112 A SYMPOSIUM ON RESPIRATORY ENZYMES 



dinium bases are readily subject to reduction processes. Concerning 

 the position where the reduction takes place, the following possi- 

 bilities had to be considered. 



FORMULA 3 

 H H Hg 



HC'^^'^C-CONH^ HC^^VCONH^ '-'9j^^^^"^^^^^2 



I ' k 



R R R 



(a) (b) (c) 



A comparison of the earlier known compounds of this group 

 showed noteworthy diflFerences between the p-dihydro compounds 

 and the reduced coenzymes, but very good agreement between the 

 latter and the model o-dihydro compounds. Whether the reduction 

 takes place according to formula 3a or 3b could not be decided 

 (23a, c). 



The next step was the preparation of nicotinamide derivatives 

 which are substituted by carbohydrate radicals on the ring nitrogen 

 atom. The properties of these compounds showed a still better 

 correspondence with those of the coenzymes. The best representa- 

 tive of these compounds which can be prepared in a pure state 

 was found to be tetra-acetyl-glucosido-nicotinamide bromide. The 

 absorption maximum of its dihydro derivative is very similar to that 

 of the reduced codehydrogenases. In addition to similarity in optical 

 properties the model nucleosides share with the coenzymes the 

 sensitivity of the glycosidic linkage toward alkali when in the oxi- 

 dized form, and stability toward alkali when reduced; also, the 

 action of strong acid on the dihydro compounds yields, according 

 to Karrer, products which no longer have the absorption band at 

 340 m[x. Furthermore, both the model compounds and the co- 

 enzymes are very sensitive toward hypoiodite, which destroys the 

 pyridine ring (24). 



When the pyridinium model compounds are reduced to the di- 

 hydro compounds, the ring nitrogen of the reaction products is 

 trivalent, and they do not contain the acid group. Corresponding to 

 this acid group in the model compounds is the phosphoric acid in 

 the coenzymes. Upon reduction an acid group is liberated according 

 to the following scheme (8, 21): 



