752 PHYSIOLOGY OF DIGESTION AND SECRETION. 



upon which an enzyme acts as its substrate, and it has been 

 assumed that the action of the enzyme, like that of the toxins, 

 takes place in two stages; first, the combination of the enzyme 

 and the substrate; second, the breaking down of this compound 

 to give the final products of the reaction. There is some reason 

 for believing that these two stages may be separated, and that 

 enzymes which on account of certain conditions, such as heating, 

 have lost their power of decomposing the substrate, may still 

 have the power of combining with it. Toxins showing a similar 

 property are designated as toxoids, and for the enzyme in this 

 condition the term zymoid has been suggested (Bayliss) 



Definition and Classification of Enzymes (Ferments) . On the 

 basis of the considerations presented in the preceding paragraphs 

 Oppenheimer suggests the following definition: An enzyme is a 

 substance, produced by living cells, which acts by catalysis. The 

 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. 

 A distinction is made frequently between endo-enzymes and exo- 

 enzymes. Under the latter group are included those enzymes which 

 are eliminated from the cells in which they are formed, and which 

 are found, therefore, in solution in the secretions, for example, 

 the ptyalin of the saliva or the pepsin of the gastric juice. By 

 endo-enzymes is meant a group of intracellular enzymes which are 

 not secreted, but are held within the cells in some form of com- 

 bination. To obtain them in solution or suspension it is necessary 

 to destroy this cell, usually by mechanical means, such as grinding 

 the tissue with sand and, in some cases, by submitting the ground 

 mass to a great pressure in a hydraulic press. The liquid obtained 

 by this latter method is known as the " press juice " of the tissue. 

 In life the endo-enzymes play their part within the bounds of the 

 cells in which they are contained, and probably constitute the 

 chief means through which are effected the metabolic processes 

 that characterize living matter. 



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 termin- 

 ation -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 en- 

 zymes which were first discovered being referred to most frequently 

 under their original names. Having in mind only the needs of 



