158 IMMUNO-CATALYSIS 



substrate is often less than one millionth of a second. That is, in one 

 second more than one million molecules come into contact with a 

 single enzyme molecule during which time they are catalyzed and 

 metabolized. Such a continuous combination, dissociation and destruc- 

 tion of the substrate at tremendous speed by its specific enzyme is in 

 no way comparable to the antigen and antibody reaction. It can be 

 seen readily that the action of an enzyme on its substrate is dynamic 

 and a catalytic reaction, and that of antigen with antibody is non- 

 catalytic and results in a static state. This process has a true counterpart 

 in the enzyme reactions.* 



In practically all enzyme reactions whether the substrates are 

 anabolized or catabolized by the action of enzymes, certain inhibitory 



*In in vitro experiments the combination between an antigen and antibody may or 

 may not result in precipitation. If the number of antigen molecules is in large excess 

 a precipitation may fail to occur (certain bacterial carbohydrates or small molecular 

 weight proteins reacting with their respective antibodies). However, there are antigen- 

 antibody reactions in which precipitation occurs even in the region of excess antigen 

 (a large molecular weight protein antigen reacting with its specific antibody). In 

 inhibition experiments, reactions which involve the participation of antibody and of 

 small molecular weight haptens, a precipitation fails to occur, despite the complete 

 neutralization of the combining groups of the two reactants. The phenomenon of 

 precipitation does not therefore accompany all the phases of, or various types of, 

 antigen-antibody reactions. 



The counterparts of the various phases of or of various types of antigen-antibody 

 reactions are encountered in reactions involving a combination between an enzyme 

 and its reaction products. These inhibitors combine with and completely inhibit the 

 activity of the enzymes. Such inhibitions (or combinations), however, may or may 

 not result in the formation of an insoluble enzyme-inhibitor complex. As in antigen- 

 antibody reactions the failure to form an insoluble complex appears to depend on the 

 molecular size of the inhibitor as well as the ratio of the number of molecules of the 

 two reactants. As discussed below the enzyme pepsin is inhibited by pepsin inhibitor; 

 the degree of inhibition is dependent on the concentration of the inhibitor. When, 

 however, one molecule of inhibitor combines with one molecule of pepsin the enzyme 

 is completely inhibited as a consequence of the formation of a soluble inhibitor-pepsin 

 complex. As in certain antigen-antibody reactions, the combination between the 

 inhibitor and pepsin does not result in a precipitation. On the other hand a solution 

 of trypsin inhibitor mixed with a solution or trypsin of equal molecular strength 

 completely inactivates trypsin with the formation of a crystalline compound with 

 trypsin. 



Straus and Goldstein (1943) have reported a study on the zone behavior of 

 enzymes. Departing from the classical treatment of the kinetics of enzyme reactions, 

 they show that under a number of common conditions systems involving the participa- 

 tion of an enzyme, specific substrate and an inhibitor, the enzyme-substrate, enzyme- 

 inhibitor systems behave in three distinct ways depending upon the concentrations of 

 the reactants and the dissociation constants of the system. An important practical con- 

 sequence of the theory of zone behavior concerns the effect of diluting a mixture of 

 enzyme and inhibitor (or substrate). They show that dilution is a crucial operation 

 which significantly afiFects the subsequent experimental observations. For a fuller 

 understanding of this study the reader is referred to the original article. 



