no PHYSIOLOGICAL ACTIVITIES 



be until their composition and constitution are known. 

 Most probably they form a combination with the substance 

 acted on {the substrate) as a result of Avhich there is a rear- 

 rangement of the atoms in such a way that new compounds 

 are formed, nearly always at least two, and the enzyme is 

 at the same time set free. It is rather remarkable that 

 chiefly optically active substances are split up by enzymes, 

 and where two modifications exist it is usually the dextro- 

 rotatory one which is attacked. No single enzyme attacks 

 both. This probably means that the structure of the 

 enzyme corresponds to that of the substrate, "fits it as a 

 key fits a lock," as Emil Fischer says. 



The production of enzymes is by no means restricted to 

 bacteria, since all kinds of living cells that have been inves- 

 tigated have been shown to produce them and presumably 

 all living cells do. Hence the number of different kinds of 

 enzymes and of substances acted upon is practically unlim- 

 ited. Nevertheless they may be grouped into a compara- 

 tively few classes based on the general character of the 

 change brought about by them. 



I. Class I is the so-called "splitting' enzymes whose 

 action is for the most part hydrolytic, that is, the substance 

 takes up water and then splits into compounds that were ap- 

 parently constituents of the original molecule. As examples 

 may be mentioned diastase, the enzyme first discovered, 

 which changes starch into a malt-sugar, hence is more com- 

 monly called amylase^ (starch-splitting enzyme) ; invertase,^ 

 which splits cane-sugar into dextrose and levulose : C12H22O11 

 + H2O = C6H12O6 + C6H12O6. Lipase,^ or a fat-splitting 

 enzyme, which decomposes fat into glycerin and fatty acid : 



C3H5(OCnH2n-lO)3 + SHsO = C3H5(OH)3 + 3CnH2n02. 



Fat. Glycerin. Fatty acid. 



Proteases, which split up proteins into proteoses and pep- 

 tones. 



Other classes of "splitting enzymes" break up the prod- 



1 It will be noted that the names of enzymes (except some of those first 

 discovered) terminate in ase, which is usually added to the stem of the name 

 of the substance acted on, though sometimes to a word which indicates the 

 substance formed by the action, as lactacidase, alcoholase. 



