The short version of Freeze-thaw fits in a sentence. The long version — which is the one that helps — is below.
Reviewed 2026-04-05. Anything still debated is marked as such rather than presented as settled.
Commercial NAD+ is supplied as a solid, often as the free acid or a salt, and purity is verified by chromatographic methods. Laboratories typically store it desiccated at minus 20 degrees Celsius or below. Working solutions are prepared fresh because even sterile aqueous solutions can lose activity over hours to days depending on pH and temperature. Documentation may include a certificate of analysis, an assay value, and a recommended retest date. Researchers should verify identity and purity when results depend on precise cofactor concentrations.
NAD+ is commonly measured by high-performance liquid chromatography with ultraviolet detection, often at 254 or 260 nm. Enzymatic cycling assays provide higher sensitivity by coupling NAD+ to a reporter reaction. Mass spectrometry can distinguish NAD+ from close analogues and confirm isotope labeling. Sample preparation usually involves rapid quenching of metabolism to prevent interconversion with NADH. Because NAD+ and NADH differ by one hydride, extraction conditions strongly affect the measured ratio.
In aqueous solution, NAD+ is most stable under mildly acidic to neutral conditions and degrades faster at high pH or elevated temperature. The molecule can hydrolyze at the pyrophosphate bond or undergo nonenzymatic cyclization. Buffers, chelating agents, and cold temperatures slow these losses during analysis. Repeated freeze-thaw cycles are generally avoided because they can promote degradation and concentration changes. Light exposure is also controlled, though NAD+ is less photolabile than some related nucleotides.
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.
| Property | Value | Notes |
|---|---|---|
| UV absorption maximum | 259–260 nm | Aqueous solution; pH-dependent |
| Common salt form | Disodium salt | Improves aqueous solubility |
| Typical storage temperature | -20 °C or lower | Desiccated and protected from light |
| Common analytical method | HPLC with UV detection | Often paired with mass spectrometry |
| Aqueous stability | pH and temperature dependent | Degrades faster at alkaline pH and high heat |
Quality control for NAD+ materials typically combines identity, purity, and water content checks. Identity may be confirmed by ultraviolet spectrum, retention time in chromatography, or mass accuracy, while purity is assessed by HPLC peak area or quantitative nuclear magnetic resonance. Residual water and solvents can affect molar calculations and enzyme assays, so Karl Fischer titration or thermogravimetric analysis may be used. Commercial materials vary in grade and counterion form, and published methods should specify the exact salt or hydrate when reporting concentrations. Regulatory status depends on intended use, with research reagents, dietary ingredients, and clinical products treated under different frameworks.
Quantification of NAD+ in biological samples usually relies on separation techniques coupled to sensitive detection. High-performance liquid chromatography with ultraviolet detection can measure the oxidized form by its absorbance near 260 nm, while mass spectrometry provides greater specificity and can distinguish NAD+ from close analogs. Enzymatic cycling assays use coupled dehydrogenase reactions to amplify signal and estimate NAD+ concentrations in cell or tissue extracts. Because NAD+ and NADH interconvert rapidly, sample preparation must quench metabolism quickly and preserve the redox state before analysis.
In glycolysis, the tricarboxylic acid cycle, and fatty acid oxidation, NAD+ is reduced to NADH at specific dehydrogenase steps. NADH then delivers electrons to the mitochondrial electron transport chain, mainly at complex I, supporting oxidative phosphorylation and ATP production. The balance between NAD+ and NADH, often expressed as a ratio, influences metabolic flux and redox homeostasis in different cellular compartments. Cytosolic and mitochondrial pools are connected but not identical, and their ratios can differ substantially because of compartment-specific enzymes and transport systems.
Beyond redox chemistry, NAD+ serves as a substrate for enzymes that cleave the molecule and transfer its ADP-ribose moiety or remove acetyl groups. Sirtuins consume NAD+ during deacetylation, poly(ADP-ribose) polymerases use it in DNA damage responses, and CD38 enzymes hydrolyze it to signaling metabolites. These consumption pathways mean that NAD+ availability can influence gene regulation, DNA repair, and calcium signaling. Cellular NAD+ concentrations decline in some tissues with age in animal models, but whether this decline is a cause or consequence of aging in humans remains an active open question.
Also in 1893, Édouard Laguesse suggested that the islet cells of the pancreas, described as "little heaps of cells" by Paul Langerhans in 1869, might play a regulatory role in digestion. These cells were named Islets of Langerhans after the original discoverer. Soon after, it was established that the role of the pancreas in carbohydrate metabolism could be localized to the islets; Eugene Lindsay Opie (1901) confirmed this connection in relation to diabetes mellitus. In 1909, Belgian physician Jean de Mayer hypothesized that the islets secrete a substance that plays this metabolic role, and termed it insulin, from the Latin insula ('island'). Sir Edward Albert Sharpey-Schafer independently proposed the same in 1916, not knowing at the time that de Meyer had made the same suggestion a few years prior. The endocrine role of the pancreas in metabolism, and indeed the existence of insulin, was further clarified between 1921 and 1922 when a group of researchers in Toronto, including Frederick Banting, Charles Best, John MacLeod, and James Collip, were able to isolate and purify the extract.
=== Metabolism === DHEA is transformed into DHEA-S by sulfation at the C3β position via the sulfotransferase enzymes SULT2A1 and to a lesser extent SULT1E1. This occurs naturally in the adrenal cortex and during first-pass metabolism in the liver and intestines when exogenous DHEA is administered orally. Levels of DHEA-S in circulation are approximately 250 to 300 times those of DHEA. DHEA-S in turn can be converted back into DHEA in peripheral tissues via steroid sulfatase (STS). The terminal half-life of DHEA is short at only 15 to 30 minutes. In contrast, the terminal half-life of DHEA-S is far longer, at 7 to 10 hours. As DHEA-S can be converted back into DHEA, it serves as a circulating reservoir for DHEA, thereby extending the duration of DHEA. Metabolites of DHEA include DHEA-S, 7α-hydroxy-DHEA, 7β-hydroxy-DHEA, 7-keto-DHEA, 7α-hydroxyepiandrosterone, and 7β-hydroxyepiandrosterone, as well as androstenediol and androstenedione.
===== Adverse effects ===== Mild or moderate adverse events are common, expressing symptoms like nausea, facial flushing, headache, and sunburn. Unlike flibanserin, bremelanotide induces fewer side effects and is not affected nor develops severe complications with alcohol. However, the drug slows down gastric emptying, which can cause an impact on oral drug uptake and the subsequent drug effectiveness. As activation of MC1R gives rise to hyperpigmentation, treatment dosage is at a maximum of eight doses per month. Besides, to minimize the chances of cardiovascular complications, the prescribed daily dosage is at one dose.
Vitamin D: insufficiency is common in diabetics. Observational studies show that serum vitamin D is inversely associated with biomarkers of diabetes; impaired insulin secretion, insulin resistance, and glucose intolerance. It has been suggested that vitamin D may induce beneficial effects on diabetic complications by modulating differentiation and growth of pancreatic β-cells and protecting these cells from apoptosis, thus improving β-cells functions and survival. In particular, vitamin D supplementation has been shown to have positive effects on people with type 1 diabetes. Vitamin D has also been suggested to act on immune system and modulate inflammatory responses by influencing proliferation and differentiation of different immune cells., Moreover, deficiency of vitamin D may contribute to diabetic complications by inducing hyperparathyroidism, since elevated parathyroid hormone levels are associated with reduced β-cells function, impaired insulin sensitivity, and glucose intolerance. Finally, vitamin D may reduce the risk of vascular complications by modulating lipid profile. Antioxidants may have beneficial effects on diabetic complications by reducing blood pressure, attenuating oxidative stress and inflammatory biomarkers, improving lipid metabolism, insulin-mediated glucose disposal, and by enhancing endothelial function. Vitamin C has been proposed to induce beneficial effects by two other mechanisms.
Sources: en.wikipedia.org
States choose to balance for two reasons. First, they place their survival at risk if they fail to curb a potential hegemon before it becomes too strong; to ally with the dominant power means placing one's trust in its continued benevolence. Secondly, joining the weaker side increases the likelihood that the new member will be influential within the alliance. States choose to bandwagon because it may be a form of appeasement as the bandwagoner may hope to avoid an attack by diverting it elsewhere—a defensive reason—or because it may align with the dominant side in wartime to share the spoils of victory—an offensive reason. Realists claim that balancing is when states ally against the prevailing threat and results in a more secure world whereas in a bandwagoning world security is scarce as rising hegemons are not kept in check. With bandwagoning, the threatened state abandons hope of preventing the aggressor from gaining power at its expense and instead joins forces with its dangerous foe to get at least some small portion of the spoils of war. The weaker the state the more likely it is to bandwagon than to balance as they do little to affect the outcome and thus must choose the winning side. Strong states may change a losing side into a winning side and thus are more likely to balance. States will be tempted to bandwagon when allies are unavailable, however excessive confidence in allied support encourages weak states to free ride relying on the efforts of others to provide security.
The first time the FDA approved the use of lipid nanoparticles as a drug delivery system was in 2018, when the agency approved the first siRNA drug, Onpattro. Encapsulating the mRNA molecule in lipid nanoparticles was a critical breakthrough for producing viable mRNA vaccines, solving a number of key technical barriers in delivering the mRNA molecule into the host cell. Research into using lipids to deliver siRNA to cells became a foundation for similar research into using lipids to deliver mRNA. However, new lipids had to be invented to encapsulate mRNA strands, which are much longer than siRNA strands. Principally, the lipid provides a layer of protection against degradation, allowing more robust translational output. In addition, the customization of the lipid's outer layer allows the targeting of desired cell types through ligand interactions. However, many studies have also highlighted the difficulty of studying this type of delivery, demonstrating that there is an inconsistency between in vivo and in vitro applications of nanoparticles in terms of cellular intake. The nanoparticles can be administered to the body and transported via multiple routes, such as intravenously or through the lymphatic system. One issue with lipid nanoparticles is that several of the breakthroughs leading to the practical use of that technology involve the use of microfluidics. Microfluidic reaction chambers are difficult to scale up, since the entire point of microfluidics is to exploit the microscale behaviors of liquids.
=== Isolation === Many biologically active natural compounds are derived from fungi, and guanacastepene A is one such compound. Attempts were made to characterize CR115, but CR115 did not form spores under any of the conditions tested, and no other morphological characteristics were observed that provided insight into its phylogeny. rDNA sequence analysis shows a similarity of 90% to an uncharacterized root basidiomycete. The fungal strain was cultivated in potato dextrose broth for 14–21 days. The culture was then extracted with hexane to obtain a crude extract containing nonpolar secondary metabolites. This extract was fractionated by C18 flash column chromatography using an acetonitrile/water (CH3CN/H2O) step gradient and subsequently purified by reverse-phase HPLC.
YAN is a measurement of the primary organic (free amino acids) and inorganic (ammonia and ammonium) sources of nitrogen that can be assimilated by S. cerevisiae. There are several nitrogenous compounds found in must and wine including peptides, larger proteins, amides, biogenic amines, pyridines, purines and nucleic acids but these cannot be directly used by yeast for metabolism. Taken together, the total nitrogen content of grape must can range from 60 to 2400 mg of nitrogen per liter, however not all of this nitrogen will be assimilable. The lack of protease enzymes, which break down larger peptides into smaller components, that can work outside the cell, limits the size of the molecules that yeast can use as a source for nitrogen. The amount of YAN that winemakers will see in their grape musts depends on a number of components including grape variety, rootstock, vineyard soils and viticultural practices (such as the use of fertilizers and canopy management) as well as the climate conditions of particular vintages. Infections by mold, such as Botrytis cinerea (known as noble rot when it is desired) can reduce the amino acid content of grape must by as much as 61%. Some regions are noted for having low YAN such as Washington State which during a typical vintage will have 90% of tested must below 400 mg N/L and nearly a quarter be below 150 mg N/L. In the vineyard, nitrogen is taken up by the grapevine as nitrate (NO3−), ammonium or urea which gets reduced into ammonia.
== Method translation == Faster GC methods have shorter times but Kovats indices of the compounds may be conserved if proper method translation is applied. Temperatures of the temperature program stay the same, but ramps and times change when using a smaller column or faster carrier gas. If column dimensions Length×diameter×film are divided by 2 and gas velocity is doubled by using H2 in place of Helium, the hold times must be divided by 4 and the ramps must be multiplied by 4 to keep the same index and the same retention temperature for the same compound analyzed. Method translation rules are incorporated in some chromatography data systems.
Sources: en.wikipedia.org
== Early life and education == Goldstein grew up on Staten Island and received his B.S. degree from Wagner College in 1959 and his M.S. and Ph.D. degrees from Rutgers University, the latter in 1964. He served on the faculty of the Albert Einstein College of Medicine from 1964 to 1972, and as director of the Division of Biochemistry at the University of Texas Medical Branch in Galveston from 1972 through 1978 before accepting the chairmanship of the Department of Biochemistry and Molecular Biology at the George Washington University School of Medicine & Health Sciences, a position he held for 31 years.
Trepanging is an economically important activity in some areas, particularly Southeast Asia. Sea cucumber is considered a delicacy in Far Eastern countries such as China, Indonesia, Japan, and Malaysia. Besides being valued for flavour-enhancing properties, sea cucumber is widely regarded as a stimulant and aphrodisiac. There is evidence that its reputed medicinal properties may be true. Based upon the belief in the healing properties of sea cucumber, pharmaceutical and cosmetics companies have developed pills, oils, and creams based on extracts. The effectiveness of sea cucumber extract in tissue repair has been the subject of scientific study.
Lee & Co as concrete contractor using a Lee's Mixer, an early example of concrete mixers used during construction on the Admiralty Pier. Further construction from 1897 onwards established the Eastern Arm of the current harbour, the Southern Breakwater and a further extension of the Admiralty Pier. During the First World War, a former cargo ship, the Spanish Prince, was deliberately sunk by the Admiralty close to the entrance to Dover Harbour to prevent German U-Boats from firing torpedoes into the harbour. During World War II, the remains of another ship, the War Sepoy, were sunk alongside the Spanish Prince.
=== Salt of hartshorn === Compositions containing ammonium carbonate have long been known. They were once produced commercially, formerly known as sal volatile or salt of hartshorn. It was obtained by the dry distillation of nitrogenous organic matter such as hair, horn, leather. In addition to ammonium bicarbonate, this material contains ammonium carbamate (NH4CO2NH2), and ammonium carbonate ((NH4)2CO3). It is sometimes called ammonium sesquicarbonate. It possesses a strong ammoniacal smell, and on digestion with alcohol, the carbamate is dissolved leaving a residue of ammonium bicarbonate. A similar decomposition takes place when the sesquicarbonate is exposed to air.
Sources: en.wikipedia.org
NAD+ and NADH can interconvert quickly after a sample is collected, which can alter the measured ratio. Rapid quenching and cold handling limit enzymatic and chemical changes.
Purity is often checked by HPLC with UV detection, sometimes paired with mass spectrometry for identity. An assay against a standard can quantify the active cofactor content.
Solid NAD+ is usually kept dry, cold, and protected from light. Aqueous working solutions are best prepared fresh because degradation depends on pH, temperature, and time.
NAD+ is the oxidized form, while NADH is the reduced form carrying an added hydride. The two form a redox pair that cells use in many energy-yielding reactions.