8 JOHN H. NORTHROP 



States that the equiHbrium constant for the reaction A=nB is defined 

 by the equation K=[Br/[A] or [A] = [B]'^/K. [A] and [B] are 

 concentrations at equiHbrium, and n is the number of molecules of B 

 formed from A. If [A] is very small compared to [B] then the concen- 

 tration of A at equilibrium will increase in proportion to the «th power 

 of the total concentration. Thus if it be assumed that a protein is hydro- 

 lyzed to lOO amino acids, increasing the total concentration lo times 

 should increase the concentration of unhydrolyzed protein 10^°° times. 

 The fact that there are undoubtedly a number of intermediate steps in 

 the hydrolysis does not change the results, since according to thermo- 

 dynamic reasoning the condition at equilibrium must be independent of 

 the path by which the equilibrium is reached. 



Effect of pH. It is possible to assume that the equilibrium is shifted 

 by changes in pH. of the solution, since the energy change is small in 

 any case and could be significantly affected by the energy of ionization 

 of the products of the reaction. 



Experimental Results. Danilewsky found that a precipitate 

 (plastein) was formed when a preparation of pepsin was added to a 

 concentrated solution of the products of pepsin hydrolysis of protein. 

 The problem was carefully investigated by Wasteneys and Borsook 

 (1930). They confirmed the importance of the concentration, although 

 the experimental result is not exactly as expected by theory since it is 

 necessary also to change the pH. In dilute acid solution the plastein is 

 digested, whereas the synthetic reaction had an optimum rate of pti 4.0. 

 There is a decrease in amino and carboxyl groups. 



The total amount of plastein formed is greater when greater amounts 

 of enzyme are added. This is apparently contrary to theory, but it is 

 very difficult to be sure that equilibrium is reached, especially with low 

 concentrations of enzyme in the presence of reaction products. The 

 products act as inhibitors and greatly decrease the reaction rate. 



Wasteneys and Borsook also found that the reaction was accelerated 

 by the presence of some emulsifying agents (benzaldehyde) and took 

 place to some extent without the enzyme. This observation is difficult 

 to understand from the point of view of synthesis unless these substances 

 help remove the protein from solution. Wasteneys and Borsook con- 

 sider that the protein is synthesized as a soluble protein which soon 

 changes to an insoluble (denatured?) form and precipitates from the 

 solution. If this is correct then the emulsifying agents may act by 

 accelerating the denaturation reaction since many proteins are rapidly 

 denatured at interfaces. 



