78 PHYSICOCHEMICAL BASIS OP PHYSIOLOGICAL PROCESSES 



It should be clearly understood that pure catalysts, such as the hydro- 

 gen ion, in accelerating a reaction like the above, do so equally in both 

 directions, so that the position of equilibrium remains unchanged. En- 

 zymes may, however, cause this position to change because of their form- 

 ing intermediate combinations. 



The reverse phase of certain reactions is probably the cause of at least 

 some of the synthetic processes which occur in the animal body. A great 

 difficulty in accepting such a view, however, is the fact that the equilib- 

 rium point of all hydrolytic reactions, in the presence of an excess of 

 water, is so near complete hydrolysis that very little synthesis can be 

 possible. That is true so long as the substance synthesized is soluble, 

 but if it is nearly insoluble in water, or if it is immediately removed 

 from the site of the reaction by diffusion, or in any other way,, then it is 

 obvious that it will go on being synthesized by the reaction. Thus, in the 

 intestine neutral fat is hydrolyzed by pancreatic lipase into fatty acid 

 and glycerin, which are absorbed into the epithelium, where they again 

 come under the influence of intracellular lipase. This latter will tend to 

 accelerate the synthesis of neutral fat from the fatty acid and glycerin 

 until the equilibrium point of the system (fat acid + glycerin ^ neutral 

 fat + H 2 0) is again reached; but this point, although it is near the right 

 hand of the equation, will really never be reached for the reason that the 

 neutral fat, as quickly as it is formed, will become deposited in insoluble 

 globules in the protoplasm and thus be removed from the equation. In 

 support of this view it has been found that lipase is present in intestinal 

 mucosa after all traces of adherent pancreatic juice have been washed 

 away. By similar reactions the fat of the tissues becomes decomposed to 

 fatty acid and glycerin and passes out of the blood when the concentra- 

 tion of fat in this fluid falls below a certain level. Provided one of the 

 substances synthesized is insoluble or can in some other way be removed 

 from the reaction, it is plain that, even though the equilibrium point is 

 very near to that of complete hydrolysis, yet the reversion will be suf- 

 ficient to do all that is required of it. 



Results such as the above have prompted many to conclude that it is 

 by such reversible action that all synthetic processes occur in the living 

 organism. But the demonstrable synthesis of an ester must not be taken 

 as evidence that all other syntheses are explainable on the same basis. 

 For example, we have seen above that in the case of cane sugar the equi- 

 librium point in the equation is so near that of complete hydrolysis, that no 

 measurable amount of cane sugar is formed when dextrose and levulose are 

 allowed to act on each other, and that cane sugar does not appear 

 when sucrase is added to the mixture. If instead of sucrase we take 

 another of the sugar enzymes namely, maltase, which accelerates the 



