Two independent lines of evidence suggest that a pathway to NAD+ from nicotinamide riboside may exist in fungi and other eukaryotes. First, though the recent literature (Panozzo et al. 2002, Sandmeier et al. 2002, Bitterman et al. 2002, Anderson et al. 2003, Gallo et al. 2004) depicts all of the metabolic flux through NaMN, the enzymes placed on this scheme as NaMN adenylyltransferases were initially classified as NMN adenylyltransferases (Kornberg 1950, Emanuelli et al. 1999, Emanuelli et al. 2003, Garavaglia et al. 2002) or as dual specificity NMN/NaMN adenylyltransferases (Zhou et al., 2002). Though it is possible that eukaryotic adenylyltransferases were misclassified, it is reasonable to ask whether eukaryotes have a pathway that produces NMN to feed into these enzymes.
Second, anticancer agents such as tiazofurin (Cooney et al., 1983) and benzamide riboside (Krohn et al., 1992) have been shown to be metabolized intracellularly to NAD+ analogs tiazofurin adenine dinucleotide and benzamide adenine dinucleotide, which inhibit IMP dehydrogenase, the rate-limiting enzyme for guanine nucleotide biosynthesis. As these compounds can be considered analogs of nicotinamide riboside (Figure 3), generation of NAD+ analogs would necessarily involve phosphorylation of the 5′ hydroxyl group and subsequent adenylylation of these intermediates. Though an NMN/NaMN adenylytransferase is thought to be the enzyme that converts the mononucleotide intermediates to NAD+ analogs and the structural basis for this has been established (Zhou et al., 2002), several different enzymes including adenosine kinase, 5′ nucleotidase (Fridland et al. 1986, Saunders et al. 1990) and a specific nicotinamide riboside kinase (Saunders et al., 1990) have been proposed to be responsible for tiazofurin phosphorylation in vivo. Indeed, a putative nicotinamide riboside kinase (Nrk) activity was reportedly purified but no amino acid sequence information was obtained and, as a consequence, no genetic test was ever performed to assess its function in nutrient or drug metabolism (Sasiak and Saunders, 1996).