ENZYMES 199 



outside of the cell. Here, as in the intestine, the pitcher of the 

 pitcher plant, etc., they do their work. The relation to the cell is 

 secondary as long as the proper environment (temperature, acidity, 

 etc.) is maintained. 



General Characters. — Enzymes have all the characteristics 

 named above for inorganic catalysts and, in addition, possess 

 others which may be enumerated as follows : 



1. They are colloidal, which means that they possess the gen- 

 eral properties of such materials as outlined in Chapter II. They 

 carry electrical charges, exhibit to a marked degree the phenome- 

 non of adsorption, and do not diffuse through parchment mem- 

 branes. They do, however, pass through porcelain filters, and this 

 fact is made use of in separating enzymes from the cells which 

 contain or secrete them. 



2. They were once thought to be proteinaceous in nature, 

 but it is extremely difficult to secure the enzyme in a pure condi- 

 tion, and it is possible that the protein material is an impurity. 

 Although urease, the first enzyme to be isolated (by Sumner, 

 1926), was found to be a protein of the class of globulins, the more 

 that enzymes have been purified, in general, the fewer protein 

 characteristics do they show; and Willstatter has shown definitely 

 that lipase, invertase, and peroxidase are not proteins. Peroxi- 

 dase, however, is spoken of as a " nitrogenous glucoside." Some 

 enzymes retain their activity when acted upon by proteolytic 

 substances which would destroy them if they were proteins. 

 This fact would indicate that the protein is an impurity, and that 

 the colloidal inorganic salts such as those of manganese and iron 

 which are often found associated with enzymes, may be the im- 

 portant constituent. It may develop that their chemical nature is 

 not so important, and that their colloidal nature is the chief fac- 

 tor in their action. It may also develop that some are proteins 

 and others are not. 



3. They are very sensitive to heat and light. Each enzyme has 

 an optimum temperature which is generally between 40° and 

 55° C. Most of them cannot be heated over 70°, and all are in- 

 activated or " killed" at the temperature of boiling water (100° 

 C.) if in solution, although, when dry, they may be able to stand 

 slightly higher temperatures for a short time. The inactivation 

 by heat may be explained most easily as a coagulation of the col- 

 loidal proteins present. The activity of the enzyme is doubled 



