ENZYME ACTION 53 



and by rcinovinf:; tlicm from the fluid about the cells favor osmotic diffusion from 

 tlie intestinal epithelium, thus facilitating absorption. 



Quite similar must be the process that takes place in the tissue cells through- 

 out the body. In the blood-serum bathing the cells is a mixture of fat and. its 

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

 the diffusible substances enter a cell containing lipase, c. r/., a liver cell, the process 

 of building and splitting will be quite the same as in the intestinal epithelium. 

 The only difference is that here the fatty acid maj' be removed from the cell by 

 being utilized by oxidation or some other chemical transformation.'* 



To summarize, it may be stated that thiouglK)iit 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 also act reversibly, then the phenomena of 

 protein 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 attempt at 

 establishment of equilibrium by enzymes, perpetuated by prevention of 

 attainment of actual equilibrium through destruction of some of the 

 participating substances by oxidation or other chemical 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. 1^ A favorite idea is that they form loose compounds 

 with the substance to be split and with water; the resulting compound 

 being unstable and breaking down, the water remains attached to 

 the components of the substance. 



Enzymes do not act catalytically on all substances by any means, 

 but show a decidedly specific nature. They affect only organic sub- 

 stances, and the actions are limited to two processes — hydrolj'sis and 

 oxidation, or the reverse processes of dehydration and reduction.'^ 

 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 substances, 

 whereas the enzymes are specific; thus hydrochloric acid will hydrolyze 

 starch or protein 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 



16 Bradley (Jour Biol. Chem., 1910 (S), 251; 1913 (13), 407-439) calls atten- 

 tion to the great concentration necessary for fat synthesis by lipase in vitro, and 

 the lack of correspondence between the amount of fat and of lipase in various 

 tissues, questioning the importance of lipase for fat synthesis in the living tissues 

 as well as the significance of reversed enzyme reaction for biological processes in 

 general. 



'^ See Euler, "Chemical Dynamics of Enzyme Reactions," Ergebnisse d. Physiol. 

 1910 (9), 241. 



18 Alcoholic fermentation may be an exception, the change being CeHnOs = 

 2C2H6OH -|- 2CO2, but it is very possibly an intramolecular oxidation. 



