NAD+ Coenzyme Research Overview: Redox Chemistry, Sirtuins, PARPs and the Salvage Pathway

NAD+ is a dinucleotide: an adenosine monophosphate and a nicotinamide mononucleotide joined through a pyrophosphate bridge. Its molecular formula is C₂₁H₂₇N₇O₁₄P₂, its molecular weight is 663.43 g/mol, and its CAS number is 53-84-9.

The reactive part of the molecule is the nicotinamide ring. In the oxidized form (NAD+) the ring carries a positive charge; in the reduced form (NADH) it has accepted a hydride ion at the C4 position. Phosphorylation of the adenosine 2′-hydroxyl by NAD kinase produces NADP+, which with NADPH forms a separate pool used mainly for reductive biosynthesis and antioxidant enzymes.

Dehydrogenases transfer a hydride from a substrate to NAD+, producing NADH. Classic examples include glyceraldehyde-3-phosphate dehydrogenase in glycolysis, lactate dehydrogenase, and several dehydrogenases of the tricarboxylic acid (TCA) pathway. NADH is reoxidized mainly by complex I of the mitochondrial electron transport chain, which links the NAD+/NADH ratio to mitochondrial respiration.

Because NADH absorbs light at 340 nm (molar absorptivity about 6,220 M⁻¹cm⁻¹) and fluoresces, while NAD+ does not, the conversion between the two is the basis of many spectrophotometric enzyme assays. The free NAD+/NADH ratio differs between the cytosol and mitochondria, and compartment-specific measurement is an active methods area.

Sirtuins (SIRT1–SIRT7 in mammals) are enzymes that remove acetyl and other acyl groups from lysine residues on histones and many other proteins. Unlike classical deacetylases, they consume NAD+ in each reaction, releasing nicotinamide and 2′-O-acyl-ADP-ribose. This makes sirtuin activity sensitive to NAD+ availability.

The isoforms differ by location: SIRT1, SIRT6 and SIRT7 are mainly nuclear, SIRT2 is mainly cytosolic, and SIRT3, SIRT4 and SIRT5 are mitochondrial. SIRT5 prefers succinyl, malonyl and glutaryl groups, and SIRT4 has ADP-ribosyltransferase and lipoamidase activities, illustrating that the family is broader than "deacetylases" alone. Nicotinamide itself is a feedback inhibitor of sirtuins and is often used as a research tool for that reason.

Poly(ADP-ribose) polymerases transfer ADP-ribose units from NAD+ onto target proteins. PARP1, the most abundant family member, is rapidly activated by DNA strand breaks and builds poly(ADP-ribose) chains that recruit DNA repair factors. Strong PARP1 activation can consume a large fraction of the cellular NAD+ pool within minutes, which is why PARP activity is a major variable in NAD+ turnover experiments.

Other family members (PARP2, tankyrases, and the mono-ADP-ribosyltransferase PARPs) add single or multiple ADP-ribose units in different cellular contexts. PARP inhibitors are widely used in research to separate PARP-dependent NAD+ consumption from other routes.

CD38 is a multifunctional ectoenzyme with NAD+ glycohydrolase and ADP-ribosyl cyclase activities. It converts NAD+ to ADP-ribose and nicotinamide and generates the calcium-mobilizing second messengers cyclic ADP-ribose (cADPR) and, from NADP+, NAADP. CD38 is expressed prominently on immune cells, and its activity is a significant NAD+ sink in many cell and tissue models.

SARM1, an axonal protein with an intrinsic NAD+ hydrolase TIR domain, is another NAD+-consuming enzyme studied in neurobiology, where its activation produces rapid NAD+ depletion in axons.

Because sirtuins, PARPs and CD38 continuously cleave NAD+, cells must resynthesize it. Three routes are recognized:

• De novo pathway from tryptophan via the kynurenine pathway to quinolinic acid, which quinolinate phosphoribosyltransferase (QPRT) converts to nicotinic acid mononucleotide (NaMN).
• Preiss–Handler pathway from nicotinic acid, converted to NaMN by nicotinic acid phosphoribosyltransferase (NAPRT). NaMN is adenylylated to NaAD and then amidated to NAD+ by NAD synthetase.
• Salvage pathway, which recycles the nicotinamide released by NAD+-consuming enzymes. Nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme, converts nicotinamide to nicotinamide mononucleotide (NMN), and NMN adenylyltransferases (NMNAT1–3, located in the nucleus, Golgi and mitochondria respectively) convert NMN to NAD+. Nicotinamide riboside enters this route after phosphorylation by NR kinases (NRK1/2).

In most cultured mammalian cells the salvage pathway is the dominant source of NAD+. The NAMPT inhibitor FK866 is a standard tool for lowering NAD+ levels experimentally. Mitochondrial NAD+ import is mediated by the transporter SLC25A51.

NAD+ and NADH have opposite chemical stabilities, which shapes extraction: NAD+ is stable in acid and destroyed in base, while NADH is stable in base and destroyed in acid. Separate acid and alkaline extractions are therefore common when both forms must be quantified. Methods include:

• enzyme-coupled colorimetric or fluorometric assays based on alcohol dehydrogenase or similar enzymes,
• HPLC with UV detection at 260 nm (and 340 nm for the reduced form),
• LC-MS/MS for NAD+ together with its precursors and breakdown products (NMN, nicotinamide, ADP-ribose), and
• genetically encoded biosensors for free NAD+ or the NAD+/NADH ratio in specific compartments.

Research-grade NAD+ is a white to off-white, hygroscopic solid. A lot is documented with HPLC purity, mass spectrometry identity and lot number on its certificate of analysis. Store the dry material sealed, desiccated and cold, and protect solutions from alkaline pH and repeated freeze–thaw. NuRev lists NAD+ as NAD+ 500mg, with lot documentation on the Certificates of Analysis page.

NuRev supplies NAD+ for laboratory research use only; it is not for human or veterinary use.