488 



2. ANALOGS OF ENZYME REACTION COMPONENTS 



in favor of ERN, and that actually competition with phosphate rather than 

 with nicotinamide riboside might be expected. A further complication is 

 the finding that the exchange reaction and the sensitivity to nicotinamide 

 depend on a cofactor, which was isolated and shown to be either ergothio- 

 neine or a closely related compound. It is possible that ergothioneine acts 

 as a ribosyl acceptor and this would modify the kinetics of the nicotin- 

 amide inhibition. It is interesting that ergothioneine will make the Neuro- 

 spora NADase inhibitable by nicotinamide. 



A variety of substances related to nicotinamide or other portions of the 

 NAD molecule are inhibitory to NADases (Table 2-29). There is a good 

 deal of variation in susceptibility between the different enzymes. In this 

 connection it may be mentioned that Handler and Klein (1942) found that 

 rabbit brain NADase is readily inhibited by 5-10 mM nicotinamide but 

 not inhibited at all by 160 mM picolinate, quinolinate, benzamide, a-amino- 

 nicotinate, trigonelline, adenine, adenosine, or pyridine. These inhibitions 

 probably involve different mechanisms. Some are not competitive and the 

 inhibitors probably participate in the transfer reaction as does nicotinamide, 

 while others are competitive and the inhibitors are bound reversibly to sites 

 at the active center, thereby preventing the binding of NAD. The inhibition 



Nicotine 



€f 



Nikethamide 



CON' 



.C2H5 



COO 



^N 



CH3 



Trigonelline 



,CH, 



4(5)-3'-Pyridyl- 

 glyoxaline 



CONHNH, 



CONHNH-CH 



Isoniazid 



CH, 



of beef spleen NADase by isoniazid exhibits unique kinetics inasmuch as 

 an increase in the NAD concentration actually increases the inhibition, this 

 suggesting that some interaction between NAD and isoniazid occurs, the an- 

 alog formed being the active inhibitor. Isonicotinamide inhibits similarly to 

 isoniazid. 3-Substituted pyridines generally inhibit somewhat more strong- 

 ly than the 4-substituted compounds. One of the most potent inhibitors is 



