EFRAIM RACKER 



Thus, every one of the intermediates of the niicrocycles of 

 glyceraldehyde-3-phosphate dehydrogenase and phosphogluco- 

 mutase can be isolated. It should be pointed out that in both 

 instances this feat has been made possible by the artificial inter- 

 ruption of the cyclic process of catalysis, either by removal of the 

 coenzymes, as with glyceraldehyde-3-phosphate dehydrogenase, 

 or by omission of a phosphate acceptor, as with phosphogluco- 

 mutase. With many enzymes, however, neither of these two 

 approaches is feasible, and, in order to change the course of 

 events, alternative methods, such as the use of inhibitors, must be 

 explored. 



INHIBITORS 



An outstanding biochemist once remarked that only un- 

 inhibited investigators use inhibitors in complex biological sys- 

 tems. Pitfalls similar to those encountered in inhibitor studies 

 of multienzyme systems also confront the investigator of a micro- 

 cycle which is catalyzed by a single enzyme. On the other hand, 

 unambiguous studies with inhibitors such as sodium fluoride and 

 iodoacetate have been landmarks in the history of the biochemis- 

 try of glycolysis and of muscular contraction. Inhibitors, used 

 cautiously, have also been valuable tools in the study of enzyme- 

 coenzyme-substrate interactions. 



The protective action of coenzymes or substrates against in- 

 hibitors has yielded important information about the mode of 

 their interaction with the enzyme. Early studies of Rapkine on 

 TDH with several SH inhibitors indicated an interaction be- 

 tween the SH groups of the enzyme and DPN. Alcohol dehy- 

 drogenase from yeast can be protected against lAA by DPN or 

 ethyl alcohol (c/. 37) ; testosterone dehydrogenase can be pro- 

 tected by DPN against /^-chloromercuribenzoate (48). It should 

 be pointed out that more complex effects may be encountered. 

 Thus, DPN, which protects TDH against iV-ethyl maleimide, 

 increases markedly the susceptibility of the enzyme to lAA. 

 Glyceraldehyde-3-phosphate protects TDH against lAA, but so 

 do some other phosphorylated compounds (e.g., 3-phospho- 



232 



