sirtuins is one of those subjects where the details matter more than the headlines. This page pulls together the background, the mechanisms, and the practical points readers ask about most.
Updated 2026-06-22. Numbers and descriptions here follow the published literature rather than marketing material.
NAD+ is a dinucleotide composed of adenine, ribose, and nicotinamide moieties linked by phosphate groups. Its oxidized form carries a positive charge on the nicotinamide ring, which enables reversible hydride transfer. The molecule functions as a coenzyme in oxidoreductase reactions rather than as a dietary vitamin in its intact form. Cells maintain separate pools in cytoplasm, mitochondria, and nucleus. This compartmentalization allows distinct redox environments while preserving a shared chemical identity.
In glycolysis, NAD+ accepts electrons during the oxidation of glyceraldehyde-3-phosphate, forming NADH. The tricarboxylic acid cycle and fatty acid oxidation also generate NADH, which donates electrons to the mitochondrial electron transport chain. This flow supports ATP synthesis and helps maintain the redox balance of the cell. Other dehydrogenases use NAD+ as a cofactor for biosynthetic reductions and detoxification reactions. NADH is later reoxidized to sustain continued flux through these pathways.
Beyond electron transfer, NAD+ serves as a substrate for enzymes that cleave the molecule and transfer ADP-ribose units. Sirtuins, poly(ADP-ribose) polymerases, and CD38 consume NAD+ in regulatory reactions. These activities link NAD+ availability to DNA repair, chromatin modification, calcium signaling, and metabolic stress responses. Because consumption can exceed biosynthesis under some conditions, cellular NAD+ levels are dynamic rather than fixed. Enzyme affinity and local synthesis also influence how much NAD+ is available for signaling.
In redox reactions, NAD+ accepts a hydride ion, which consists of two electrons and one proton. The hydride adds to the nicotinamide ring at a specific carbon, converting NAD+ into NADH. Dehydrogenase enzymes use this step in glycolysis, the citric acid cycle, and fatty acid oxidation. NADH later donates electrons to the mitochondrial electron transport chain, helping to drive ATP synthesis. The balance between NAD+ and NADH reflects the metabolic state of a cell, and shifts in that balance can alter how pathways operate.
Beyond electron transfer, NAD+ serves as a substrate for enzymes that cleave it and attach its ADP-ribose portion to other molecules. This group includes poly(ADP-ribose) polymerases, CD38, and sirtuins. Such reactions consume NAD+ and can influence its availability for metabolism. Cells replenish NAD+ through a salvage pathway that recycles nicotinamide and through routes starting from tryptophan or vitamin B3 forms. How these synthesis and consumption routes are coordinated across tissues remains an active area of study, and compartment-specific concentrations are difficult to measure directly.
| Property | Value | Notes |
|---|---|---|
| Chemical name | Nicotinamide adenine dinucleotide | Oxidized form abbreviated NAD+ |
| Molecular formula | C21H27N7O14P2 | Free acid form |
| Molar mass | 663.43 g/mol | Calculated for free acid |
| CAS Registry Number | 53-84-9 | Common entry for beta-NAD+ |
| Appearance | White to off-white powder | Hygroscopic solid |
NAD+ stands for nicotinamide adenine dinucleotide, the oxidized form of a coenzyme found in all living cells. The molecule consists of two nucleotides, adenine and nicotinamide ribose, joined through phosphate groups. Its chemical formula is C21H27N7O14P2, and the free acid has a molar mass near 663.43 grams per mole. In redox reactions, NAD+ accepts a hydride ion and becomes NADH. The pair NAD+ and NADH participates in hundreds of metabolic reactions, including steps in glycolysis, the citric acid cycle, and oxidative phosphorylation.
In cells, NAD+ functions primarily as an electron carrier. Dehydrogenase enzymes in glycolysis and the citric acid cycle transfer hydride from substrates to NAD+, producing NADH. NADH then delivers electrons to the mitochondrial respiratory chain, supporting ATP synthesis. In fermentation, NADH is reoxidized to NAD+ so that glycolysis can continue. The balance between NAD+ and NADH helps set metabolic flux. Beyond redox, NAD+ serves as a substrate for enzymes that cleave it, including sirtuins, poly(ADP-ribose) polymerases, and CD38. These reactions consume NAD+ and release nicotinamide and ADP-ribose products.
Biosynthesis occurs through salvage, Preiss-Handler, and de novo pathways. In mammals, the salvage pathway from nicotinamide predominates, and NAMPT is often described as rate-limiting. Nicotinamide riboside and nicotinic acid enter related routes that converge on NAD+ production. Tissue NAD+ concentrations vary widely and are maintained by a balance of synthesis and consumption. Some studies report age-related declines in certain tissues, but whether these changes cause disease or can be reversed to improve human health remains an open question.
Beyond redox catalysis, NAD+ is a substrate for enzymes that transfer ADP-ribose or remove acetyl groups from proteins. Sirtuins and poly(ADP-ribose) polymerases consume NAD+ and release nicotinamide as a byproduct. These reactions connect cellular energy status to gene regulation, DNA repair, and stress responses. Because NAD+ is used rather than merely recycled in such signaling, its concentration reflects both biosynthesis and consumption. The balance between salvage and de novo synthesis pathways determines available pools in different tissues.
Biosynthesis of NAD+ starts from nicotinamide, nicotinic acid, or nicotinamide riboside through salvage pathways. A rate-limiting enzyme, nicotinamide phosphoribosyltransferase, converts nicotinamide to nicotinamide mononucleotide. Further coupling with ATP yields NAD+. In mammals, the liver and muscle can synthesize NAD+ from dietary precursors, but tissue levels vary widely. Researchers study these pathways to understand age-related changes, metabolic disorders, and neurodegeneration. Direct causal links between NAD+ decline and disease remain an active area of investigation.
NAD+ is a dinucleotide composed of adenine, ribose, and nicotinamide groups joined by phosphate linkages. It serves as a coenzyme in oxidoreductase reactions, cycling between oxidized NAD+ and reduced NADH. The molecule is water-soluble and occurs in all living cells. Its nicotinamide ring accepts hydride ions during catabolic reactions, linking substrate oxidation to electron transport. This redox couple supports ATP production and helps maintain cytosolic and mitochondrial redox balance in many cell types.
NAD+ is a dinucleotide composed of nicotinamide, ribose, and adenine linked by phosphate groups. Its full name is nicotinamide adenine dinucleotide, with "+" denoting the oxidized form. The molecule acts as a coenzyme in redox reactions, cycling between NAD+ and NADH. In cells, it participates in electron transfer during glycolysis, the citric acid cycle, and oxidative phosphorylation. It is distinct from NADP+, which carries an additional phosphate group and supports different biosynthetic reactions.
Beyond redox chemistry, NAD+ serves as a substrate for enzymes that transfer ADP-ribose or remove acetyl groups. Sirtuins, PARPs, and CD38-family enzymes consume NAD+ and produce nicotinamide and ADP-ribose-related products. These reactions link NAD+ availability to DNA repair, chromatin modification, and cellular signaling. Because the molecule is central to energy metabolism and regulation, changes in its concentration are studied in aging, immunity, and metabolic research. The balance between synthesis and consumption varies by tissue, developmental stage, and physiological state.
In humans, NAD+ can be synthesized from nicotinic acid, nicotinamide, nicotinamide riboside, and tryptophan through overlapping pathways. The salvage pathway recycles nicotinamide back to NAD+ and is often considered a major route in many tissues. Dietary precursors and intracellular recycling both contribute to the pool, but the quantitative importance of each source remains an active research question. NAD+ levels are not uniform across organs or cell compartments. Measurements in blood do not necessarily reflect concentrations inside tissues.
== History == The proposal that proteins were linear chains of α-amino acids was made nearly simultaneously by two scientists at the same conference in 1902, the 74th meeting of the Society of German Scientists and Physicians, held in Karlsbad. Franz Hofmeister made the proposal in the morning, based on his observations of the biuret reaction in proteins. Hofmeister was followed a few hours later by Emil Fischer, who had amassed a wealth of chemical details supporting the peptide-bond model. For completeness, the proposal that proteins contained amide linkages was made as early as 1882 by the French chemist E. Grimaux. Despite these data and later evidence that proteolytically digested proteins yielded only oligopeptides, the idea that proteins were linear, unbranched polymers of amino acids was not accepted immediately. Some scientists such as William Astbury doubted that covalent bonds were strong enough to hold such long molecules together; they feared that thermal agitations would shake such long molecules asunder. Hermann Staudinger faced similar prejudices in the 1920s when he argued that rubber was composed of macromolecules. Thus, several alternative hypotheses arose. The colloidal protein hypothesis stated that proteins were colloidal assemblies of smaller molecules. This hypothesis was disproved in the 1920s by ultracentrifugation measurements by Theodor Svedberg that showed that proteins had a well-defined, reproducible molecular weight and by electrophoretic measurements by Arne Tiselius that indicated that proteins were single molecules.
It was proposed by the researchers that low maximal receptor activation, or lower-efficacy partial agonism, could explain the atypical effects of biased MOR agonists like SR-17018 rather than biased agonism. These results were subsequently reanalyzed and disputed by other researchers. In any case, the exact mechanisms underlying the atypical effects of SR-17018 remain unclear. For example, they may be related to G protein over β-arrestin2 bias, or to differences in G protein-mediated signaling. SR-17018 produces rewarding effects as assessed by conditioned place preference (CPP) in rodents. Its rewarding effects in this assay were comparable to those of morphine. In addition to rewarding effects, SR-17018 produces modest hyperlocomotion in rodents, albeit to a far lesser extent than morphine or fentanyl. Addition of SR-17018 to morphine in rodents attenuates morphine-induced hyperlocomotion whilst increasing analgesic efficacy in mice. Unlike morphine and fentanyl, SR-17018 does not produce locomotor sensitization with chronic administration. A self-administration study of SR-17018 in mice was limited by the drug's poor solubility. However, in a subsequent study, the drug produced self-administration in mice, but was less efficacious than oxycodone or fentanyl. SR-17018 has also been found to produce self-administration in rhesus monkeys, but along with buprenorphine, was less efficacious than heroin in this test.
=== Legal status === In August 2011, the US Food and Drug Administration (FDA) granted accelerated approval to the biologics license application (BLA) submitted by Seattle Genetics for the use of brentuximab vedotin in the treatment of relapsed HL and ALCL. In October 2012, the European Medicines Agency (EMA) gave it conditional marketing authorization for relapsed or refractory HL and ALCL. In November 2017, the FDA approved brentuximab vedotin as a treatment for patients with cutaneous T-cell lymphoma (CTCL) who have received prior systemic therapy. This approval is for patients with primary cutaneous anaplastic large cell lymphoma (pcALCL) and CD30-expressing mycosis fungoides (MF). In March 2018, the FDA approved brentuximab vedotin to treat adults with previously untreated stage III or IV classical Hodgkin lymphoma (cHL) in combination with chemotherapy. In November 2018, the FDA expanded the approved use of brentuximab vedotin in combination with chemotherapy for adults with certain types of peripheral T-cell lymphoma (PTCL). This is the first FDA approval for treatment of newly diagnosed PTCL. In November 2022, the FDA approved brentuximab vedotin in combination with doxorubicin, vincristine, etoposide, prednisone, and cyclophosphamide for people aged two years of age and older with previously untreated high risk classical Hodgkin lymphoma. This is the first pediatric approval for brentuximab vedotin.
=== Machine Learning applications === Machine learning models trained on these molecular representations have been applied to predict various chirality-related properties. One practical application is forecasting the elution order of enantiomers in chiral chromatography. Models trained on experimental retention data from chiral stationary phases can learn structure-retention relationships. Random Forest and other ensemble methods have been applied to predict which enantiomer elutes first on columns such as Chiralpak AD-H using both traditional circular fingerprints and neural network-derived descriptors. Another application is the prediction of optical rotation, a fundamental chiral property. Machine learning models have been developed to predict specific rotation values for chiral molecules based on their structure, with applications to both organic compounds and specialized classes such as chiral fluorinated molecules. These predictions can assist in structural characterization and quality control in pharmaceutical development. While these machine learning approaches show promise, several limitations remain. Model accuracy depends heavily on training data quality and coverage of chemical space. Neural network architectures, particularly Transformers, face inherent challenges in learning stereochemical features from string-based representations like SMILES.
Sources: en.wikipedia.org
Calcitriol is produced in the cells of the proximal tubule of the nephron in the kidneys by the action of 25-Hydroxyvitamin D 1-alpha-hydroxylase, a mitochondrial oxygenase and an enzyme which catalyzes the hydroxylation of calcifediol (25-hydroxycholecalciferol, 25-OH D3) in the 1-alpha position. The activity of this enzyme is stimulated by PTH. This is an important control point in Ca2+ homeostasis. Additional effects on the production of calcitriol include an increase by prolactin, a hormone which stimulates lactogenesis (the formation of milk in mammary glands), a process which requires large amounts of calcium. Activity is also decreased by high levels of serum phosphate and by an increase in the production of the hormone FGF23 by osteocyte cells in bone. Calcitriol is also produced outside the kidney in small amounts by many other tissues including placenta and activated macrophages. Analogous to the conversion of D3 to 25-OH D3 and then to calcitriol, ergocalciferol (vitamin D2) is converted to 25-OH D2 (ercalcidiol) and then to 1,25-(OH)2D2 (ercalcitriol or ergocalcitriol), a product with similar effects. When the drug alfacalcidol is used, 25-hydroxylation in the liver produces calcitriol as the active metabolite. This will produce greater effects than other vitamin D precursors in patients with kidney disease who have loss of the renal 1-alpha-hydroxylase.
=== Anti-angiogenic drugs === Osteonecrosis of the jaw has been identified as one of the possible complications of taking anti-angiogenic drugs; the association of the disease with the medication is known as MRONJ. This has been stated in the Drug Safety Updates by the MHRA. Angiogenesis inhibitors interfere with blood vessel formation by interfering with the angiogenesis signalling cascade. They are used primarily to treat cancer. These cancer-fighting agents tend to hinder the growth of blood vessels that supply the tumour, rather than killing tumour cells directly. They prevent the tumour from growing. For example, bevacizumab/aflibercept is a monoclonal antibody that specifically binds to vascular endothelial growth factor (VEGF), preventing VEGF from binding to receptors on the surface of normal endothelial cells. Sunitinib is a different example of an anti-angiogenic drug; it inhibits cellular signalling by targeting multiple receptor tyrosine kinases. It reduces the blood supply to the tumour by inhibiting new blood vessel formation in the tumor. The tumour may stop growing or even shrink.
Swift's attempts to sell Chase's design to major railroads were rebuffed, as the companies feared that they would jeopardize their considerable investments in stock cars, animal pens, and feedlots if refrigerated meat transport gained wide acceptance. In response, Swift financed the initial production run on his own, then—when the American roads refused his business—he contracted with the GTR (a railroad that derived little income from transporting live cattle) to haul the cars into Michigan and then eastward through Canada. In 1880, the Peninsular Car Company (subsequently purchased by ACF) delivered the first of these units to Swift, and the Swift Refrigerator Line (SRL) was created. Within a year, the line's roster had risen to nearly 200 units, and Swift was transporting an average of 3,000 carcasses a week to Boston, Massachusetts. Competing firms such as Armour and Company quickly followed suit. By 1920, the SRL owned and operated 7,000 of the ice-cooled rail cars. The General American Transportation Corporation would assume ownership of the line in 1930.
Sources: en.wikipedia.org
NAD+ is the oxidized form, while NADH is the reduced form carrying an additional hydride equivalent. The pair participates in reversible electron transfer reactions. Their ratio helps indicate the redox state of a compartment.
NAD+ itself is not classified as a vitamin, but its precursor niacin is an essential nutrient in humans. Cells synthesize NAD+ from niacin, nicotinamide, nicotinamide riboside, or tryptophan. The intact dinucleotide is not obtained directly from typical diets in meaningful amounts.
Age-related studies often examine whether NAD+ levels decline in tissues and whether that decline affects mitochondrial function or DNA repair. Interventions using precursor molecules raise open questions about cause and effect. Current evidence does not establish that changing NAD+ levels slows human aging.
It indicates the oxidized form, which has a positive charge on the nicotinamide nitrogen. The reduced partner NADH lacks that charge and carries added electrons. The plus sign is part of the standard abbreviation, not a separate ion.