FIG. 4.

Nicotinamide salvage. (A) Nicotinamidases homologous to E. coli pncA convert nicotinamide to nicotinic acid for Preiss-Handler salvage. (B) Nicotinamide phosphoribosyltransferases homologous to the H. ducreyi and F. tularensis nadV products convert nicotinamide to NMN for adenylylation to NAD. Vertebrate nadV homologs are also termed NAMPT, PBEF, and Visfatin. Bacterial NMN adenylyltransferase, encoded by nadM, converts NMN to NAD. Eukaryotic enzymes do not discriminate well between NaMN and NMN adenylylation, such that most eukaryotic NaMN adenylyltransferases, such as S. cerevisiae Nma1 and Nma2 (depicted in Fig. 3A), are also NMN adenylyltransferases.