See Figure 1 for abbreviations. Further kinetic analysis showed that the K m value towards NAM was 5.81 mM, and the V max was at
400 nmol/min/mg protein. The kinetic data indicated that xapA in E. coli was much less efficient in using NAM to synthesize NR than using typical substrate (K m at 5.81 mM on NAM vs. 72 μM on xanthosine) , or when compared with other NAD+ salvaging enzymes (e.g., K m values at 70 μM and 2 μM for pncA and pncB on NAM and NA, respectively) [39, 40], but similar to those of deoD www.selleckchem.com/products/gs-9973.html (PNP-I) from calf and E. coli (i.e., 1.48 mM and 0.62 mM, respectively) in converting the non-typical substrate NR to NAM . The contribution of xapA in NAD+ salvaging was also confirmed in bacterial mutants cultured in M9/NAM medium, in which the consumption of extracellular NAM by the triple-deletion (BW25113ΔnadCΔpncAΔxapA) was reduced by 95% in comparison to that by the double-deletion BW25113ΔnadCΔpncA (Figure 5A). The consumption of extracellular NAM was restored when vector expressing xapA (but not the EGFP control) was reintroduced to the triple-deletion (Figure 5A). The level of intracellular NAD+ was detectable in BW25113ΔnadCΔpncA (150 ng), GF120918 ic50 but virtually undetectable in BW25113ΔnadCΔpncAΔxapA (Figure 5B). Again, the intracellular NAD+ level could be restored by reintroducing xapA into the triple-deletion,
but not by EGFP (Figure 5B). Figure 5 Consumption of extracellular NAM (A) to form intracellular NAD + (B) by four strains of Escherichia many coli derived from BW25113 cultured in M9/NAM medium until the strain BW25113Δ nadC Δ pncA reached the mid-log phase. Strain 1, BW25113ΔnadCΔpncA; strain 2, BW25113ΔnadCΔpncAΔxapA; strain 3, BW25113ΔnadCΔpncAΔxapA/pBAD-xapA;
and strain 4, BW25113ΔnadCΔpncAΔxapA/pBAD-EGFP. Discussion Contribution of xapA to an alternative NAD+ salvage pathway from NAM Xanthosine phosphorylase (xapA, EC 22.214.171.124) is a second purine nucleoside phosphorylase (PNP-II) in E. coli. Similar to PNP-I (deoD), it mainly functions in the purine metabolism by carrying out both phosphorylation and synthesis of purine and purine deoxy-/ribonucleosides . Here we first obtained genetic evidence that xapA was probably involved in NAD+ salvage in E. coli. We also provided more direct biochemical evidences that xapA was able to synthesize NR from NAM. Both bacterial growth experiments and enzyme kinetic data indicated that xapA used NAM in a much less efficient way than using its typical substrates (i.e., purine analogs), suggesting that NAM ACP-196 served only as a non-typical substrate, which was comparable to the PNP-I. Therefore, the capability to convert NAM to NR appeared to be a “side effect” for xapA. However, such a side-effect was sufficient to maintain the survival of E. coli by feeding NAM into the salvage pathway III when all other NAD+ synthetic pathways were unavailable and only NAM was present in the minimal medium.