A practical reference on freeze-thaw: what it is, how it behaves, what the literature reports, and where the honest uncertainties sit.
This page was last updated on 2026-03-29 and is reviewed periodically as new material appears.
Cells synthesize glutathione through two ATP-dependent enzymatic steps. The first step combines glutamate and cysteine to form gamma-glutamylcysteine, catalyzed by glutamate-cysteine ligase. The second step adds glycine, producing the complete tripeptide, catalyzed by glutathione synthetase. Glutathione itself can inhibit the first enzyme, providing negative feedback when levels are high. Because cysteine is often limiting, its availability influences how quickly the pathway proceeds. These reactions occur in the cytosol, and the resulting glutathione can be distributed to other compartments.
Glutathione functions in redox balance, detoxification, and sulfur amino acid storage. It participates in reactions that help maintain ascorbate and protein thiol status. The molecule serves as a cofactor for several enzymes, including glutathione peroxidases and glutathione S-transferases. These enzymes reduce peroxides and conjugate electrophiles, respectively. Glutathione also contributes to the metabolism of xenobiotics and to the transport of cysteine between tissues. How interorgan transport and tissue-specific regulation shape whole-body pools remains an active area of study.
Glutathione is a tripeptide composed of glutamate, cysteine, and glycine. Its cysteine residue carries a thiol group, which allows the molecule to participate in reduction and oxidation reactions. The compound exists in most living cells, where the reduced form, often abbreviated GSH, is usually more abundant than the oxidized disulfide form, GSSG. Intracellular concentrations are commonly in the millimolar range, while extracellular concentrations are much lower. This uneven distribution supports its role as a major cellular redox buffer.
Storage recommendations for glutathione reagents usually specify a cool, dry, dark environment because the thiol oxidizes in air and light. Solid material is often kept desiccated at low temperature, while solutions are prepared fresh or stored frozen in aliquots. Repeated freeze-thaw cycles can accelerate degradation, and metal ions can catalyze oxidation. Quality control may include purity assays, water content, and identity confirmation. Stability limits are method-specific, so a stated shelf life applies only to defined conditions and packaging.
Laboratory measurement of glutathione requires attention to oxidation before analysis. Blood, tissue, or cell samples can lose reduced glutathione as it converts to GSSG or forms mixed disulfides with proteins. Acid extraction, rapid freezing, and thiol-blocking reagents are common strategies to preserve the original distribution. Reported concentrations therefore depend on collection protocol, extraction method, and the time between sampling and analysis. Comparisons across studies are most reliable when these pre-analytical variables are described.
| Property | Value | Notes |
|---|---|---|
| Chemical formula | C10H17N3O6S | Reduced glutathione (GSH) |
| Molar mass | 307.32 g/mol | Calculated for C10H17N3O6S |
| Appearance | White to off-white powder | Typical solid form |
| Solubility | Water-soluble | Polar tripeptide |
| Common synonyms | GSH; L-glutathione | Gamma-glutamylcysteinylglycine |
Biosynthesis proceeds in two ATP-dependent steps. First, glutamate-cysteine ligase joins glutamate and cysteine. Second, glutathione synthetase adds glycine to the intermediate. The pathway is regulated by cysteine availability, enzyme expression, and feedback inhibition by glutathione itself. Liver tissue has a particularly high capacity for synthesis and export. Because the molecule is made inside cells, circulating glutathione reflects a balance of release, uptake, and breakdown rather than simple dietary supply.
Functionally, glutathione supports redox balance by donating electrons and becoming oxidized. It also serves as a cofactor for enzymes such as glutathione peroxidases and glutathione S-transferases. These enzymes participate in peroxide reduction and in conjugation reactions that help process reactive molecules. Separate from antioxidant roles, glutathione can modify protein cysteines through S-glutathionylation, influencing enzyme activity and signaling. Research continues to examine how these chemical roles translate into whole-organism effects.
Quantifying glutathione requires distinguishing GSH from GSSG and preventing oxidation during sample preparation. Common approaches include the enzymatic recycling assay, often called the Tietze method, which measures total glutathione after converting GSSG to GSH. HPLC with ultraviolet or fluorescence detection and LC-MS/MS can separate and quantify both forms, sometimes after derivatization of the thiol group. Blood, plasma, and tissue samples differ in matrix and baseline concentrations, so method validation must account for recovery, linearity, and interference. No single assay is universally standard.
Glutathione is most stable as a dry powder stored cool and dry, but its thiol group is readily oxidized in solution. Aqueous preparations at neutral or alkaline pH lose GSH faster because the thiolate form reacts with dissolved oxygen and metal ions. Acidic conditions, chelating agents, and oxygen exclusion can slow oxidation, while repeated freeze-thaw cycles promote degradation. Light exposure and trace metals also contribute to loss. Laboratories typically validate stability for their own matrices because degradation rates depend on pH, temperature, concentration, and container materials.
Glutathione is a small tripeptide made of glutamic acid, cysteine, and glycine. Its cysteine thiol group allows reversible oxidation and reduction, making it central to cellular redox chemistry. The reduced form, often abbreviated GSH, predominates inside most cells, while the oxidized disulfide form, GSSG, forms when two GSH molecules react. The ratio of GSH to GSSG is widely used as an indicator of oxidative stress in laboratory research, though it does not by itself diagnose a clinical condition.
Biosynthesis occurs in two ATP-dependent steps. The enzyme glutamate-cysteine ligase joins glutamate and cysteine, forming gamma-glutamylcysteine; glutathione synthetase then adds glycine to produce the complete tripeptide. Because the peptide bond from glutamate uses the gamma-carboxyl group, glutathione resists digestion by many ordinary peptidases. Tissues vary in synthesis capacity, and the liver generally contains high concentrations relative to many other organs. This uneven distribution contributes to organ-specific differences in redox buffering and affects how experimental results are interpreted across tissue types.
Glutathione participates in detoxification reactions, amino acid transport, and the maintenance of protein thiols. It serves as a cofactor for several enzymes, including glutathione peroxidases and glutathione S-transferases. In research literature, altered glutathione status appears in studies of aging, infection, metabolic stress, and environmental exposure. Whether low glutathione is a cause, consequence, or marker of such conditions often remains unresolved. Direct measurement in blood or tissue provides a snapshot, but results depend on sample handling, timing, and the method used.
Quantification of glutathione in biological or food samples commonly uses liquid chromatography coupled to ultraviolet, fluorescence, electrochemical, or mass spectrometric detection. Because the thiol group oxidizes readily, samples are often acidified or derivatized immediately after collection to stabilize reduced glutathione. Enzymatic recycling assays and colorimetric kits offer higher throughput but generally lower specificity than chromatographic methods. Mass spectrometry can distinguish glutathione from related thiols and allow simultaneous measurement of oxidized forms. Reported concentrations depend strongly on sample type, extraction procedure, and analytical platform.
Glutathione reference materials are sensitive to oxygen, light, and elevated temperature. Solid material is typically stored desiccated at -20 °C or below, while solutions require tighter control because thiol oxidation proceeds faster in liquid form. Aqueous solutions are often prepared fresh, kept cold, and protected from air; some protocols add acid or chelating agents to slow metal-catalyzed oxidation. Repeated freeze-thaw cycles can accelerate degradation and should be avoided. Stability data vary by matrix, so laboratories usually verify performance with their own storage conditions.
== History == Deslorelin was successfully trialed in the U.S. and was approved for veterinary use under certain circumstances. In Europe, it was approved for use in equine assisted reproduction. Ovuplant was withdrawn from the U.S. market following issues with mares which did not become pregnant failing to return to estrus in a timely manner. Techniques were developed where the implant was removed 48 hours after implantation in the mare, however compounded biorelease Deslorelin products were at the time available as well as more commonly used ovulation promoters such as hCG, which did not produce the same failure effect. Upon "Sucromate Equine" receiving FDA-approval, the compounded products were no longer legally available within the U.S., however they remain available in Australia and New Zealand where an approved version is marketed. It is also being trialed in humans to study its efficacy in treatment of breast cancer in women, and in treating precocious puberty and congenital adrenal hyperplasia in male and female children. As of August 2011 this drug was not approved for general use outside the FDA-licensed functions in the U.S., other than within approved clinical trials. Orphan drug status has been designated in the U.S., though approval had not been issued as of 2011.
In 2010, Ian Read was named CEO. In February 2011, Pfizer announced the closure of its UK research and development facility (formerly also a manufacturing plant) in Sandwich, Kent, which at the time employed 2,400 people. In March 2011, Pfizer acquired King Pharmaceuticals for $3.6 billion in cash. King produced emergency injectables such as the EpiPen. On September 4, 2012, the FDA approved bosutinib (Bosulif) for chronic myelogenous leukemia (CML), a rare type of leukemia and a blood and bone marrow disease that affects primarily older adults. In November 2012, Pfizer received approval from the Food and Drug Administration for Xeljanz, a tofacitinib, for rheumatoid arthritis and ulcerative colitis. The drug had sales of $1.77 billion in 2018, and in January 2019, it was the top drug in the United States for direct-to-consumer advertising, passing adalimumab (Humira). In 2023, the Institute for Clinical and Economic Review (ICER) identified Xeljanz (tofacitinib) as one of five high-expenditure drugs that experienced significant net price increases without new clinical evidence to justify the hikes. Specifically, Xeljanz's wholesale acquisition cost rose by 6%, leading to an additional $72 million in costs to U.S. payers. On February 1, 2013, Zoetis, the Agriculture Division of Pfizer and later Pfizer Animal Health, became a public company via an initial public offering, raising $2.2 billion. Later in 2013, Pfizer completed the corporate spin-off of its remaining stake in Zoetis.
== WADA proscription == In 2015, Radwa Arafa Abd Elsalam, an athlete from Egypt, underwent a doping control where ractopamine was identified in the sample. Elsalam was sanctioned by the Egyptian Anti-Doping Organisation (EGY-NADO) with a two year ineligibility period for non-intentional presence of a prohibited substance, in violation of Article 2.1 of the EGY-NADO rules. While not mentioned by name in the 2015 Prohibited List, ractopamine was determined to — much like clenbuterol, another β2-adrenergic agonist — constitute an Other Anabolic Agent prohibited under S1.2. Elsalam appealed the decision, claiming that ractopamine is used in the meat industry overseas, so she must have inadvertently consumed food contaminated with ractopamine. EGY-NADO accepted her appeal, reduced the sanction to six months, and provided the World Anti-Doping Agency (WADA) with records pertaining to the case. WADA filed an appeal with the Court of Arbitration for Sport (CAS) against EGY-NADO and Elsalam with respect to the appealed decision. WADA asserted ractopamine was "undisputed[ly]" an Other Anabolic Agent, that "[i]t is not sufficient for an athlete merely to make protestations of innocence and suggest that the prohibited substance must have entered her body in inadvertently", and that "her explanations are nothing more than mere speculation".
In the cooking process, Maillard reactions can produce hundreds of different flavor compounds depending on the chemical constituents in the food, the temperature, the cooking time, and the presence of air. These compounds, in turn, often break down to form yet more flavor compounds. Flavorists have used the Maillard reaction over the years to make artificial flavors, the majority of patents being related to the production of meat-like flavors. According to chemistry Nobel Prize winner Jean-Marie Lehn "The Maillard is, by far, the most widely practiced chemical reaction in the world".
The RCC's early economic policy has been characterized as being state capitalist in orientation. Many initiatives were established to aid entrepreneurs and develop a Libyan bourgeoisie. Seeking to expand cultivatable acreage, in September 1969 the government launched a "Green Revolution" to increase agricultural productivity and lessen Libyan reliance on imported food. They hoped to make Libya self-sufficient in food production. All land expropriated from Italian settlers or unused was repossessed and redistributed. Irrigation systems were established along the northern coastline and various inland oases. Production costs often surpassed produce value, keeping production in deficit and relying on state subsidies. With crude oil as the country's primary export, Gaddafi sought to improve Libya's oil sector. In October 1969, he proclaimed the current trade terms unfair, benefiting foreign corporations more than the Libyan state, and threatened to decrease production. In December, Jalloud successfully increased the price of Libyan oil. In 1970, other OPEC states followed suit, leading to a global increase in the price of crude oil. The RCC followed with the Tripoli Agreement of 1971, in which they secured income tax, back-payments and better pricing from the oil corporations; these measures brought Libya an estimated $1 billion in additional revenues in its first year. Increasing state control over the oil sector, the RCC began a program of nationalization, starting with the expropriation of British Petroleum's share of the British Petroleum-N.B.
Sources: en.wikipedia.org
=== Ra === Efraim Racker (1913–1991). Austrian-American biochemist at Cornell University, notable for work on ATP synthase. Member Natl. Acad. Sci. USA. George Radda FRS (1936–2024). Hungarian biochemist at Oxford University, known for applying nuclear magnetic resonance to complex biological material, and many other contributions. Ronald T. Raines (b. 1958). American biochemist at the University of Wisconsin–Madison and MIT, known for work on enzymes and other proteins. Venkatraman Ramakrishnan FRS (President) (b. 1952). Indian-British-American structural biologist at the MRC Laboratory of Molecular Biology, Cambridge, known for work on the ribosome. Nobel Prize in Chemistry in 2009. Philip Randle (1926–2006). British biochemist at the University of Bristol known for work on diabetes. Samuel Mitja Rapoport (1912–2004). Austrian and German biochemist at the Humboldt University, Berlin noted for studies of mitochondria, and for discovering a method for preserving blood for transfusions. Leader of biochemistry in the German Democratic Republic. Member of the German Academy of Sciences at Berlin. Tom Rapoport (b. 1947). German-American cell biologist at Harvard Medical School who studies protein transport in cells.
=== Luminous flux: watt equivalent === With the phaseout of the incandescent lamp in the United States and European Union in the early 21st century, manufacturers and sellers of more energy-efficient lamps have compared the visible light output of their lamps to commonly used incandescent lamp sizes with the watt equivalent or watt incandescent replacement (usually with a lowercase w as a unit symbol, as opposed to capital W for the actual wattage). 1 watt incandescent replacement corresponds to 15 lumens. Thus, a 72-watt halogen lamp, a 23-watt compact fluorescent lamp and a 14-watt light-emitting diode lamp, all of which emit 1500 lumens of visible light, are all marketed as "100 watt incandescent replacement" (100w).
Fatty acids with an odd number of carbons are found in the lipids of plants and some marine organisms. Many ruminant animals form a large amount of 3-carbon propionate during the fermentation of carbohydrates in the rumen. Long-chain fatty acids with an odd number of carbon atoms are found particularly in ruminant fat and milk. Chains with an odd-number of carbons are oxidized in the same manner as even-numbered chains, but the final products are propionyl-CoA and acetyl-CoA. Propionyl-CoA is first carboxylated using a bicarbonate ion into a D-stereoisomer of methylmalonyl-CoA. This reaction involves a biotin co-factor, ATP and the enzyme propionyl-CoA carboxylase. The bicarbonate ion's carbon is added to the middle carbon of propionyl-CoA, forming a D-methylmalonyl-CoA. However, the D-conformation is enzymatically converted into the L-conformation by methylmalonyl-CoA epimerase. It then undergoes intramolecular rearrangement, which is catalyzed by methylmalonyl-CoA mutase (requiring B12 as a coenzyme) to form succinyl-CoA. The succinyl-CoA formed then enters the citric acid cycle. However, whereas acetyl-CoA enters the citric acid cycle by condensing with an existing molecule of oxaloacetate, succinyl-CoA enters the cycle as a principal in its own right. Thus, the succinate just adds to the population of circulating molecules in the cycle and undergoes no net metabolization while in it.
On March 10, 2023, Silicon Valley Bank (SVB) failed after a bank run, marking the third-largest bank failure in United States history and the largest since the 2008 financial crisis. It was one of three bank failures, along with Silvergate Bank and Signature Bank, in March 2023 in the United States. Seeking higher investment returns from its burgeoning deposits, SVB had dramatically increased its holdings of long-term securities since 2021, accounting for them on a hold-to-maturity basis. The market value of these bonds decreased significantly through 2022 and into 2023 as the Federal Reserve raised interest rates to curb an inflation surge, causing unrealized losses on the portfolio. Higher interest rates also raised borrowing costs throughout the economy and some Silicon Valley Bank clients started pulling money out to meet their liquidity needs. To raise cash to pay withdrawals by its depositors, SVB announced on Wednesday, March 8 that it had sold over US$21 billion worth of securities, borrowed $15 billion, and would hold an emergency sale of some of its treasury stock to raise $2.25 billion. The announcement, coupled with warnings from prominent Silicon Valley investors, caused a bank run as customers withdrew funds totaling $42 billion by the following day. On the morning of March 10, the California Department of Financial Protection and Innovation seized SVB and placed it under the receivership of the Federal Deposit Insurance Corporation (FDIC). An additional $100 billion were expected to be withdrawn during Friday.
Sources: en.wikipedia.org
Glutathione is built from three amino acids: glutamate, cysteine, and glycine. The linkage involves the gamma-carboxyl group of glutamate rather than the alpha-carboxyl group, which is unusual for peptides. This structure protects the bond from some common peptidases.
It is present in nearly all cells, with notable amounts in the liver, kidneys, and red blood cells. The highest intracellular concentrations are usually in the millimolar range. Levels differ by tissue, age, and physiological state.
It is not classified as an essential nutrient because cells can synthesize it from amino acids. Dietary sources exist, but their contribution to tissue pools is not fully established. The body's production depends on enzyme activity and precursor availability.
Pre-analytical handling, extraction chemistry, and detection method all influence reported glutathione values. Oxidation during sample processing can shift the measured GSH/GSSG ratio. Standardized protocols and reference materials help reduce, but do not eliminate, these differences.