A xanthine oxidase inhibitor is any substance that inhibits

Enzyme inhibitor

An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...

 the activity of xanthine oxidase

Xanthine oxidase

Xanthine oxidase Xanthine oxidase Xanthine oxidase (XO (sometimes 'XAO'), a form of xanthine oxidoreductase that generates reactive oxygen species. Is an enzyme that catalyzes the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid...

, an enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 involved in purine metabolism

Purine metabolism

-Biosynthesis:Purines are biologically synthesized as nucleotides and in particular as ribotides, i.e. bases attached to ribose 5-phosphate. A key regulatory step is the production of 5-phospho-α-D-ribosyl 1-pyrophosphate by PRPP synthetase, which is activated by inorganic phosphate and...

. In human

Human

Humans are the only living species in the Homo genus...

s, inhibition of xanthine oxidase reduces the production of uric acid

Uric acid

Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...

, and several medication

Medication

A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...

s that inhibit xanthine oxidase are indicated

Indication (medicine)

In medicine, an indication is a valid reason to use a certain test, medication, procedure, or surgery. The opposite of indication is contraindication.-Drugs:...

 for treatment of hyperuricemia

Hyperuricemia

Hyperuricemia is a level of uric acid in the blood that is abnormally high. In humans, the upper end of the normal range is 360 µmol/L for women and 400 µmol/L for men.-Causes:...

 and related medical conditions including gout

Gout

Gout is a medical condition usually characterized by recurrent attacks of acute inflammatory arthritis—a red, tender, hot, swollen joint. The metatarsal-phalangeal joint at the base of the big toe is the most commonly affected . However, it may also present as tophi, kidney stones, or urate...

. Xanthine oxidase inhibitors are being investigated for management of reperfusion injury

Reperfusion injury

Reperfusion injury is the tissue damage caused when blood supply returns to the tissue after a period of ischemia or lack of oxygen. The absence of oxygen and nutrients from blood during the ischemic period creates a condition in which the restoration of circulation results in inflammation and...

.

Xanthine oxidase inhibitors are of two kinds: purine analogue

Purine analogue

Purine analogues are antimetabolites which mimic the structure of metabolic purines.* Azathioprine is the main immunosuppressive cytotoxic substance. It is widely used in transplantations to control rejection reactions. It is nonenzymatically cleaved to 6-mercaptopurine that acts as a purine...

s and others. Purine analogues include allopurinol

Allopurinol

Allopurinol is a drug used primarily to treat hyperuricemia and its complications, including chronic gout.- Mechanism of action :...

, oxypurinol

Oxypurinol

Oxypurinol is an inhibitor of xanthine oxidase. It is an active metabolite of allopurinol and it is cleared renally. In cases of renal disease, this metabolite will accumulate to toxic levels....

, and tisopurine

Tisopurine

Tisopurine is a drug used in the treatment of gout....

. Others include febuxostat

Febuxostat

Febuxostat is a urate lowering drug, an inhibitor of xanthine oxidase that is indicated for use in the treatment of hyperuricemia and gout....

 and inositol

Inositol

Inositol or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula 6126 or 6, a sixfold alcohol of cyclohexane. It exists in nine possible stereoisomers, of which the most prominent form, widely occurring in nature, is cis-1,2,3,5-trans-4,6-cyclohexanehexol, or myo-inositol...

s (phytic acid

Phytic acid

Phytic acid is the principal storage form of phosphorus in many plant tissues, especially bran and seeds. Phytate is not digestible to humans or nonruminant animals, however, so it is not a source of either inositol or phosphate if eaten directly...

 and myo-inositol

Inositol

Inositol or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula 6126 or 6, a sixfold alcohol of cyclohexane. It exists in nine possible stereoisomers, of which the most prominent form, widely occurring in nature, is cis-1,2,3,5-trans-4,6-cyclohexanehexol, or myo-inositol...

).

In experiments, numerous natural product

Natural product

A natural product is a chemical compound or substance produced by a living organism - found in nature that usually has a pharmacological or biological activity for use in pharmaceutical drug discovery and drug design...

s have been found to inhibit xanthine oxidase in vitro

In vitro

In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...

 or in model animals (mice

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

, rat

Rat

Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s). These include three flavonoid

Flavonoid

Flavonoids , are a class of plant secondary metabolites....

s that occur in many different fruits and vegetables: kaempferol

Kaempferol

Kaempferol is a natural flavonol, a type of flavonoid, that has been isolated from tea, broccoli, Delphinium, Witch-hazel, grapefruit,cabbage, kale, beans, endive, leek, tomato, strawberries, grapes, brussels sprouts, apples and other plant sources. Kaempferol is a yellow crystalline solid with a...

, myricetin

Myricetin

Myricetin is a naturally occurring flavonol, a flavonoid found in many grapes, berries, fruits, vegetables, herbs, as well as other plants. Walnuts are a rich dietary source. Trace amounts can be found as glycosides. It is one of the phenolic compounds present in red wine.Myricetin has antioxidant...

, and quercetin

Quercetin

Quercetin , a flavonol, is a plant-derived flavonoid found in fruits, vegetables, leaves and grains. It also may be used as an ingredient in supplements, beverages or foods.-Occurrence:...

. More generally, planar flavones and flavonols with a 7-hydroxyl group inhibit xanthine oxidase. An essential oil

Essential oil

An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...

 extracted from Cinnamomum osmophloeum

Cinnamomum osmophloeum

Cinnamomum osmophloeum, commonly known as pseudocinnamomum or indigenous cinnamon, is an evergreen tree in the genus Cinnamomum. It is native to Taiwan....

inhibits xanthine oxidase in mice

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

. The natural product propolis

Propolis

Propolis is a resinous mixture that honey bees collect from tree buds, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces in the hive. Propolis is used for small gaps , while larger spaces are usually filled with beeswax. Its color varies depending on its...

 from selected sources inhibits xanthine oxidase in rat

Rat

Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s; the specific substance responsible for this inhibition has not been identified, and the generality of these findings is unknown. An extract of leaves of Pistacia

Pistacia

Pistacia is a genus of flowering plants in the cashew family, Anacardiaceae. It contains ten to twenty species that are native to Africa and Eurasia from the Canary Islands, whole Africa, and southern Europe, warm and semi-desert areas across Asia, and also North America from Mexico to warm and...

 integerrima also inhibits xanthine oxidase at a level that appears to merit further research.

In folk medicine

Folk medicine

-Description:Refers to healing practices and ideas of body physiology and health preservation known to a limited segment of the population in a culture, transmitted informally as general knowledge, and practiced or applied by anyone in the culture having prior experience.All cultures and societies...

 the tree fern Cyathea spinulosa (formerly Alsophila spinulosa) has been used for gout, but its most active component, caffeic acid

Caffeic acid

Caffeic acid is a hydroxycinnamic acid, a naturally occurring organic compound. This yellow solid consists of both phenolic and acrylic functional groups...

, is only a weak inhibitor of xanthine oxidase.