NAD turnover in microplasmodia of physarum polycephalum

NAD deamidation "a new reaction" by an enzyme from Aspergillus terreus DSM 826

NAD deamidation is a non-previously recognized reaction. This reaction has been found to be catalyzed by extracts of Aspergillus terreus DSM 826. Conversion of NAD to the biosynthetic intermediate, deamido NAD, by these extracts, at the optimum pH and temperature did not exceed about 55 of the amount of the substrate added. Completion of the reaction was achieved when the extracts were pre-heated at 50 degrees C for 15 min in absence of the substrate. In a very similar manner, the extracts catalyzed hydrolytic cleavage of the amide linkages of different biomolecules such as nicotinamide, nicotinamide riboside, nicotinamide mononucleotide, L-glutamine, L-asparagine and acetamide. Polyacrylamide was also deamidated under the same conditions. In addition, complete dephosphorylation of the dinucleotide molecule was also effected by the same extracts. Separation of the NAD deamidating enzyme from the NAD dephosphorylating enzyme was achieved on using either DEAE - Sephadex A-25 or Sephadex G-200 column chromatography. The obtained phosphohydrolase-free-deamidase showed optimum activity at pH 8 of 0.1 M phosphate buffer and 50 degrees C. It exhibited broad substrate specificity and hyperbolic substrate saturation kinetics. It was isosterically inhibited by the product of its activity and this inhibition was prevented by heating the extracts at 50 degrees C for 15 min. Its activity was not affected in presence of sodium fluoride, partially inhibited in presence of magnesium chloride and was retained in the freezer for some months.

Telomeric and rDNA silencing in Saccharomyces cerevisiae are dependent on a nuclear NAD(+) salvage pathway

The Sir2 protein is an NAD(+)-dependent protein deacetylase that is required for silencing at the silent mating-type loci, telomeres, and the ribosomal DNA (rDNA). Mutations in the NAD(+) salvage gene NPT1 weaken all three forms of silencing and also cause a reduction in the intracellular NAD(+) level. We now show that mutation of a highly conserved histidine residue in Npt1p results in a silencing defect, indicating that Npt1p enzymatic activity is required for silencing. Deletion of another NAD(+) salvage pathway gene called PNC1 caused a less severe silencing defect and did not significantly reduce the intracellular NAD(+) concentration. However, silencing in the absence of PNC1 was completely dependent on the import of nicotinic acid from the growth medium. Deletion of a gene in the de novo NAD(+) synthesis pathway BNA1 resulted in a significant rDNA silencing defect only on medium deficient in nicotinic acid, an NAD(+) precursor. By immunofluorescence microscopy, Myc-tagged Bna1p was localized throughout the whole cell in an asynchronously growing population. In contrast, Myc-tagged Npt1p was highly concentrated in the nucleus in approximately 40% of the cells, indicating that NAD(+) salvage occurs in the nucleus in a significant fraction of cells. We propose a model in which two components of the NAD(+) salvage pathway, Pnc1p and Npt1p, function together in recycling the nuclear nicotinamide generated by Sir2p deacetylase activity back into NAD(+).

A phylogenetically conserved NAD+-dependent protein deacetylase activity in the Sir2 protein family

The yeast Sir2 protein, required for transcriptional silencing, has an NAD(+)-dependent histone deacetylase (HDA) activity. Yeast extracts contain a NAD(+)-dependent HDA activity that is eliminated in a yeast strain from which SIR2 and its four homologs have been deleted. This HDA activity is also displayed by purified yeast Sir2p and homologous Archaeal, eubacterial, and human proteins, and depends completely on NAD(+) in all species tested. The yeast NPT1 gene, encoding an important NAD(+) synthesis enzyme, is required for rDNA and telomeric silencing and contributes to silencing of the HM loci. Null mutants in this gene have significantly reduced intracellular NAD(+) concentrations and have phenotypes similar to sir2 null mutants. Surprisingly, yeast from which all five SIR2 homologs have been deleted have relatively normal bulk histone acetylation levels. The evolutionary conservation of this regulated activity suggests that the Sir2 protein family represents a set of effector proteins in an evolutionarily conserved signal transduction pathway that monitors cellular energy and redox states.

Isolation of NAD cycle mutants defective in nicotinamide mononucleotide deamidase in Salmonella typhimurium

The NAD or pyridine nucleotide cycle is the sequence of reactions involved in the breakdown of NAD to nicotinamide mononucleotide (NMN) and regeneration of NAD. This cycle is fivefold more active during aerobic growth of Salmonella typhimurium and under this condition breaks down half of the NAD pool every 90 min. DNA ligase is known to convert NAD to NMN but is only a minor contributor to the NAD cycle during aerobic growth. The dominant aerobic route of NMN formation is otherwise uncharacterized. Accumulated NMN generated by either of these routes is potentially dangerous in that it can inhibit the essential enzyme DNA ligase. The reactions which recycle NMN to NAD may serve to minimize the inhibition of ligase and other enzymes by accumulated NMN. The predominant recycling reaction in S. typhimurium appears to be NMN deamidase, which converts NMN directly to the biosynthetic intermediate nicotinic acid mononucleotide. Mutants defective in this recycling step were isolated and characterized. By starting with a ligase-deficient (lig mutant) parent strain that requires deamidase to assimilate exogenous NMN, two classes of mutants that are unable to grow on minimal NMN media were isolated. One class (pncC) maps at 83.7 min and shows only 2% of the wild-type levels of NMN deamidase. Under aerobic conditions, a lig+ allele allows a pncC mutant to grow on NMN and restores some deamidase activity. This growth ability and enzyme activity are not found in lig+ strains grown without oxygen. This suggests that the existence of a second NMN deamidase (pncL) dependent on ligase and stimulated during aerobic growth. The second class of mutants (pncD) gains a requirement for isoleucine plus valine with growth in the presence of exogenous NMN. We propose that pncD mutations reduce the activity of an ilv biosynthetic enzyme that is naturally sensitive to inhibition by NMN.