Catechin

Catechin is a natural phenol antioxidant plant secondary metabolite. The term catechins is also commonly used to refer to the related family of flavonoids and the subgroup flavan-3-ols (or simply flavanols).

The name of the catechin chemical family derives from catechu, which is the juice or boiled extract of Mimosa catechu (Acacia catechu L.f)

Chemistry
Catechin possesses two benzene rings (called the A- and B-rings) and a dihydropyran heterocycle (the C-ring) with a hydroxyl group on carbon 3. The A ring is similar to a resorcinol moiety while the B ring is similar to a catechol moiety. There are two chiral centers on the molecule on carbons 2 and 3. Therefore, it has four diastereoisomers. Two of the isomers are in trans configuration and are called catechin and the other two are in cis configuration and are called epicatechin.

The most common catechin isomer is the (+)-catechin. The other stereoisomer is (-)-catechin or ent-catechin. The most common epicatechin isomer is (-)-epicatechin (also known under the names L-epicatechin, epicatechol, (-)-epicatechol, l-acacatechin, l-epicatechol, epi-catechin, 2,3-cis-epicatechin or (2R,3R)-(-)-epicatechin).

The different epimers can be distinguished using chiral column chromatography.

Making reference to no particular isomer, the molecule can just be called catechin. Mixtures of the different enantiomers can be called (+/-)-catechin or DL-catechin and (+/-)-epicatechin or DL-epicatechin.

Moreover, the flexibility of the C-ring allows for two conformation isomers, putting the B ring either in a pseudoequatorial position (E conformer) or in a pseudoaxial position (A conformer). Studies confirmed that (+)-catechin adopts a mixture of A- and E-conformers in aqueous solution and their conformational equilibrium has been evaluated to be 33:67.

Regarding the antioxidant activity, (+)-catechin has been found to be the most powerful scavenger between different members of the different classes of flavonoids. The ability to quench singlet oxygen seems to be in relation with the chemical structure of catechin, with the presence of the catechol moiety on ring B and the presence of a hydroxyl group activating the double bond on ring C.

Catechin exists in the form of a glycoside. Antioxidant properties can also be provided using a catechin associated with a sugar. In 1975-76, a group of USSR scientists of Kaz ssr discovered first the catechin rhamnoside using the plants of Filipendula that grow in that region. Pioneer and head of the discovery was PhD N. D. Storozhenko born in 1944. Though not thoroughly studied, the rhamnoside of catechin can enter the blood cell without breaking the outer layer.

Oxidation
Electrochemical experiments show that (+)-catechin oxidation mechanism proceeds in sequential steps, related with the catechol and resorcinol groups and the oxidation is pH-dependent. The oxidation of the catechol 3′,4′-dihydroxyl electron-donating groups occurs first, at very low positive potentials, and is a reversible reaction. The hydroxyl groups of the resorcinol moiety oxidised afterwards were shown to undergo an irreversible oxidation reaction.

In food
l-Epicatechin can be found in cacao beans and was first called kakaool. The different other enantiomers can as well be found in chocolate where the different processes of fabrication can lead to epimerisation by heating. The kola nut, a related species, contains epicatechin and D-catechin. Açaí oil, obtained from the fruit of tha açaí palm (Euterpe oleracea), is rich in (+)-catechin (66.7 +/- 4.8 mg/kg).


 * see also : List of phytochemicals in food, List of micronutrients and List of antioxidants in food

Taste
The taste associated with monomeric (+)-catechin or (-)-epicatechin is described as not exactly astringent, nor exactly bitter. It is envisaged to encapsulate catechin in cyclodextrins to mask its taste to use it as an additive.

Biosynthesis
Leucocyanidin reductase (LCR) uses 2,3-trans-3,4-cis-leucocyanidin to produce (+)-catechin and is the first enzyme in the proanthocyanidins (PA)-specific pathway. Its activity has been measured in leaves, flowers, and seeds of the legumes Medicago sativa, Lotus japonicus, Lotus uliginosus, Hedysarum sulfurescens, and Robinia pseudoacacia. The enzyme is also present in Vitis vinifera (grape).

Biodegradation
Catechin oxygenase, a key enzyme in the degradation of catechin, is present in fungi and bacteria.

Among bacteria, degradation of (+)-catechin can be achieved by Acinetobacter calcoaceticus. Catechin is metabolized to protocatechuic acid (PCA) and phloroglucinol carboxylic acid (PGCA). It is also degraded by Bradyrhizobium japonicum. Phloroglucinol carboxylic acid is further decarboxylated to phloroglucinol, which is dehydroxylated to resorcinol. Resorcinol is hydroxylated to hydroxyquinol. Protocatechuic acid and hydroxyquinol undergo intradiol cleavage through protocatechuate 3,4-dioxygenase and hydroxyquinol 1,2-dioxygenase to form β-carboxy cis, cis-muconic acid and maleyl acetate.

Among fungi, degradation of catechin can be achieve by Chaetomium cupreum.

In rats, all plasma catechin metabolites are present as conjugated forms and mainly constituted by glucuronidated derivatives. In the liver, the concentrations of catechin derivatives are lower than in plasma, and no accumulation is observed when the rats are adapted for 14 days to the supplemented diets. The hepatic metabolites are intensively methylated (90–95%), but in contrast to plasma, some free aglycones can be detected. Rats fed with (+)-catechin and (-)-epicatechin exhibit (+)-catechin 5-O-β-glucuronide and (-)-epicatechin 5-O-β-glucuronide in their body fluids. The primary metabolite of (+)-catechin in plasma is glucuronide in the nonmethylated form. In contrast, the primary metabolites of (-)-epicatechin in plasma are glucuronide and sulfoglucuronide in nonmethylated forms, and sulfate in the 3'-O-methylated forms (3'OMC). Catechin is absorbed into intestinal cells and metabolized extensively because no native catechin can be detected in plasma from the mesenteric vein. Mesenteric plasma contains glucuronide conjugates of catechin and 3'-O-methyl catechin, indicating the intestinal origin of these conjugates. Additional methylation and sulfation occur in the liver, and glucuronide or sulfate conjugates of 3'OMC are excreted extensively in bile. Circulating forms are mainly glucuronide conjugates of catechin and 3'OMC. Another study shows that catechin undergoes enzymatic oxidation by tyrosinase in the presence of glutathione (GSH) to form mono-, bi-, and tri-glutathione conjugates of catechin and mono- and bi-glutathione conjugates of a catechin dimer.

In the crab eating macaque Macaca iris, (+)-catechin administered orally or intraperitonally leads to the formation of 10 metabolites and notably to m-hydroxyphenylhydracrylic acid excreted in the urine.

In man, (+)-catechin absorbed orally is metabolized largely within 24 hours with the production of eleven metabolites detected in the urine.

Biotransformation
Biotransformation of (+)-catechin into taxifolin by a two-step oxidation can be achieved by Burkholderia sp.

The laccase/ABTS system oxidizes (+)-catechin to oligomeric products of which proanthocyanidin A2 is a dimer.

(+)-Catechin and (-)-epicatechin are transformed by the endophytic filamentous fungus Diaporthe sp. into the 3,4-cis-dihydroxyflavan derivatives, (+)-(2R,3S,4S)-3,4,5,7,3',4'-hexahydroxyflavan (leucocyanidin) and (-)-(2R,3R,4R)-3,4,5,7,3',4'-hexahydroxyflavan, respectively, whereas (-)-catechin and (+)-epicatechin with a 2S-phenyl group resisted the biooxidation.

Leucoanthocyanidin reductase (LAR) uses (2R,3S)-catechin, NADP+ and H2O to produce 2,3-trans-3,4-cis-leucocyanidin, NADPH, and H+. Its gene expression has been studied in developing grape berries and grapevine leaves.

Catechin and epicatechin are the building blocks of the proanthocyanidins, a type of condensed tannin.

Bioactivity studies
It is reported that catechin induced longevity in the nematode worm Caenorhabditis elegans. Transcriptomic studies shows that catechin reduces atherosclerotic lesion development in apo E-deficient mice. (+)- and (−)-catechin seem to have stereospecific opposite effects on glycogen metabolism in isolated rat hepatocytes. (+)-Catechin inhibits intestinal tumor formation in mice. (+)-Catechin inhibits the oxidation of low density lipoprotein. (-)-Catechin suppresses expression of Kruppel-like factor 7. Catechin shows an enhancement of the antifungal effect of amphotericin B against Candida albicans.

Incubation experiments with (+)-catechin show a prevention of human plasma oxidation.

Interactions with human genes
Catechin interacts the most with the PTGS2, IL1B, CAT, CYP1A1, SOD, BAX, CASP3, MAPK1, MAPK3 and S100B human genes.

PTGS2 (aka COX-2 for cyclooxygenase-2) is a dioxygenase. The presence of catechin seems to increase its expression. IL1B induces the formation of cyclooxygenase-2 (PTGS2/COX2). Catechin increases its expression. CAT is a catalase. Catechin decreases its expression. CYP1A1 (Cytochrome P450, family 1, member A1) is an enzyme implied in the metabolism of xenobiotics. Catechin decreases its expression. SOD (Superoxide dismutase) is an enzyme that catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide. Catechin increases its expression. BAX (Bcl-2–associated X protein) is a protein of the Bcl-2 gene family. It promotes apoptosis by competing with Bcl-2 proper. Catechin increases its expression. CASP3 (Caspase 3) is a protein that plays a central role in the execution-phase of cell apoptosis. Catechin increases its expression. MAPK1 (Mitogen-activated protein kinase 1) and MAPK3 (Mitogen-activated protein kinase 3) are enzymes that are extracellular signal-regulated kinases (ERKs) and act as an integration point for multiple biochemical signals, involved in a wide variety of cellular processes such as proliferation, differentiation, transcription regulation, and development. Catechin seems to increase their expression. S100B (S100 calcium binding protein B) is an pro-inflammatory enzyme specific of mature astrocytes that ensheath the blood vessels. Catechin decreases the expression of the gene and could regulate S100B-activated oxidant stress-sensitive pathways through blocking p47phox protein expression. Treatment with catechin could eliminate reactive oxygen species (ROS) to reduce oxidative stress stimulated by S100B. Catechin decreases its expression.

Experiments on human Caco-2 cells show changes in the expression of genes like STAT1, MAPKK1, MRP1 and FTH1 genes, which are involved in the cellular response to oxidative stress, are in agreement with the antioxidant properties of catechin. In addition, the changes in the expression of genes like C/EBPG, topoisomerase 1, MLF2 and XRCC1 suggest novel mechanisms of action at the molecular level.

Detail for all tested genes : (dec : decreased expression, inc : increased expression, = : does not affect the activity, expression assayed in human if not specified otherwise) ABCG2 : (-)-catechin decreases the expression of ABCG2 ACE (in Rattus norvegicus) : (+)-catechin or (-)-epicatechin do not affect the activity of the angiotensin-converting enzyme ACTB (in Rattus norvegicus) decrease AKT1 decrease ANXA2 increase ARHGAP4 decrease ATF4 increase BAT2 increase BAX (rattus norvegicus) increase BCL2 decrease BRCC3 decrease BTG1 increase CASP3 increase CAT (mus musculus) decrease CCL2 increase CCND1 decrease CD81 increase CD9 increase CEBPG increase CXCL10 increase CYP19A1 (rattus norvegicus) increase CYP1A1 decrease CYP1A2 = DEK decrease DFFA (mus musculus) decrease DNMT1 decrease EWSR1 increase FLT3LG decrease FTH1 increase GRN increase HCFC1 increase HEAB decrease HMOX1 increase HOXD3 increase HSPD1 decrease ICAM1 increase IL10 increase IL1B increase IL2RA decrease IL32 decrease IRF4 decrease ITGAL increase ITGB2 increase LYN decrease MAP2K1 decrease ? MAPK1 increase ? MAPK3 increase ? MIF decrease NCF1 ? NFE2L2 increase NFKBIA decrease NOS2 (mus musculus) increase NOTCH1 increase NPM1 decrease PARP1 (mus musculus) increase PECAM1 increase PLAT increase PLAU increase PON1 = PTGS2 increase? RAC1 decrease RARB decrease RELA decrease RPL6 increase S100B decrease SERPINE1 decrease SF1 decrease SLC20A1 increase SOD (Drosophila melanogaster) increase SOD2 (Drosophila melanogaster) increase STAT1 decrease STAT5B increase STAT6 increase SULT1A1 increase : sulfation of catechin TCF7 increase TK1 decrease TNF increase TNFRSF8 decrease TOP1 decrease TOP2A decrease TRP53 increase XCR1 decrease ZNF593 increase

Protection of the mouse brain after a stroke
Ninety minutes after feeding mice a single modest dose of epicatechin, a compound found naturally in dark chocolate, the scientists induced an ischemic stroke by, in essence, cutting off blood supply to the animals' brains. They found that the animals that had preventively ingested the epicatechin suffered significantly less brain damage than the ones that had not been given the compound. While most treatments against stroke in humans have to be given within a two- to three-hour time window to be effective, epicatechin appeared to limit further neuronal damage when given to mice 3.5 hours after a stroke. Given six hours after a stroke, however, the compound offered no protection to brain cells.

Histidine decarboxylase inhibitor
(+)-Catechin is a histidine decarboxylase inhibitor. Thus, it inhibits the conversion of histidine to histamine, and, so, is thought to be beneficial through reduction of potentially damaging, histamine-related local immune response(s).

Monoamine oxidase inhibitor
(+)-Catechin and (-)-epicatechin are also selective monoamine oxidase inhibitors (MAOIs) of type MAO-B. They could be used as part of the treatment of Parkinson's and Alzheimer's patients.

Ecological effects
Catechin also has ecological functions.

It is released into the ground by some plants to hinder the growth of their neighbors, a form of allelopathy. Centaurea maculosa, the spotted knapweed, is the most studied plant showing this behaviour, catechin isomers, both released into the ground through its root exudates, have effects ranging from antibiotic to herbicide. It causes a reactive oxygen species wave through the target plant's root starting in the apical meristem rapidly followed by a Ca2+ spike that kills the root cells through apoptosis. Most plants in the European ecosystem have defenses against catechin, but few plants are protected against it in the North-American ecosystem where Centaurea maculosa has been introduced causing uncontrolled growth of this weed.

(+)-Catechin acts as an infection-inhibiting factor in strawberry leaf. Epicatechin and catechin may prevent coffee berry disease by inhibition of appressorial melanization of Colletotrichum kahawae.

Other uses
It has been suggested that (+)-catechin could be used as a scavenger for indoor air pollutents such as volatile organic compounds (VOC) to adapt for instance as filters to air conditioners or to air purifiers.