JOHN H. NORTHROP 487 



however, and finally becomes almost negligible in spite of the fact 

 that there is still a large amount of egg albumin in solution and that 

 the pepsin still retains its activity (as may be demonstrated by dilut- 

 ing the solution, after which digestion will continue). The addition 

 of a further amount of pepsin to the solution will now have little or 

 no effect. The albumin solution is "immune" to the pepsin. This 

 is due to the fact that a small amount of pepsin can cause the pro- 

 duction of a very large amount of peptone. Each unit of peptone 

 produced decreases the amount of free pepsin somewhat; but as may 

 be seen from equation (2) it would require an infinite concentration 

 of peptone (d) to reduce the concentration of free pepsin (Q) to 0. 

 Practically, the reaction stops owing to the destruction of the pepsin.^* 

 Referring again to Table VI, it will be noted that Qd, the product 

 of the concentration of active enzyme into the concentration of pep- 

 tone, approaches a constant value as d increases. In other words 

 the concentration of active enzyme becomes nearly inversely pro- 

 portional to the concentration of peptone, when the latter is present 

 in great excess. This is a well known property of mass action equi- 

 libria and follows from the formula, as may be seen below. The for- 

 mula used in this connection is 



Q.[d-(E-Q)] 



where Q is the concentration of active (free) enzyme, [d—{E—Q)] 

 the concentration of free peptone, and E—Q the concentration of 

 combined pepsin or peptone. It is obvious that as d increases Q 

 must decrease so that the value of the term E — Q approaches the 

 constant value E. When d becomes very large compared with E the 

 term d—{E — Q) will not differ significantly from d. The equation 

 may then be written 



KE 



d in this equation represents the amount of peptone present at the 

 beginning of the reaction. If the equation is to hold throughout the 

 reaction the concentration of peptone will be represented by d-\-x. 



15 Northrop, J. H., /. Gen. Physiol, 1919-20, ii, 465. 



