| Abstract: | The biosynthesis of pyridine nucleotides has been examined in a number of mammalian cell lines in culture. In all lines examined, nicotinamide is incorporated by a biochemical pathway distinct from the Preiss-Handler pathway for nicotinic acid. In at least the human cell line D98/AH2, there is no detectable endogenous synthesis of the pyridine ring from tryptophan. Although most cell lines examined (hamster BHK 21/13, mouse L929 and human D98/AH2) use either nicotinic acid or nicotinamide as a precursor for DPN and TPN, two mouse cell lines, 3T3-4E and LM CIID, are unable to utilize nicotinic acid as a source of the pyridine ring. If nicotinic acid is present in the medium, substantial amounts of intracellular desamido DPN accumulate suggesting that the last step (desamido DPN→DPN) is limiting in the Preiss-Handler pathway. With nicotinamide, the only compound which accumulates in substantial amounts apart from DPN and TPN is nicotinamide ribose; there is no detectable NMN. The results of pulse-labeling experiments suggest that nicotinamide ribose may be an intermediate in the nicotinamide pathway. Following growth of D98/AH2 cells in high concentrations of niacin, biosynthesis of DPN from nicotinamide was completely inhibited for at least six hours. The converse experiment revealed no inhibition of niacin incorporation. This observation suggests that a niacin pathway intermediate, which present evidence indicates is desamido-DPN. can inhibit nicotinamide utilization. Newly synthesized DPN turns over with a half-life of two hours in azaserine-treated D98/AH2 cells. In the absence of azaserine, the nicotinamide moiety of newly synthesized DPN is lost from D98/AH2 cells to the medium with a half-life of eight hours. About 80% of the nicotinamide is lost to medium as nicotinamide ribose. |
