70 A TEXTBOOK OF PHYSIOLOGY 



such cases be detected in the urine. It is supposed that there are 

 intracellular enzymes normally contained within the tissues; these 

 enzymes, under the varying conditions of life, build up and break 

 down the tissues according to the need of that tissue and of the body 

 as a whole. Under adverse circumstances, such as the shutting off of 

 the blood-supply or the presence of toxins and poisons, the action 

 may proceed mainly in the direction of disintegration. 



Besides the enzyme concerned with the cell proteins, we have 

 evidence of enzymes acting upon carbohydrates and fats. A striking 

 example is the enzyme glycogenase, which forms glycogen from the 

 dextrose brought to the liver cells, and as occasion needs reconverts 

 this glycogen into dextrose. There are intracellular enzymes con- 

 cerned in the formation of urea, uric acid, etc. From the liver alone 

 at least fifteen enzymes have been isolated, which shows the great 

 importance of the intracellular enzymes in the metabolic processes of 

 the body. 



Whether enzymes be extracellular or intracellular, they have the 

 following well-marked properties: 



1. Enzymes perform their action best at an optimum temperature. 

 For the enzymes of our body this is 37 C., the body temperature. Cold 

 inihibits their action, but does not kill them, even when they are 

 subjected (as has been the case with some unicellular organisms) to 

 the great cold produced by evaporation of liquid air. Warming 

 above the temperature of the body tends to inhibit, while temperatures 

 varying from 55 C. to 70 C. destroy their action altogether. 



2. They have an optimum medium in which they act. This is 

 usually faintly alkaline (to litmus), and corresponds in the case of 

 intracellular enzymes to the reaction of the body tissue fluids. The 

 pepsin contained in gastric juice acts best in an acid medium; others 

 apparently w r ork best in a neutral medium. The enzymes of some 

 micro-organisms work best in the absence of free oxygen, and are 

 termed " anaerobic," in contradistinction to the enzymes requiring 

 free oxygen for their activity, which are called "aerobic." 



3. They are specific in action. Enzymes are classified according 

 to the substrate upon which they act. There are, for example, pro- 

 teolytic (protein-splitting), lipolytic (fat-splitting), amylolytic (starch- 

 splitting), sucrolytic (sugar-splitting) enzymes, as well as several others. 

 It is found that the proteolytic act only on protein, the starch-splitting 

 only upon starch, and so forth. 



Nevertheless, the active powers of some enzymes, which are secreted 

 together, correspond so closely (for example, pepsin and remain) that 

 the double action may be manifestations of one parent substance. We 

 may regard the parent substance as having different groups of " side- 

 chains " attached to it, one group of side-chains acting as pepsin, the 

 other as rennin. This idea is even more applicable where the sphere 

 of action of some of them appears to be so limited that it is difficult 

 to conceive of the existence of a separate enzyme for each action. 



Proteolytic (Protein-splitting) are pepsin, trypsin, and erepsin. 

 Pepsin is the active proteolytic enzyme of the stomach, trypsin 



