ENZYMES AS REAGENTS 



glycerate) which are not substrates and do not react directly 

 with SH groups (24). 



The inhibitor may block a partial reaction of the cyclic proc- 

 ess of enzyme catalysis and permit a functional separation of a 

 multistep system. For example, iodoacetate inhibits the oxido- 

 reduction catalyzed by TDH, but arsenolysis of the acyl enzyme 

 can still take place (38). Potassium cyanide stabilizes acyl 

 enzyme even in the presence of DPN, without interfering with 

 arsenolysis (26). Tetra alkyl pyrophosphate has been shown 

 (54) to inhibit cholinesterase by forming a phospho-enzyme in- 

 stead of an acyl enzyme. In contrast to the acyl enzyme, the 

 phospho-enzyme is not readily hydrolyzed, and nucleophiiic re- 

 placement agents must be used to reactivate the active center of 

 the enzyme. 



Oxidized (S-S) TDH, which is devoid of dehydrogenase 

 activity, has been shown to hydrolyze acetyl phosphate (12). A 

 stimulation of the phosphatase action of TDH has also been 

 noted after treatment with iodoacetate (26). An inhibitor can, 

 therefore, not only block a cyclic process but channel it into a 

 side reaction, Iodoacetate alters the properties of this catalyst 

 so fundamentally that instead of acting as an acyl transfer agent, 

 it becomes a catalyst of hydrolysis. This exhibition of new 

 catalytic activities in the presence of inhibitors requires most 

 cautious interpretation of observations obtained with enzymes 

 which have been exposed to maltreatment by enzymologists or 

 histochemists. The activation of unspecific phosphatases by 

 iodoacetate or by maleate, reported in the older literature {cf. 

 53), may represent a similar alteration of functional SH groups 

 resulting in a change in catalytic activity. 



EXCHANGE REACTIONS 



The use of isotopically labeled compounds in biochemical 

 research has led to the discovery of a new group of reactions 

 often referred to as exchange reactions. They may be defined 

 as reactions in which an isotopic label is incorporated into a 

 compound, although no synthesis of the latter has occurred. 



233 



