On the structure of triphosphopyridine nucleotide.

As a result of the work of Warburg (l), von Euler (2), and their associates, it is known that triphosphopyridine nucleotide (TPN) and diphosphopyridine nucleotide (DPN) are dinucleotides and differ only in that TPN contains one additional phosphate group. The structure proposed by Schlenk and von Euler for DPN, in which nicotinamide ribose phosphate and adenosine-5-phosphate are linked by a pyrophosphate bond, has been confirmed by subsequent studies (3-5). The findings of von Euler and coworkers (6) of an interconversion of TPN and DPN in yeast maceration juice provided further evidence that these nucleotides were very similar in structure and it remained only to determine the disposition of the additional (third) phosphate group of TPN. It was considered that the three phosphates were either linked in a chain (7) or, as-later suggested by Schlenk et al. (S), that the third phosphate group was esterified to the pentose of the adenosine portion of the molecule. The unavailability of adequate amounts of pure TPN postponed a definitive solution of this question (9). Recently, a study of the action of nucleotide pyrophosphatase (10,4), an enzyme which breaks the pyrophosphate bonds in several nucleotides, revealed that the products of TPN cleavage were nicotinamide ribose phosphate and a diphosphoadenosine fragment which was not adenosine pyrophosphate. The present paper is a study of the structure of this diphosphoadenosine fragment. On the basis of evidence already available it appears rather certain that one of the phosphates of the diphosphoadenosine fragment is esterified at carbon 5 of the ribose. DPN has been shown to contain adenosine-5-phosphate and there is accumulating evidence for a direct interconversion of DPN and TPN. In addition to the atudies with yeast juice (6), potato extracts are known to convert TPN to DPN (4) and the mechanism of synthesis of TPN from DPN has been shown with a purified yeast enzyme to involve a direct phosphorylation by ATP (11). Also, TPN cleavage by washed kidney particles (12) yielded adenosinedphosphate in almost stoichiometric amounts (unpublished experiments). Concerning the point of attachment of the remaining phosphate, carbons 2 and 3 are the only likely sites, since carbon 1 is in glycosidic linkage with adenine and carbon 4 is part of a furanose ring. As a result of fur-