DIGESTION 69 



obscure way with ordinary chemical reagents. These have been 

 called unorganized or unformed ferments, to distinguish 

 them from such organized ferments as bacteria, yeast, fungi, 

 etc. They are not themselves possessed of any vital activity, 

 though formed in living organisms, like plants or animals. They 

 are of indefinite chemical composition, contain nitrogen and are 

 supposed to be of proteid structure. The characteristic point in 

 their action has been supposed to be that they produce a chem- 

 ical change without themselves being affected by that change. This 

 is doubtless practically true, but it is found in experimental work 

 that "a given solution of enzyme cannot be used over and over 

 again indefinitely." It finally loses its identity. 



According to the foods on which they act and the effects they 

 produce, enzymes are classified as: (i) Proteolytic enzymes, 

 which convert proteids into soluble peptones; examples are 

 pepsin and trypsin. (2) Amylolytic enzymes, which convert 

 starches into sugar; examples are ptyalin and amytopsin. (3) 

 Fat-splitting enzymes, which convert neutral fats into 

 glycerine and fatty acids; an example is steapsin. (4) 

 Sugar-splitting enzymes, which convert the non-absorbable 

 (saccharose) into absorbable (dextrose) sugar; an example is 

 invertase. (5) Coagulating enzymes, which precipitate solu- 

 ble proteids; an example is rennin. 



Characteristics of Enzymes. Some of the characteristics of 

 enzymes are as follows: (i) They are soluble in water and in 

 glycerine. (2) In solution they are destroyed before the boiling 

 point is reached (140 to 180 Fahrenheit). Very low temper- 

 atures do not destroy them, but suspend their action. (3) 

 They never completely convert the substance upon which they 

 act. It is supposed that the substance produced, as peptones for 

 example, have an inhibitory action upon the enzyme. If these 

 substances be removed as they are formed, the action of the 

 enzyme continues. (4) The particular result is independent of 



