508 PROTOPLASM 



thesis of sugar from carbon dioxide and water ; we call this some- 

 thing an enzyme and name it chlorophyllase. We assume a 

 great deal when we do this. But in many other instances the 

 enzyme is definitely known, as in the case of invertase (a sugar- 

 splitting enzyme) and pepsin (a protein-splitting enzyme). 



The enzymes are classified either according to the kind of 

 substances that they break down or the way in which they do it. 

 There are (1) hydrolytic enzymes, including (a) the esterases 

 (lipases) or fat-splitting enzymes, (6) the carbohydrate-splitting 

 enzymes (cellulase attacking cellulose, and maltase attacking 

 maltose), and (c) the protein-splitting enzymes (pepsin and 

 trypsin — these were known before the ending -ase was adopted 

 for enzymes) — (2) oxidative enzymes, including (a) the oxidases, 

 (b) peroxidases, (c) the zymases (splitting hexose sugars such as 

 glucose and fructose), and (d) the catalases. The oxidases and 

 perioxidases are enzymes of particular biological significance. 



A few enzymes have been obtained in pure crystalline form. 

 Northrop has crystallized trypsin (the pancreatic protease), 

 pepsin, and urease. 



Willstatter believes that enzymes are not single substances 

 but are always associated with a colloid carrier, i.e., a protein 

 that carries the active principle, or enzymatic component. The 

 two constitute an adsorption complex. 



Enzymes have long been thought to be highly specific (i.e., 

 a definite enzyme for every specific reaction in the plant and 

 animal body). Pasteur believed them to be vital substances 

 capable of being produced only by the living cell. Many of 

 them are proteins. Enzyme activity and enzyme specificity 

 thus become the reactions of highly specialized protein groups. 

 The degree of specificity of enzymes is uncertain. Some, such 

 as invertase, which splits sucrose into dextrose and levulose; and 

 catalase, an oxidizing enzyme, which splits peroxides into oxygen 

 and water, seem to be highly specific, while others, such as the 

 lipases, which split neutral fats into glycerin and fatty acids, 

 attack large groups of related substances. With a change in the 

 nature of the medium there may also be a change in the nature 

 of the reaction; i.e., the enzyme may function as a catalyst in 

 more than one way. R. B. Harvey, therefore, states that it is 

 not necessary to assume the presence of a separate specific enzyme 

 for each process. Indeed, certain "enzymes" (e.g., the oxidases) 



