CHEMISTRY OF NUCLEOSIDES AND NUCLEOTIDES 181 



b. Di- and Triphosphopyridine Nucleotides {DPN and TPN) 



The hydrogen-transporting coenzyme, DPN, was called cozymase when 

 first discovered'-" and is sometimes knoAMi as coenzyme I or codehydro- 

 genase I. The closely related TPN"-^ differs from it only in possessing an 

 extra phosphate group.-'' On acid hydrolysis DPN gives adenine (1 mol.), 

 nicotinamide (1 mol.), pentose (2 mol.), and phosphoric acid (2 mol.).--^"-^" 

 The amino group in the adenine residue of both coenzymes is unsubstituted, 

 since they are dcaminated with nitrous acid to the hydroxy analogues with- 

 out loss of other groups. They are easily reduced to dihydro derivatives 

 which, unlike the coenzymes themselves, are very labile towards acids but 

 relatively stable to alkali. These properties, together with the accompany- 

 ing changes in ultraviolet spectra, are typical for ciuaternary pyridinium 

 compounds, and it has been shown that synthetic A^-glycosides of nico- 

 tinamide are very similar in properties to DPN and TPN.^^'-^^ Furthermore, 

 enzymic hydrolysis of DPN gives A-(D-ribofuranosyl)nicotinamide-^''"-" 

 (XLII) which, in its dihydro form, is identical with the synthetic glycoside 

 prepared from dihydronicotinamide and acetobromoribofuranose.-^^ Only 

 one sugar phosphate (ribose-5-phosphate) is formed by hydrolysis of DPN, 

 and a pyrophosphate linkage is present as shown by titration and the isola- 

 tion from a hydrolysate of adenosine-5'-pyrophosphate.^^^ These facts are 

 consistent with formula XLIII for DPN.^"" 



The structures proposed for these coenzymes are strongly supported by 

 enzymic hydrolysis studies.-^' • -"- The pyrophosphate linkage of DPN is 



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