NAD+

Nicotinamide adenine dinucleotide is a dinucleotide coenzyme present in every living cell. It is not a peptide, despite frequently appearing alongside peptides in research catalogs, and its chemistry and handling requirements differ accordingly. The oxidized form is written NAD+ because the nicotinamide ring nitrogen carries a positive charge; the reduced form, NADH, is the same molecule carrying two additional electrons and a proton.

NAD+ occupies two distinct roles in cellular biochemistry, and conflating them is the most common source of confusion in this area. In its first role it is a redox cofactor, accepting and donating a hydride ion in hundreds of dehydrogenase reactions across glycolysis, the citric acid cycle, and fatty acid oxidation. In that role it is cycled continuously and is not consumed. In its second role it is a substrate that is cleaved and destroyed, supplying the ADP-ribose moiety used by sirtuins, poly-ADP-ribose polymerases, and the NAD glycohydrolases CD38 and CD157. That second role is why cells must continuously resynthesize NAD+, and it is why NAD+ concentration became a subject of aging research rather than remaining a topic in metabolic enzymology.

Response BioLabs supplies NAD+ as a lyophilized powder for laboratory research use only. It is a standard biochemical reagent used in enzyme assays, cofactor regeneration systems, and cell culture work. Nothing on this page describes a use in humans.

The molecule was identified in 1906 by Arthur Harden and William Young, who found that yeast extract lost its fermenting capacity when dialyzed and regained it when the small heat-stable fraction was added back. They called the factor cozymase. Hans von Euler-Chelpin later determined its structure, and he and Harden shared the 1929 Nobel Prize in Chemistry for this line of work. In the 1930s Otto Warburg established that the nicotinamide portion of the molecule is the site of hydride transfer, which explained mechanistically what cozymase was doing.

The nutritional connection arrived in 1937, when Conrad Elvehjem showed that nicotinamide resolved canine black tongue, linking the vitamin niacin to the human deficiency disease pellagra and to this coenzyme. In 1958 Preiss and Handler described the three-step route from nicotinic acid to NAD+ that still bears their names, and subsequent decades mapped the salvage pathway through nicotinamide phosphoribosyltransferase and the de novo route from tryptophan through the kynurenine pathway.

The modern research literature dates from 2000, when Imai, Guarente, and colleagues reported that the yeast silencing protein Sir2 is an NAD-dependent histone deacetylase. That result reclassified NAD+ from a passive electron carrier into a signaling currency whose availability could regulate gene expression, and it connected metabolic state to chromatin regulation in a way that launched the sirtuin and aging literature. Bieganowski and Brenner subsequently described nicotinamide riboside and its kinase pathway in 2004, adding a further biosynthetic entry point, and reports of age-associated declines in tissue NAD+ in rodents through the 2010s produced the current interest in NAD metabolism as a target of investigation.

NAD+ consists of two nucleotides joined head to head through a pyrophosphate bridge. One half is adenosine monophosphate, the other is nicotinamide mononucleotide. The molecular formula is C21H27N7O14P2 for the free acid inner salt, with an average molecular weight of approximately 663.4 g/mol; suppliers commonly list the cationic form as C21H28N7O14P2 at approximately 664.4 g/mol, and both figures refer to the same compound described under different conventions. The CAS number is 53-84-9.

Two bonds in the molecule matter functionally. The first is the carbon at position four of the nicotinamide ring, which accepts a hydride ion during reduction to NADH. This is a two-electron transfer, distinguishing NAD from one-electron carriers such as flavins, and it is fully reversible, which is what allows the coenzyme to cycle. The second is the glycosidic bond linking nicotinamide to its ribose. This bond carries substantial free energy of hydrolysis, and cleaving it is what NAD-consuming enzymes exploit: they release nicotinamide and transfer the remaining ADP-ribose to a substrate.

For laboratory work, the stability characteristics of NAD+ and NADH are opposite, which is a practical detail that catches people out. NAD+ is relatively stable in acidic solution and degrades quickly in alkali. NADH is stable in alkali and degrades in acid. The dry powder is hygroscopic and light sensitive. Aqueous NAD+ solutions lose titer over days even when refrigerated, so assays that depend on accurate cofactor concentration should use freshly prepared solutions and should verify concentration spectrophotometrically. The standard readout is absorbance at 340 nm, where NADH absorbs strongly and NAD+ does not, which is the basis of most coupled enzyme assays using this cofactor.

The redox role is settled biochemistry. NAD+ accepts a hydride at glyceraldehyde-3-phosphate dehydrogenase in glycolysis, at three points in the citric acid cycle, and throughout fatty acid oxidation, delivering electrons to complex I of the respiratory chain. The phosphorylated relative NADP+ serves the opposite function, supplying reducing equivalents as NADPH for biosynthesis and for regeneration of the glutathione and thioredoxin antioxidant systems. The ratio of NAD+ to NADH, rather than absolute concentration, is the parameter that reports cellular redox state.

The consuming enzymes are where the current research interest sits. The sirtuins, SIRT1 through SIRT7, are NAD-dependent deacylases that remove acetyl and other acyl groups from lysine residues on histones and on many non-histone substrates, producing nicotinamide and 2 prime-O-acyl-ADP-ribose in the process. Because they require NAD+ stoichiometrically and are inhibited by their own nicotinamide product, their activity is sensitive to NAD availability. Reported substrates studied in this context include PGC-1 alpha, FOXO transcription factors, p53, and mitochondrial enzymes acted on by SIRT3.

Poly-ADP-ribose polymerases, particularly PARP1 and PARP2, are activated by DNA strand breaks and attach long chains of ADP-ribose to acceptor proteins as part of the DNA damage response. Under genotoxic stress PARP activity becomes the largest acute consumer of cellular NAD+, and the resulting competition with sirtuins for a shared substrate pool is one of the more interesting proposed links between DNA damage and metabolic regulation. A third class, the ectoenzymes CD38 and CD157, hydrolyze NAD+ and its precursors; CD38 expression increases with age in mouse tissues and has been reported as a significant driver of age-associated NAD decline in that model. The axon degeneration enzyme SARM1 was later shown to be an NAD-cleaving enzyme as well.

One mechanistic point deserves particular emphasis because it is routinely misunderstood. NAD+ is a large, highly charged molecule and is not efficiently transported across the plasma membrane intact. Extracellular NAD+ is instead processed at the cell surface by CD38 and by the nucleotidase CD73 into nicotinamide mononucleotide and then nicotinamide riboside, and it is those smaller species that enter cells and are reassembled into NAD+ inside. This means supplying NAD+ extracellularly is mechanistically distinct from raising intracellular NAD+ directly, and any reasoning that skips this step is skipping the part where the biology actually happens.

What is established: the structure, redox chemistry, biosynthetic routes, and consuming enzymes of NAD+ are textbook biochemistry, independently confirmed across decades and thousands of laboratories. Structures of sirtuins and PARPs with bound cofactor are solved. The stoichiometric NAD requirement of the sirtuins and the role of PARP activation in acute NAD depletion are not in dispute. This is the most solidly characterized compound in this library by a wide margin.

What is reasonably well supported in animal models: NAD+ concentrations decline with age in multiple rodent tissues, CD38 upregulation contributes to that decline in mice, and administration of the precursors nicotinamide riboside and nicotinamide mononucleotide raises tissue NAD metabolites and produces metabolic changes in various rodent models. That body of work is substantial, though replication across laboratories and models has been uneven, and effect sizes vary considerably with the model used.

What is preliminary or absent: essentially everything about NAD+ as an administered intervention. Human trials in this area have studied precursors, principally nicotinamide riboside and nicotinamide mononucleotide, not NAD+ itself. Those trials have generally been small and short, have demonstrated that the precursors raise blood NAD metabolite levels and are tolerated over the durations tested, and have produced mixed and often null results on functional endpoints. There is no controlled human trial evidence supporting NAD+ administration itself, and given the extracellular degradation route described above, there is a clear mechanistic question about what such administration would even accomplish that a precursor would not.

The honest summary is that this is a compound with impeccable biochemistry and an unfinished interventional story. Confidence about what NAD+ does inside a cell should not be transferred into confidence about what supplying it from outside does to an organism. Those are different questions, and only the first one has been answered.

The lyophilized powder is hygroscopic and light sensitive and should be stored desiccated at -20 C or below for long-term inventory, with 2 to 8 C acceptable for material in active use. Vials are equilibrated to room temperature before opening so that moisture does not condense onto cold powder, and are resealed promptly. Absorbed water accelerates degradation of the dry material and also introduces weighing error.

Aqueous solutions should be prepared close to the time of use. NAD+ degrades rapidly at alkaline pH, so buffers should be checked before dissolution rather than after, and any solution held at elevated pH should be considered compromised regardless of appearance. Refrigerated solutions lose titer over days, and freeze and thaw cycling accelerates the process. For assay work where cofactor concentration is a measured variable rather than an excess reagent, concentration should be verified spectrophotometrically instead of calculated from mass.

The standard analytical handle is absorbance at 340 nm, where NADH absorbs and NAD+ does not, with an extinction coefficient of approximately 6220 per molar per centimeter. This gives a direct readout of the oxidation state of a stock and is the basis of most coupled dehydrogenase assays. A yellow tint in a supposedly white powder or a solution that fails to give the expected 260 nm absorbance is a reason to discard the material rather than to proceed. Aliquot, label with compound, lot, concentration, buffer, and preparation date, and record the date because with this compound time in solution is the dominant variable.

Researchers frequently ask why precursors dominate the literature if NAD+ is the molecule of interest. The answer is transport. NAD+ carries two negative charges on its pyrophosphate bridge and is not efficiently taken up intact by cells; the smaller precursors nicotinamide riboside and nicotinamide mononucleotide are handled by defined transport and salvage routes and can therefore raise intracellular NAD+ through the pathway the cell already uses. Study designs in this field reflect that biology rather than a preference for one reagent over another.

A second common question concerns the relationship between NAD+, NADH, NADP+, and NADPH. They are four species in two related pools. NAD+ and NADH are the oxidized and reduced forms of the catabolic pool, kept largely oxidized to accept electrons from fuel breakdown. NADP+ and NADPH are the phosphorylated anabolic pool, kept largely reduced to donate electrons for biosynthesis and antioxidant regeneration. They are interconverted by NAD kinase and by nicotinamide nucleotide transhydrogenase, and they are not interchangeable in assays.

A third question concerns measurement. Reported tissue NAD+ concentrations vary widely between publications, in part because the molecule degrades quickly during sample processing and in part because extraction methods differ in how well they preserve the oxidized and reduced species separately. Anyone comparing NAD quantification across papers should check the extraction and detection method before comparing the numbers, since methodological differences in this field are often larger than the biological effects being reported.