I40 ORGANIC AGRICULTURAL CHEMISTRY 



fluids of animals such as ptyalin in saliva, pepsin in gastric 

 juice, and diastase in the leaves and seeds of plants, etc. These 

 enzymes were also termed unorganized ferments to distinguish 

 them from yeast and certain bacteria which were called organ- 

 ized ferments and in which the fermentation action was con- 

 sidered to be due to the living organism itself. In 1897 Buch- 

 ner isolated from yeast an enzyme, which he called zymase, which 

 without the presence of the living yeast organism produced the 

 alcoholic fermentation. This proved that in this case what 

 had been considered an organized ferment was, in fact, an un- 

 organized ferment or enzyme similar to the others mentioned. 



This was the beginning of the idea that all fermentation, 

 whether connected with an animal or a vegetable organism or 

 not, is due to the activity of substances, not of the organism itself. 

 These substances or enzymes, however, are produced by the 

 organism so that indirectly the organism is essential to the fer- 

 mentation. Pasteur, the great French chemist and biologist, 

 showed the connection between fermentation and living organ- 

 isms leading to the vitalistic theory of fermentation, while 

 Buchner, in isolating an enzyme from yeast, showed the relation 

 of a non-living substance, produced by the organism, to the 

 fermentation, thus establishing the enzyme theory of fermentation. 



Definition of Enzyme. — These facts which have been given 

 lead to the definition of an enzyme as follows : 



An enzyme is a substance which acts catalytically in bringing 

 about a chemical reaction of the ordinary type, the enzyme being a 

 product of a living cell but acting independently of that cell. 



General Nature of Enzymes. — The exact nature of enzymes 

 is unknown, though it is supposed that they are of protein char- 

 acter. They are colloidal substances, non-diffusible, soluble 

 in water, dilute alcohol, dilute glycerol and sodium chloride 

 solution, but precipitated by strong alcohol and by a solution 

 of ammonium sulphate. In general they follow the physico- 

 chemical laws applying to colloids and catalysers. They act 

 best at a certain optimum temperature which is, in most cases, 

 approximately the temperature of the animal body, 37° C. 



