PROSTHETIC GROUPS, COENZYMES AND ENZYMES 



DPNH we do not know, whether by a hydrogen bond — SH 



N+, as suggested by Kaplan and Ciotti, or by a covalent 



Hnkage, or otherwise. 



The whole question of the 340 m/x band shifts are probably 

 of very great importance for elucidating the working mech- 

 anism of DPN and TPN-enzymes. 



Meyerhof, Ohlmeyer, and Mohle (9) already in 1938 

 observed that DPN forms compounds with cyanide and bisulfite, 

 at that time presumed to be 



,r"^CONH2 , f^CONH2 



l^XH-CN ^"^ l^/CH-SOa-Na- 



and these compounds showed a band shift from 340 to 320 or 

 325 mju. 



Pullman et al. (14) and Rafter and Colowick (16) have 

 presented very good evidence suggesting that of four possible 

 resonating structures in the pyridine ring of DPN the one with a 

 positive charge on C4 is preferred 



CH 



o 



N 



This means that the chemical addition should occur primarily 

 at C4 instead of at Ce- Kaplan and Ciotti observed that the 

 325-m)u band of DPNCN was shifted further to 310 m/i when 

 it was coupled with ADH; and the 315-m)u band of the DPN- 

 hydroxylamine complex, studied by Kaplan and his co-workers, 

 moved further to 300 m/x when ADH was added. This seems to 

 us to indicate that, for example, cyanide and ADH are bound to 

 different sites at the pyridine ring. Perhaps addition reactions 

 may occur at more than one of the double bonds of the pyridine 

 ring. The developments in this special field are awaited with 

 much interest. 



Now, let us turn back to the question of the validity of our 



301 



