SYNTHETIC ACTION OF HYDROLYZING ENZYMES 225 



agent in accomplishing the synthesis is a modification of pepsin, arising 

 from it by loss of water : 



Hydrolyzing Pepsin ^ Synthesizing Pepsin + H2O 



and that in bringing about the hydrolysis of protein the hydrolyzing 

 form may partially lose water and be transformed into the synthesizing 

 form and vice versa, high temperatures, as usual, favoring the forma- 

 tion of the anhydride or synthesizing form. If this were so, of course, 

 pepsin would be far from being a true catalyzer, since it would enter 

 into and be modified by the reactions which it accelerates. Such 

 modification has, however, not yet been shown to occur. 



It will be seen, therefore, that while in some instances the hydrolyzing 

 enzymes appear to act as genuine catalyzers, in other instances their 

 behavior appears to be inconsistent with this view. In any case, the 

 synthetic activity of the hydrolyzing enzymes, so far as it has yet been 

 demonstrable in vitro is very inferior in point of speed and completeness 

 to the synthetic processes which actually and continually occur in 

 living tissues. As we shall see, glucose is converted into glycogen 

 almost as rapidly as it can be absorbed and transported, dissolved in 

 minute concentration in the. blood, to the cells of the liver. Fat is 

 synthesized from glycerol and fatty acids in the intestinal mucosa 

 within a few moments after absorption, there are strong reasons for the 

 belief that the synthesis of protein from amino-acids in the tissues is 

 not much less rapid. The contrast betweea these phenomena and the 

 prolonged periods of action and high concentrations both of the enzyme 

 and the products required to resynthesize the substrate by a hydrolyzing 

 enzyme, and the fragmentary yield which results, point very strongly 

 to the existence, in living tissues of a decisively different synthesizing 

 mechanism to that involved in the reversed action of catalyzers. Only 

 two alternatives appear to be open to us in interpreting this dispro- 

 portion. Either the tissues employ enzymes which selectively accele- 

 rate syntheses and therefore are consumed by the syntheses which they 

 accomplish, or else the syntheses in living tissues, composed though 

 they are of over eighty per cent, of water, take place under conditions 

 approximating to almost complete desiccation. This latter alternative 

 is not so inconceivable as it might appear, because the reactions in 

 question may possibly take place at the surface of lipoid granules 

 which are emulsified in the protoplasm, but which, being insoluble in 

 water, afford a medium which is almost water-free. The most serious 

 difficulty attaching to this view, however, is that so many of the 

 products so rapidly synthesized in living tissues are as insoluble in 

 oils as water is itself, for example the proteins. On the other hand if 

 the former alternative be adopted we are faced with the difficulty that 

 the hypothetical synthesizing enzymes have never been obtained apart 

 from living tissue, so that either their action is intimately bound up 

 with the uninjured Structure of the protoplasm, or else we have not yet 

 hit upon the right methods of extracting them from living tissues and 

 conserving their synthetic activity. ' 

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