838 LIGHT AND LIFE 



rescence is enhanced by lactic dehydrogenase. That this new form 

 SH 



is a thiol (R— C=NH) was rendered probable (a) by complexing 

 S 



the thione (R— C— NHo with parachloromercuribenzoate, whereupon 

 the shift in absorption from 398 ni/^ to 341 m^ upon illumination 

 was prevented; and (b) by methylating the thione S atom, which is 



S-CH:, 



stabilized as an analogue of the enol form (R— C^NHs) , and which 

 was found likewise to have maximum absorption at 341 \w^. This 

 is then a case of photo-induced tautomerism. 



There is little indication that adenine and the pyridine rings in- 

 teract in DPN. Aminopyridine-DPN has been prepared and its ab- 

 sorption characteristics studied; but since the amino group of this 

 analogue is an electron-donating group whereas the carboxamide of 

 DPN is an electron acceptor, the analogue is not a good model for 

 DPN action. 



The chief point of the foregoing considerations is that in the oxida- 

 tion and reduction of DPN there is not only a change on the charge 

 of the pyridinium ring N but also a change of the molecule's con- 

 formation. Yet both oxidized and reduced forms of the coenzyme 

 occupy the same sites when bound in an enzyme complex. It must 

 be that if one form of the dinucleotide fits closely the other Avill fit 

 less well and the binding interaction will be weaker. In most cases 

 where the interaction has been measured, it is stronger between 

 enzyme and DPNH than between enzyme and DPN. 



In discussing the reduced pyridine nucleotide enzyme complexes, 

 Velick has compared the jjroperties and behavior of the glyceralde- 

 hyde-3-phosphate dehydrogenase (GPD) and lactic dehydrogenase 

 (LDH) . Studies were made of the persistence and the disappearance 

 of the 260 ni/x excitation band, serving respectively as criteria for the 

 closed and open conformations of the dinucleotide. These studies 

 reveal that DPNH is bound ni the folded conformation to GPD, but 

 in LDH and all other dehydrogenases examined it is bound in the 

 open conformation. Corresjjondingly, GPD has a greater affinity for 

 DPN than for DPNH, just the reverse of the behavior of all the other 

 tested enzymes. Moreover, in the GPD-DPNH complex, unlike all 



