SEVERO OCHOA 



nitely known to function with TPN, and one at least can function with 

 either coenzyme (Table I). 



Table I 

 Pyridine Nucleotide Dehydrogenases 



The pyridine in the coenzymes acts by cyclic addition and re- 

 moval of hydrogen (see scheme I). Through the reversible change 

 pyridine ^ dihydropyridine, it functions as a hydrogen carrier. The 

 protein component of the dehydrogenase, upon whose nature depends 

 the specificity for its substrate, binds both substrate and coenzyme, and 

 in this complex hydrogen from the substrate is transferred to the 

 pyridine of the coenzyme; the substrate is thus oxidized and the pyri- 

 dine reduced. This transfer of hydrogen is reversible, so that dihydro- 

 pyridine and oxidized substrate when bound by the protein can react 

 to form pyridine and reduced substrate. Thus, the action of a typical 

 pyridine nucleotide dehydrogenase can be represented as follows: 



RHa + P-Go :^R + P-CoHa 



where RH2 and R represent reduced and oxidized substrate and P • Co 

 and P-CoHa represent oxidized and reduced protein-coenzyme com- 

 plex, respectively. 



The reduced coenzymes have a sharp absorption band at 340 

 m^, whereas there is no absorption at this wave length by the oxidized 

 form. This distinction is important because the action of the pyridine 

 nucleotide dehydrogenases can be conveniently followed by changes 

 in the absorption of light of wave length 340 myu (27). 



170 



