elements, and is able to set 

 up the alcoholic fermentation of solutions of glucose. The 

 property by which yeast cells ferment glucose is therefore due, 

 not to a true vital process, but to the action of an enzyme or 

 zymase, produced by the cell. Specific enzymes of other micro- 

 organisms formerly held to be organised ferments (the bacilli of 

 lactic fermentation, of acetic fermentation, etc.) have also been 

 isolated. 



All enzymes are now regarded as organic substances (most 

 probably of the nature of proteins) which are elaboration products 

 of the living cells, from which they can be separated and extracted 

 by various methods without losing their activity. Generally 

 speaking they can be extracted from the cells and the tissues, on 

 treating these with water or glycerin. The latter solvent, in 

 particular, yields solutions that remain active for a considerable 

 time, and has been largely employed in practice to extract these 

 enzymes. 



It should be stated that the enzymes are frequently not found 

 pre-formed within the cells which produce them, but are as it were 

 in a potential state. The complete development of their specific 

 enzyme activity necessitates the further action of oxygen and 

 other chemical compounds known as kinases. The mother- 

 substances from which the enzymes are derived are called 

 zymogens or pro-enzymes. We shall discuss these at length in 

 speaking of the digestive ferments, since there is in the intestine 

 a substance which activates the pancreatic enzymes (entero-kinase). 



No characteristic chemical reactions are common to all enzymes; 

 generally speaking, they are precipitated from their colloidal 

 solutions by alcohol, and are destroyed by high temperatures from 

 + 80 to + 100 C. In order to recognise them, it is necessary 

 to observe the properties which characterise their mode of 

 -action. In the first place enzymes, in consequence of their 



