MULTIPLE ENZYME SYSTEMS 



a Study of the interaction of the resuking two enzymes did not 

 clarify this picture. These workers had considered the pos- 

 sibility that one enzyme might form a high-energy phosphate 

 intermediate and that the other would bring about condensa- 

 tion perhaps as follows : 



citrulline + ATP , citruIline-P + ADP (1) 



citruIIine-P + aspartate > argininosuccinate + Pi (2) 



However, neither enzyme by itself under any circumstances 

 liberated phosphate from ATP. Both amino acids and both 

 enzymes had to be present. They have correctly pointed out 

 that this does not necessarily eliminate a mechanism such as 

 shown above, for if the equilibrium constant for the first reaction 

 were to favor citrulline, no activity could be demonstrated in the 

 absence of the second reaction, which must necessarily lie far 

 toward the synthesis of argininosuccinate. Since the quantity 

 of Pi liberated was related to the level of ATP and the concen- 

 tration of yeast enzyme, definitive resolution of the problem was 

 not possible. Thus it could not be ascertained clearly in what 

 sequence the two enzymes acted. However, it appears likely 

 that the over-all reaction is represented by the sum of reactions 

 (1) and (2). The difficulties encountered in this very careful 

 study certainly could not have been foreseen, and it is to be ex- 

 pected that further pursuit of this problem will lead to an eluci- 

 dation of an important mechanism. 



Frequently an intermediate in a reaction sequence is de- 

 graded to a by-product. 



Consider the situation, A — > J -^ P and also J — > Y: 



J * Y > A > J 



A > J > J > P 



If the J intermediate is transformed to Y more rapidly than it is 

 formed, it follows that J will not accumulate but the degradation 

 product Y and some of the product P will. The question arises 

 as to how the investigator is to know this since he can show only 

 that two somewhat related compounds occur. In the conver- 



551 



