freeze-thaw comes up often in conversation and rarely with the context attached. Here we lay out the basics in order, then work through the practical considerations.
Last reviewed on 2026-01-21. Where a claim depends on a specific study, the study is described rather than over-claimed.
Cells produce NAD+ through several biosynthetic routes. The salvage pathway recycles nicotinamide, while the Preiss-Handler pathway uses nicotinic acid, and a de novo route can start from tryptophan in some organisms. In mammals, the salvage pathway is generally considered the main source under ordinary conditions. Tissue concentrations vary widely by cell type and compartment, and measured declines with age have been reported in some studies. Whether such changes drive aging or mainly accompany it remains an open question.
Nicotinamide adenine dinucleotide, commonly abbreviated NAD+, is a coenzyme present in all living cells. The molecule consists of two nucleotides linked by phosphate groups, with adenine and a nicotinamide ring as its principal features. In its oxidized form, the nicotinamide ring can accept a hydride ion, becoming NADH. This reversible conversion places NAD+ at the center of many electron-transfer reactions. Its role as a redox carrier is well established across bacteria, plants, fungi, and animals.
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.
| Property | Value | Notes |
|---|---|---|
| Chemical formula | C21H27N7O14P2 | Oxidized free acid form; charge depends on pH. |
| Molar mass | 663.43 g/mol | Calculated for the free acid. |
| CAS Registry Number | 53-84-9 | For the anhydrous free acid; salts have different identifiers. |
| Appearance | White to off-white powder | Solid material; hygroscopic. |
| Solubility | Water-soluble | Dissolves in aqueous buffers; solubility varies with pH and salt. |
Laboratory handling of NAD+ follows standard practices for hygroscopic fine chemicals. Personnel typically avoid inhalation and skin contact, use gloves and eye protection, and work in a ventilated area. Quality control may include ultraviolet absorbance at the nicotinamide maximum, chromatographic purity, water content, and identity confirmation by mass spectrometry. Because commercial preparations can contain counterions, residual solvents, or related nucleotides, a certificate of analysis helps verify the material. Researchers should confirm that the form supplied matches the intended assay.
Measuring NAD+ in biological samples requires care because the molecule is chemically reactive and present at low concentrations in some tissues. Common approaches include enzymatic cycling assays, high-performance liquid chromatography, and liquid chromatography coupled to mass spectrometry. Each method has different sensitivity and specificity, and sample preparation can affect results. Acidic or alkaline extraction steps are used in some protocols, but the choice depends on the analyte and matrix. No single method is universally optimal for every tissue or fluid.
Solid NAD+ is relatively stable when kept dry, cold, and protected from light. Aqueous solutions are more vulnerable to hydrolysis and can lose activity during repeated freeze-thaw cycles or prolonged storage at ambient temperature. Stability depends on pH, ionic strength, and the presence of degrading enzymes or metal ions. For many laboratory uses, aliquots are stored frozen and thawed only once. Exact degradation rates vary by matrix, so stability should be checked for each application rather than assumed.
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.
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.
Research on NAD+ spans biochemistry, aging biology, and metabolism. Studies often examine how NAD+ levels change with age, diet, exercise, or disease states, and whether precursor supplementation alters those levels. Findings in animal models do not automatically translate to humans, and measurement methods vary across studies. Questions about tissue-specific effects, long-term consequences, and causal relationships remain open. NAD+ itself is not established as a single therapeutic agent with a broad clinical role.
Nicotinamide adenine dinucleotide, abbreviated NAD+, is a dinucleotide composed of two nucleotides joined by phosphate groups. One nucleotide contains adenine; the other contains nicotinamide. The molecule exists in oxidized (NAD+) and reduced (NADH) forms, and the reversible hydride transfer between them underlies many metabolic oxidation-reduction reactions. In cells, NAD+ serves as an electron acceptor in pathways such as glycolysis, the citric acid cycle, and oxidative phosphorylation. Its concentration and redox ratio vary by compartment, tissue, and metabolic state.
Beyond redox chemistry, NAD+ is consumed as a substrate by enzymes that transfer ADP-ribose or remove acetyl groups. Sirtuins use NAD+ in deacylation reactions, poly(ADP-ribose) polymerases use it in DNA damage responses, and CD38 hydrolases convert it to signaling metabolites. Because these enzymes compete for the same pool, changes in NAD+ availability can influence multiple cellular processes. The relative contribution of each consumption route differs by cell type and condition, and precise quantitative links remain an active area of study.
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.
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.
The carbon footprint of cattle rearing (see environmental impact of meat production) Use of chemicals in the tanning process (e.g., chromium, phthalate esters, nonyl phenol ethoxylate soaps, pentachlorophenol and solvents) Air pollution due to the transformation process (hydrogen sulfide is formed during mixing with acids and ammonia liberated during deliming, solvent vapors)
Minerals are generally oxidized derivatives of metals. Iron is mined as ores such as magnetite (Fe3O4) and hematite (Fe2O3). Titanium is mined as its dioxide, usually in the form of rutile (TiO2). These oxides must be reduced to obtain the corresponding metals, often achieved by heating these oxides with carbon or carbon monoxide as reducing agents. Blast furnaces are the reactors where iron oxides and coke (a form of carbon) are combined to produce molten iron. The main chemical reaction producing the molten iron is:
=== Importance of physiochemical properties === For drug delivery systems to bypass the blood brain barrier, modifications of physiochemical properties can enhance safety and efficacy. Size, surface charge, and lipophilicity play a major role in substance bypassing the blood brain barrier. Smaller, positively charged, or more lipophilic molecules enhance efficacy of nose-to-brain delivery. Decrease in delivery system size increases permeation. As the membrane is negatively charged, a particle with positive surface charge interacts electrostatically which enhances bioadhesion. Carriers with more lipophilicity exert better mucoadhesion and residence time. Drug system pH, solubility, and hydrogen bonding potential are other physiochemical properties which should be evaluated.
== Academic career == He received the Standard Oil Foundation for Excellence in Undergraduate Teaching Award in 1970; the Tanner Award for Excellence in Undergraduate Teaching in 1986; and he was elected by students to membership in the Society of the Golden Fleece in 1989. He collaborated with 55 M.S. and Ph.D. colleagues resulting in over 160 referred publications in journals and edited volumes. Following Hiskey's retirement in 1996, the department created the Richard G. Hiskey Graduate Student Fellowship dedicated to the recruitment and retention of outstanding graduate students. Hiskey received Outstanding Alumnus Awards from Kansas State University in 1973, Wayne State University in 1978, Emporia State University in 1979, and the UNC-CH General Alumni Association Faculty Service Award in 1992. He also was a John Simon Guggenheim Foundation Fellow and a Kenan Research Leave Fellow in 1970–71 at the Max Planck Institute for Cell Chemistry. Hiskey's initial research concerned the development of methods for the production of "mixed sulfides" and the features which stabilized or destabilized these molecules. Mixed disulfides occur in the folding and stabilization of proteins via cystine-cystine interactions. Pursuit of the synthesis of insulin led to the development of methods for the production of mixed disulfides containing two or more cystine residues differently S-protected. The second major phase of Hiskey's research program concerned the role of protein-bound X-Carboxyglutamic (GLA) residues in blood clot formation.
== Signs and symptoms == Degenerative disc disease can result in lower back or upper neck pain. The amount of degeneration does not correlate well with the amount of pain patients experience. Many people experience no pain, while others with the same amount of damage have severe, chronic pain. Whether a patient experiences pain or not largely depends on the location of the affected disc and the amount of pressure that is being put on the spinal column and surrounding nerve roots. Degenerative disc disease is one of the most common sources of back pain and affects approximately 30 million people every year. With symptomatic degenerative disc disease, the pain can vary depending on the location of the affected disc. A degenerated disc in the lower back can result in lower back pain, sometimes radiating to the hips, and pain in the buttocks, thighs, or legs. If pressure is being placed on the nerves by exposed nucleus pulposus, sporadic tingling or weakness through the knees and legs can occur. A degenerated disc in the upper neck will often result in pain to the neck, arm, shoulders and hands; tingling in the fingers may also result if nerve impingement is occurring. Pain is most commonly felt or worsened by movements such as sitting, bending, lifting, and twisting. After an injury, some discs become painful because of inflammation and the pain comes and goes. Some people have nerve endings that penetrate more deeply into the anulus fibrosus (outer layer of the disc) than others, making discs more likely to generate pain.
Sources: en.wikipedia.org
The Samoyed ( SAM-ə-yed or sə-MOY-ed; Russian: самое́дская соба́ка, romanized: samoyédskaya sobáka, or самое́д, samoyéd) is a breed of herding dog with a thick, white, double-layered coat. They are spitz-type dogs which take their name from the Samoyedic peoples. Descending from the Nenets Herding Laika, they are domesticated animals that assist in herding, hunting, protection and sled-pulling.
Musculoskeletal The first joint symptoms that patients with scleroderma have are typically nonspecific joint pains, which can lead to arthritis, or cause discomfort in tendons or muscles. Joint mobility, especially of the small joints of the hand, may be restricted by calcinosis or skin thickening. Patients may develop muscle weakness, or myopathy, either from the disease or its treatments.
=== Gender === Women typically make up some 80% of autoimmune disease patients. Whilst many proposals have been made for the cause of this high weighting, no clear explanation is available. A possible role for hormonal factors has been suggested. For example, some autoimmune diseases tend to flare during pregnancy (possibly as an evolutionary mechanism to increase health protection for the child), when hormone levels are high, and improve after menopause, when hormone levels decrease. Women may also naturally have autoimmune disease trigger events in puberty and pregnancy. Under-reporting by men may also be a factor, as men may interact less with the health system than women.
The Ancient Greek word for bone is ὀστέον ("osteon"). In anatomical terminology, including in the Terminologia Anatomica, the word for a bone is os (for example, os breve, os longum, os sesamoideum). (This is not to be confused with the alternative medical use of os to mean orifice, from the Latin ōs, mouth.)
Solvent extraction may also be used to separate out the thorium and uranium, by dissolving the resultant filter cake in nitric acid. The presence of titanium hydroxide is deleterious as it binds thorium and prevents it from dissolving fully.
Sources: en.wikipedia.org
=== "The Dark Alliance" === In 1996, the journalist Gary Webb published a series of articles for the San Jose Mercury News tying the Reagan administration to the trafficking of illegal narcotics into the United States. The three-part expose by Webb, "The Dark Alliance," claimed that in an effort to support the Sandinista National Liberation Front (FDN) financially, the CIA supported narcotics trafficking into the US by top members of the Nicaraguan rebel group, the Contras. Webb alleged that the fallout from the operation was the large scale spread of the crack epidemic in the United States. A California-based drug dealer, "Freeway," Rick Ross testified that Norwin Meneses and Danillo Blandon Reyes, two members of the FDN, supplied him with cocaine for much of the 1980s. He testified that at its peak, the drug ring operated from California to Michigan and as far south as Louisiana. The State Department knew of the operation and interfered with local police investigation to prevent the prosecution of FDN-linked trafficking. In 1990 Reyes testified that his "cocaine sales were for a time C.I.A. approved." Blandón testified that the CIA notified him of a raid on his home in 1986. Neither Meneses or Blandón ever received prison sentences for their roles in the scandal. The response to Webb's writings were mixed. Journalists from both The Washington Post and The Los Angeles Times credited Webb with an important piece of journalism. David Corn of The Washington Post wrote that Webb "deserves credit for pursuing an important piece of recent history and forcing the C.I.A.
Evidence of a consistent pattern of basalt acquisition by the Middle Pleistocene hominins for the production of Acheulian tools from the Gesher Benot Ya'aqov site, including evidence for different mode of procurement of basalt for the production of handaxes and cleavers, is presented by Golan, Ben Dor & Goren-Inbar (2026). Evidence of selective procurement of flint by Middle Pleistocene hominins from the Gesher Benot Ya'aqov site, as well as evidence of stability of their procurement preferences throughout 50,000 years, is presented by Ekshtain et al. (2026). A study on animals remains from the Caijiagou-C site in the Nihewan Basin (China), providing evidence of systematic processing of ungulate carcasses by early Middle Pleistocene hominins that involved diverse butchery stages, is published by Du et al. (2026). Evidence supporting the reliability of dating of hominin traces from the Quesang site (China) described by Zhang et al. (2021) to the Middle Pleistocene is presented by Gao et al. (2026). A study on Acheulean tools from the Revilleja de Valparaíso (Spain), interpreted as indicative of presence of diverse technological traditions with distinct origins in Western Europe around 700,000 years ago, is published by García-Vadillo et al. (2026). Approximately 430,000-years-old wooden tools are identified from the Marathousa site in the Megalopolis Basin (Greece) by Milks et al. (2026). The conclusions about age of the Petralona skull from the study of Falguères et al. (2025) are contested by Kostopoulos & Lazaridis (2026) and reaffirmed by Falguères et al. (2026).
=== Classification of abdominal and pelvic structures === The structures in the abdomen are classified as intraperitoneal, mesoperitoneal, retroperitoneal or infraperitoneal depending on whether they are covered with visceral peritoneum and whether they are attached by mesenteries (mensentery, mesocolon).
The diagnostic tests of ptaquiloside inside cells include gene mutation detection, immunohistochemical detection of tumor biomarkers, chromosomal aberrations, oxidative stress for EBH, PCR, real-time PCR and DNase-SISPA (sequence-independent single primer amplification).
In 1857, Cuban Freemasons started on the path to form a Grand Lodge for their country. The basic requirements to form any Grand Lodge or Orient in Freemasonry is to possess three Lodges, however, there were only enough Freemasons in Cuba to form two new Lodges. Initially, they sought a charter from the Reformed Grand Orient of Hesperia (Spanish: Gran Oriente Hespérico Reformado) or the Reformed Grand Orient of Spain, but like in Cuba, Freemasonry in mainland Spain had also gone through a period of persecution and no single Grand Lodge existed anymore on the Iberian Peninsula. The Cuban Freemasons therefore found their charters in the Grand Lodge of Louisiana. That year, the Grand Lodge of Louisiana chartered Prudencia Lodge No. 2 and Fraternidad Lodge No. 1 in Santiago de Cuba.
Sources: en.wikipedia.org
NAD+ is a coenzyme found in living cells and is the oxidized form of nicotinamide adenine dinucleotide. It accepts electrons in redox reactions and also serves as a substrate for certain signaling and repair enzymes.
NAD+ becomes NADH when it accepts a hydride ion during oxidation-reduction reactions. NADH then donates electrons to other molecules, after which the carrier can return to the NAD+ form.
No, nicotinamide is a smaller molecule and a component of NAD+. Cells can use nicotinamide to rebuild NAD+ through the salvage pathway.
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.