Pyridine nucleotide metabolism in imaginal discs of Drosophila melanogaster

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.

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.

NAD turnover in microplasmodia of physarum polycephalum

The rate of NAD turnover in microplasmodia of Physarum polycephalum was investigated using a double labeling technique with (14C)-adenine or adenosine and (3H)-nicotinamide. The half-life of an NAD molecule in Physarum was estimated to be 25 min, which is shorter than in either E. coli or human cell lines. The half-life of NAD in the presence of an inhibitor of NADase and poly ADPR synthase, 5-methylnicotinamide, was also investigated, but found to be indistinguishable from controls. The possible reasons for this and for the rapid turnover is discussed in the light of the known functions for NAD in prokaryotes and eukaryotes.