68 ENZYMES 



constituents, probably nearly in equilibrium, since lipase accompanies them. 

 If the diffusible substances enter a cell containing lipase, e. g., a liver cell, the 

 process of building and splitting will be quite the same as in the intestinal epi- 

 thelium. The only difference is that here the fatty acid may be removed 

 from the cell by being utilized by oxidation or some other chemical transforma- 

 tion. 



To summarize, it may be stated that throughout the body 

 there is constantly taking place both splitting and building of 

 fat. Fat enters the cells, leaves them, and is utilized only in 

 the form of its acid and alcohol, never as the fat itself. Fat 

 constitutes a resting stage in its own metabolism. 



If proteolytic enzymes are also reversible, then the phenom- 

 ena of proteid metabolism are similarly explained, for there 

 is no doubt that every cell and body fluid contains proteolytic 

 enzymes. 



All metabolism, then, may be considered as a continuous at- 

 tempt at establishment of equilibrium by enzymes, pet^petuated by 

 prevention of attainment of actual equilibrium through destruction 

 of some of the participating substances by oxidation or other chem- 

 ical processes, or by removal from the cell or entrance into it of 

 materials which overbalance one side of the equation. 



In just what manner the enzymes accomplish their catalytic 

 effect is yet unknown. A favorite idea is that they form loose 

 compounds with the body to be split and with water ; the result- 

 ing compound being unstable and breaking down, the water 

 remaining attached to the components of the substance. There 

 is some evidence, but not conclusive, indicating that the enzyme 

 does enter into combination with its object. Euler has suggested 

 that enzymes increase ionization, which is at the bottom of the 

 chemical changes. 



Enzymes do not act catalytically on all substances by any 

 means, but show a decidedly specific nature. They affect only 

 organic substances, and the actions are limited to two processes 

 hydrolysis and oxidation, or the reverse processes of dehydration 

 and reduction. 1 The most essential difference between the 

 enzymes and the chemicals that can accomplish hydrolysis or 

 oxidation is this : the ordinary chemical reagents produce their 

 effects on many sorts of substance, the enzymes are specific; 

 thus hydrochloric acid will hydrolyze starch or proteid with 

 equal facility, but pepsin will not affect starch at all. 



The very specific nature of the enzymes, their activation by 

 other body products, the fact that they seem to be bound to the 

 substance upon which they act, that they are susceptible to heat, 



J Alcoholic fermentation may be an exception, the change being C 6 H 12 O 6 = 

 2C 2 H 5 OH + 2CO 2 , but it is very possibly an intramolecular oxidation. 



