6 PHYSIOLOGICAL CHEMISTRY 



investigator is attempting to isolate them. For this reason we have 

 absolutely no proof that the final product obtained is, or is not, in the 

 same state of purity it possessed in the original cell. Some of the en- 

 zymes are more or less closely associated with the proteins from the fact 

 that they are both formed in every cell as the result of the cellular ac- 

 tivity, both may be removed from solution by " salting-out," both are 

 for the most part non-diffusible and are probably very similar as re- 

 gards elementary composition. Hence in the preparation of some 

 enzymes it is extremely difficult to make an absolute separation from 

 the protein. Most of the evidence points to the protein character of 

 enzymes. 1 Under certain conditions enzymes are readily adsorbed by 

 shredded protein material, such as fibrin, and may successfully resist 

 the most prolonged attempts at washing them free. We may sum- 

 marize some of the properties of the great body of enzymes as follows : 

 Enzymes are soluble in dilute glycerol, sodium chloride solution, 

 dilute alcohol and water, and precipitable by ammonium sulphate 

 and strong alcohol. Their presence may be proven from the nature 

 of the end-products of their action and not through the agency of any 

 chemical test. They are colloidal and non-diffusible, and occur closely 

 associated with protein material with which they generally possess many 

 properties in common. Each enzyme shows the greatest activity at a 

 certain temperature called the optimum temperature; there is also a 

 minimum and a maximum temperature for each specific enzyme. Their 

 action is inhibited by sufficiently lowering the temperature, although 

 some activity may be shown at o or even at lower temperatures 

 and freezing does not, in most cases, permanently injure enzymes. 

 Most enzymes, if in solution, are entirely destroyed by subjecting 

 them to a temperature of 7o-iooC. The best known enzymes, whether 

 derived from warm-blooded or cold-blooded animals, are most active 

 between 35-45C. The nature of the surrounding media alters the 

 velocity of the enzymatic action, some enzymes being more active in 

 acid solution whereas others require an alkaline fluid. 



Many of the more important enzymes do not occur preformed 

 within the cell, but are present in the form of a zymogen or mother- 

 substance. In order to yield the active enzyme this zymogen must be 

 transformed in a certain specific manner and by a certain specific sub- 

 stance. This transformation of the inactive zymogen into the active 

 enzyme is termed activation. For instance, the zymogen of the enzyme 

 pepsin of the gastric juice, termed pepsinogen, is activated by the hydro- 

 chloric acid secreted by the gastric cells (see page 141), whereas the acti- 

 vation of the trypsinogen of the pancreatic juice is brought about by a 



1 Others seem to be like the substrate on which they act, e.g., carbohydrate. 



