The biosynthesis of NAD in eukaryotes involves a step catalyzed by a nuclear
enzyme, NAD pyrophosphorylase (HoGEBOOM and SCHNEIDER, 1952). Such a
specific localization is difficult to understand with respect to the function of
NAD as a coenzyme in many biological oxidation-reduction reactions. However,
this coenzyme also participates in other important biological processes for which
it is structurally predestined. The NAD molecule contains two energy-rich bonds,
a pyrophosphate bond and a glycosidic linkage at the quarternary nitrogen
of the pyridine ring (Fig. 1 ; ZATMAN et al., 1953). In the presence of appropriate
catalysts, the high chemical potential will therefore allow group transfer reactions 2 H, HILZ and P. STONE:
at both positions (Fig. 1). An example of transfer reaction of type I is the
reaction of NAD with polynucleotide ligase in Escherichia coIi and in mitochon￾dria: transfer of AMP to the enzyme with concomitant release of NMN is
the first step in an overall reaction in which phosphodiester bonds are formed
between the 5'-phosphoryl and 3'-hydroxyl termini of "nicked" double-stranded
DNA chains (LITTLE et al., 1967; OLW~RA and LEHMAN, 1967).