COMPOSITION OF FOOD AND ACTION OF ENZYMES. 683 



enzyme itself remains unchanged in this process, and it acts specifi- 

 cally, that is, each enzyme exerts its activity only upon substances 

 whose molecules have a certain definite structural and stereochemi- 

 cal arrangement. The enzymes of the body are organic substances 

 of a colloid structure whose chemical composition is unknown. 



With regard to the names and classification of the different 

 enzymes, much difficulty is experienced. There is no consensus 

 among workers as to the system to be followed. Duclaux has sug- 

 gested that an enzyme be designated by the name of the body on 

 which its action is exerted, and that all of them be given the termina- 

 tion ase. The enzyme acting on fat on this system would be named 

 lipase; that on starch, amylase; that on maltose, maltase, etc. The 

 suggestion has been followed in part only, the older enzymes which 

 were first discovered being referred to most frequently under their 

 original names. An entirely satisfactory classification is impossible 

 at present. Having in mind only the needs of animal physi- 

 ology, the following classification will be used in the treatment of 

 the subjects of digestion and nutrition: 



1. The proteolytic or protein-splitting enzymes. Examples: pepsin 



of gastric juice, trypsin of pancreatic juice. They cause a hydro- 

 lytic cleavage of the protein molecule. 



2. The amylolytic or starch-splitting enzymes. Examples: ptyalin 



or salivary diastase, amylopsin or pancreatic diastase. Their 

 action is closely similar to that of the classical enzyme of this group 

 diastase found in germinating barley grains. They cause a 

 hydrolytic cleavage of the starch molecule. 



3. The lipolytic or fat -splitting enzymes. Example: the lipase found 



in the pancreatic secretion, in the liver, connective tissues, blood, 

 etc. They cause a hydrolytic cleavage of the fat molecule. 



4. The sugar-splitting enzymes. These again fall into two subgroups: 



(a) The inverting enzymes, which convert the double sugars or di- 

 saccharids into the monosaccharids. Examples: maltase, which 

 splits maltose to dextrose; invertase, which splits cane-sugar to 

 dextrose and levulose; and lactase, which splits milk-sugar (lactose) 

 to dextrose and galactose. (6) The enzymes which split the mono- 

 saccharids. There is evidence of the presence in the tissues of an 

 enzyme capable of splitting the sugar of the blood and tissues 

 (dextrose) into lactic acid. 



5. The coagulating enzymes, which convert soluble to insoluble pro- 



teins. Examples: fibrin ferment (thrombin or thrombase), that 

 causes clotting of the blood, and rennin, that causes clotting of milk. 



6. The oxidizing enzymes or oxidases. A group of enzymes which set 



up oxidation processes. Some of the details of the activity of 

 these enzymes are considered in the discussion of physiological 

 oxidations (p. 866). 



The enzymes contained in the first, second, third, and fourth (a) 

 of these groups are the ones that play the chief roles in the digestive 

 processes, and it will be noticed that they all act by hydrolysis, 

 that is, they cause the molecules of the substance to undergo de- 

 composition or cleavage by a reaction with water. Thus, in the 

 conversion of maltose to dextrose by the action of maltase the re- 

 action may be expressed so : 



