CHAPTER VI I L 



THE COMPOSITION OF THE DIGESTIVE JUICES 

 AND THE ACTION OF CERTAIN ENZYMES. 



The digestive enzymes or ferments are bodies that 

 have the power of accelerating the rate of hydrolysis of 

 certain substances. They are divided into groups depend- 

 ing on the nature of the substance on which the}^ act 

 (the so-called substrate). Thus those acting on starch 

 are called amylolytic ; on proteins, proteolytic ; on fats, 

 lipolytic, etc. The enzymes are sometimes named in 

 such a way as to indicate their origin and their action, the 

 termination -ase being employed. Thus ptyalin, the 

 amylolytic enzyme of saliva, can be termed salivary 

 amylase, to distinguish it from pancreatic amylase (amy- 

 lopsin). Gastric lipase, the lipolytic enzyme of the gastric 

 juice, is similarly distinguished from pancreatic lipase 

 (steapsin). 



The chemical composition of the enzymes is at present 

 uncertain, owing to the extreme difficulty of preparing 

 them in a pure state. The proteolytic enzymes are either 

 proteins, or compounds so readily adsorbed by proteins 

 that it is impossible to separate them. The enzymes acting 

 on certain of the carbohydrates are possibly themselves 

 of a carbohydrate nature. 



The properties of the enzymes as a class are as follows : 

 they are soluble in water, dilute salt solutions, dilute 

 alcohol and glycerol. They are precipitated by saturation 

 with ammonium sulphate and by strong alcohol. They 

 are readily carried down by different precipitates, probably 

 by a process of adsorption. They are colloidal and non- 

 diffusible. They are most active at a certain temperature, 



