freeze-thaw cycle 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 2026-07-15. Numbers and descriptions here follow the published literature rather than marketing material.
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.
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 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.
Nicotinamide adenine dinucleotide, abbreviated NAD+, is a dinucleotide built from adenine, nicotinamide, two ribose sugars, and two phosphate groups. The oxidized form carries a positive charge on the nicotinamide ring, while the reduced form, NADH, is neutral. This pair acts as a reversible electron carrier in cells. NAD+ is present in bacteria, plants, animals, and fungi. Its structure allows it to accept and donate electrons without being consumed in the reactions it supports.
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.
| Property | Value | Notes |
|---|---|---|
| Common name | Nicotinamide adenine dinucleotide (oxidized) | Often shortened to NAD+ |
| Chemical class | Dinucleotide | Contains nicotinamide and adenine moieties |
| Molecular formula | C21H27N7O14P2 | Free acid form; charge depends on pH |
| Molar mass | About 663.43 g/mol | Calculated for C21H27N7O14P2 |
| CAS number | 53-84-9 | Common identifier for beta-NAD+ |
The nicotinamide ring undergoes reversible reduction at the para position, converting NAD+ to NADH. This reaction transfers a hydride equivalent, not a free hydrogen atom or electron alone. Because the redox pair has a defined reduction potential, it links oxidation of fuels to respiratory chain activity. Many dehydrogenases use NAD+ as a co-substrate and produce NADH. The ratio of NAD+ to NADH reflects metabolic state and influences flux through several pathways.
NAD+ also serves as a substrate for enzymes that cleave it, including sirtuins, PARPs, and CD38. These enzymes consume NAD+ and release nicotinamide and ADP-ribose or related products. The dual roles as redox cofactor and signaling substrate connect NAD+ to DNA repair, circadian regulation, and calcium signaling. Cellular NAD+ concentrations vary by tissue, time of day, and stress exposure. How these consumption pathways interact with redox balance remains an active area of research.
NAD+ is a dinucleotide composed of two nucleotides joined by a pyrophosphate linkage. One nucleotide contains adenine, and the other contains nicotinamide. The oxidized form carries a positive charge on the nicotinamide ring and is abbreviated NAD+. It functions as a cofactor in hydride-transfer reactions, accepting electrons in catabolic pathways. In cells, it interconverts with reduced NADH, forming a redox couple central to energy metabolism. The molecule is water-soluble and does not cross cell membranes freely without specific transport or precursor pathways.
Measuring NAD+ in biological samples requires rapid processing because the compound can degrade or interconvert after collection. Common approaches include enzymatic cycling assays, high-performance liquid chromatography, and mass spectrometry. Each method has different sensitivity, specificity, and susceptibility to interference from related nucleotides. Sample type matters: cultured cells, animal tissues, and human blood present distinct challenges. Reported values can vary widely across laboratories because of differences in extraction, normalization, and analytical platform. Standardization remains an open issue in the field.
NAD+ is relatively unstable in aqueous solution, especially at neutral or alkaline pH and at elevated temperatures. It is typically stored dry, protected from light and moisture, and kept cold or frozen for long-term use. Solutions are often prepared fresh or buffered to mildly acidic pH to slow hydrolysis. Repeated freeze-thaw cycles can reduce integrity. Laboratories may verify concentration using ultraviolet absorbance at 259 nm or by enzymatic assay. These handling practices are general laboratory conventions rather than universal rules.
Stability of NAD+ depends on form, temperature, pH, and water content. The solid is generally more stable than solutions, and it should be kept dry and cold. In solution, hydrolysis can cleave the dinucleotide, especially under alkaline conditions or at elevated temperature. Light exposure may also contribute to degradation. Buffers, chelating agents, and sterile handling can reduce losses, but no single condition preserves all preparations indefinitely. Researchers often prepare working solutions shortly before use and verify activity or purity after storage.
Quality control for NAD+ relies on identity, purity, and functional tests. A certificate of analysis may report high-performance liquid chromatography purity, ultraviolet spectrum, water content, and residual solvents. Because NAD+ is hygroscopic, gravimetric values can shift as material absorbs water, so purity should be interpreted alongside storage history. Mass spectrometry confirms molecular identity, while enzymatic assays show whether the material supports dehydrogenase activity. Commercial material is available as the free acid and as salts, and the counterion affects molecular weight, solubility, and how concentrations are calculated.
Laboratory measurement of NAD+ usually begins with rapid sample quenching because the molecule can change form after collection. Enzymatic cycling assays amplify signal through coupled reactions and are suited to small samples. High-performance liquid chromatography with ultraviolet detection separates NAD+ from related nucleotides. Liquid chromatography-mass spectrometry offers higher specificity and can distinguish NAD+ from close analogs. Each method has trade-offs in sensitivity, throughput, and equipment needs, so reported values depend heavily on extraction and detection choices.
=== 2016 AFL season === The CAS verdict meant that Essendon's squad was suddenly twelve players short for the 2016 season. The club received permission to recruit ten top-up players from lower levels on contracts lasting until 31 October 2016 to supplement its list. The top-ups had to have either been on an AFL list in 2014 or 2015, with no more than one player taken from any single state-level club; or have been a VFL-listed player from Essendon's own reserves team. An allowance for the top-ups was made in Essendon's salary cap. The top-up players were: Ryan Crowley, James Kelly, Matthew Stokes, Matt Dea, James Polkinghorne, Jonathan Simpkin, Mark Jamar, Sam Grimley, Nathan Grima and Sam Michael. The four other AFL clubs with suspended Essendon players on their lists (Melbourne, Port Adelaide, St Kilda and the Western Bulldogs) were allowed to upgrade a rookie to the senior list, which would not normally be allowed to cover a suspended player, but were not granted top-up players. The twelve suspended players still at Essendon continued to be paid about 95% of their salaries by the club during their suspensions. Stewart Crameri continued to be paid by the Western Bulldogs, with the Bulldogs suing Essendon to recover the cost. Melbourne, Port Adelaide and St Kilda ceased payments to their suspended players, those players suing Essendon privately for their salaries.
=== Prevalence of use === The use of herbal remedies is more prevalent in people with chronic diseases, such as cancer, diabetes, asthma, and end-stage kidney disease. Multiple factors such as gender, age, ethnicity, education and social class are also shown to have associations with the prevalence of herbal remedy use.
Single nucleotide polymorphisms (SNPs) in the MTRR gene impair MTR activity, resulting in elevated homocysteine levels due to compromised methylation to methionine. Elevated homocysteine levels are associated with birth defects in addition to pregnancy complications, cardiovascular disease, cancer, megaloblastic anemia, Alzheimer's diseases and cognitive dysfunction in the elderly. Presence of the mutant variant (66A>G) is associated with significantly lower, up to 4 fold, plasma cobalamin and folate levels in cardiac transplant patients. A consequent decrease in S-adenosylmethionine availability results in DNA hypomethylation. Low folate limits one carbon metabolism and homocysteine metabolism as vitamin B12 interacts with folate in this pathway. Additionally, this mutation is associated with an increased risk in type 2 diabetes.
In molecular biology, SUMO (Small Ubiquitin-like MOdifier) proteins are a family of small proteins that are covalently attached to and detached from other proteins in cells to modify their function. This process is called SUMOylation (pronounced soo-muh-lā-shun and sometimes written sumoylation). SUMOylation is a post-translational modification involved in various cellular processes, such as nuclear-cytosolic transport, transcriptional regulation, apoptosis, protein stability, response to stress, and progression through the cell cycle. In human proteins, there are over 53,000 SUMO binding sites, making it a substantial component of fundamental biology. SUMO proteins are similar to ubiquitin and are considered members of the ubiquitin-like protein family. SUMOylation is directed by an enzymatic cascade analogous to that involved in ubiquitination. In contrast to ubiquitin, SUMO is not used to tag proteins for degradation. Mature SUMO is produced when the last four amino acids of the C-terminus have been cleaved off to allow formation of an isopeptide bond between the C-terminal glycine residue of SUMO and an acceptor lysine on the target protein. SUMO-interacting motifs (SIMs) are binding regions on proteins that interact with SUMO groups. SIMs are typically composed of short stretches of hydrophobic amino acids flanked by acidic amino acids. SUMO family members often have dissimilar names; the SUMO homologue in yeast, for example, is called SMT3 (suppressor of mif two 3). Several pseudogenes have been reported for SUMO genes in the human genome.
Nanomaterials describe, in principle, chemical substances or materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale). Nanomaterials research takes a materials science-based approach to nanotechnology, leveraging advances in materials metrology and synthesis which have been developed in support of microfabrication research. Materials with structure at the nanoscale often have unique optical, electronic, thermo-physical or mechanical properties. Nanobiomaterials are a specialization of nanomaterials; nanobiomaterials are biomaterials (both natural and synthetic compatible with the human body and typically used in Medicine and its specializations such as dentistry and veterinary) in the nanoscale. Nanobiomaterials are typically used in the field of nanomedicine since they can both actively interact with biological systems and mimic biological structures. Nanobiomaterials are different from bionanomaterials: whilst nanobiomaterials are biomaterials in the nanoscale, bionanomaterials are all the products of bionanotechnology except nanobiomaterials, i.e., bionanomaterials are all its products that are not used as (nano)biomaterials. Nanomaterials are slowly becoming commercialized and beginning to emerge as commodities.
Sources: en.wikipedia.org
=== December === 1 December – NHS Grampian suspends visiting at Dr Gray's Hospital in Elgin following an outbreak of vomiting and diarrhoea. 3 December – A teenage boy is sentenced to life imprisonment with a minimum of 18 years for the murder of Danielle Davidson, who was stabbed with a Rambo-style knife following an argument over a e-scooter in Edinburgh in May 2023. A report by Audit Scotland says that fundamental changes are required in the way NHS Scotland provides services are "urgently needed" and that "difficult decisions" about whether some services can continue to be provided may need to be taken. 4 December – Finance Secretary Shona Robison delivers the 2024 budget, which includes planned changes to taxation and a pledge to scrap the two child benefit cap in Scotland. Rocket manufacturer Orbex announces plans to pause the construction of a spaceport in Sutherland in order to focus on the construction of its SaxaVord Spaceport site in Shetland. 6 December – First Minister John Swinney and Prime Minister Keir Starmer hold private talks about the Scottish Government's plans to scrap the two child benefit cap, which are later described as "helpful". A single case of bovine spongiform encephalopathy (BSE) has been identified on a farm in Dumfries and Galloway, authorities confirm. Professor Iain Gillespie resigns as principal of Dundee University following controversy over a £7,000 business trip he and a colleague took to Hong Kong. 7 December – Storm Darragh: An amber warning for high winds, and yellow warnings for wind and rain, are in place in Scotland.
==== Epicardial fat ==== Epicardial adipose tissue (EAT) is a particular form of visceral fat deposited around the heart and found to be a metabolically active organ that generates various bioactive molecules, which might significantly affect cardiac function. Marked component differences have been observed in comparing EAT with subcutaneous fat, suggesting a location-specific impact of stored fatty acids on adipocyte function and metabolism.
An approximation of 0.3 g/kg/day divided into 4 equal spaced intervals has been suggested since creatine needs may vary based on body weight. It has also been shown that taking a lower dose of 3 grams a day for 28 days can also increase total muscle creatine storage to the same amount as the rapid loading dose of 20 g/day for 6 days. However, a 28-day loading phase does not allow for ergogenic benefits of creatine supplementation to be realized until fully saturated muscle storage. This elevation in muscle creatine storage has been correlated with ergogenic benefits discussed in the research section. However, higher doses for longer periods of time are being studied to offset creatine synthesis deficiencies and mitigating diseases.
=== Skin === The most common side effects are mucocutaneous: dry lips, skin, and nose. Other common mucocutaneous side effects are inflammation and chapping of the lips (cheilitis), redness of the skin (erythema), rashes, peeling, eczema (dermatitis), itching (pruritus) and nose bleeds (epistaxis). Absence of dryness of the lips is considered an indication of non-compliance with treatment (not taking the drug as advised), as it occurs in almost all people who take it. Regular use of lip balm and moisturizer is recommended throughout treatment to reduce these problems. The dose may need to be decreased to reduce the severity of these side effects. The skin becomes more fragile—especially to frictional forces—and may not heal as quickly as normal. Wound healing is delayed. For this reason, elective surgery, waxing of hair, tattooing, tattoo removal, piercings, dermabrasion, exfoliation, etc., are not recommended during treatment. Treatment of acne scars is generally deferred until 12 months after completion of a course of isotretinoin.
Sources: en.wikipedia.org
=== January === 1 January – In his New Year address, Archbishop of Canterbury Justin Welby urges politicians to treat their opponents as human beings rather than enemies. Secretary of State for Defence Grant Shapps says that British forces are ready to act against Houthi rebels targeting cargo ships in the Red Sea. Writing on X, Doug Barrowman, husband of Michelle, Baroness Mone, said it "suits the agenda" of ministers to "scapegoat" him and his wife as a means of distracting from government "incompetence" at failing to procure personal protective equipment during the COVID-19 pandemic. 2 January – The Home Office says it has fulfilled a pledge to clear a "legacy" backlog of 92,000 asylum applications lodged before July 2022. But after it subsequently emerges that over 4,000 cases are still waiting for a decision, the Office for Statistics Regulation (OSR) announces an examination of the figures the next day. 3 January – Sir Ed Davey, leader of the Liberal Democrats, launches the party's election campaign by targeting constituencies with Conservative MPs. Former chancellor of the exchequer Nadhim Zahawi makes a guest appearance in the ITV drama Mr Bates vs The Post Office as himself, questioning Post Office chief executive Paula Vennells in a 2015 House of Commons committee inquiry into the Horizon computer system. Richard Tice, leader of Reform UK, rules out a pact with the Conservative Party at the next general election. 4 January – Prime Minister Rishi Sunak says his "working assumption" is that the general election will take place in the second half of this year.
=== Discovery === Enrico Fermi and a team of scientists at the University of Rome reported that they had discovered element 94 in 1934. Fermi called the element hesperium and mentioned it in his Nobel Lecture in 1938. The sample actually contained products of nuclear fission, primarily barium and krypton. Nuclear fission, discovered in Germany in 1938 by Otto Hahn and Fritz Strassmann, was unknown at the time.
In gastric tissue, a loss of parietal cells due to chronic inflammation has been shown to affect chief cell differentiation and can induce chief cells to transdifferentiate back into neck cells and can lead to the formation of mucus cell metaplasia known as spasmolytic polypeptide expressing metaplasia (SPEM) that could be precancerous. If parietal cells are lost, mature chief cells do not form. Parietal cells may secrete factors that lead to transdifferentiation of chief cells, so if lost, chief cells do not normally develop.
Sources: en.wikipedia.org
The plus sign indicates the oxidized form of nicotinamide adenine dinucleotide, which can accept electrons. When it accepts electrons, it becomes NADH. The two forms together support redox reactions in cells.
No. NAD+ is the oxidized form and NADH is the reduced form. They differ by two electrons and a proton equivalent, and cells interconvert them during metabolism.
Yes. NAD+ is present in all living cells and is required for fundamental metabolic reactions. Its concentration varies by tissue, compartment, and time.
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.