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| {{infobox enzyme
| | Hi there. Allow me begin by introducing [http://www.ninfeta.tv/blog/99493 over the counter std test] writer, her name is Myrtle Cleary. My working day job is a librarian. One of the extremely very best issues in the globe for him is to collect badges but he is having difficulties to find time for it. Minnesota has always been his house but his spouse wants them to transfer. |
| | Name = Thioglucosidase (Myrosinase)
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| | EC_number = 3.2.1.147
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| | CAS_number = 9025-38-1
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| | IUBMB_EC_number = 3/2/1/147
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| | GO_code = 0019137
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| | image = 1e4m.png
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| | width =
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| | caption = Myrosinase from [[Sinapis alba]]. PDB {{PDBe|1e4m}}<ref>{{cite pmid|10978344}}</ref>
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| }}
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| '''Myrosinase''' ({{EC number|3.2.1.147}}, ''thioglucoside glucohydrolase'', ''sinigrinase'', and ''sinigrase'') is a family of [[enzyme]]s involved in [[Plant defense against herbivory|plant defense against herbivores]]. The three-dimensional structure has been elucidated and is available in the [[Protein Data Bank|PDB]] (see links in the infobox).
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| A member of the [[glycoside hydrolase]] family, myrosinase possesses several similarities with the more ubiquitous O-[[glycosidases]].<ref name=Halkier>{{cite journal |author=Halkier, B. A. and Gershenzon, J. |year=2006 |title=Biology and Biochemistry of Glucosinolates |journal=The Annual Review of Plant Biology |volume=57 |pages= 303–333 |doi=10.1146/annurev.arplant.57.032905.105228 |url=http://www.annualreviews.org/doi/pdf/10.1146/annurev.arplant.57.032905.105228}}</ref>
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| <ref name=bones>{{cite journal |author=Bones, A. M. and Rossiter, J. T. |year=2006 |title=The enzymic and chemically induced decomposition of glucosinolates |journal=Phytochemistry |volume=67 |pages=1053–1067 |doi=10.1016/j.phytochem.2006.02.024 |url=http://www.sciencedirect.com/science/article/pii/S0031942206001221}}</ref>
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| However, myrosinase is the only known [[enzyme]] found in nature that can cleave a [[Thio-|thio]]-linked [[glucose]]. Its known biological function is to catalyze the [[hydrolysis]] of a class of compounds called [[glucosinolates]].
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| <ref name=ascorbate>{{cite journal |author=Shikita, M., Fahey, J. W., Golden, T. R., Holtzclaw, D., and Talalay, P. |year=2000 |title=An unusual case of "uncompetitive activation" by ascorbic acid: Purification and kinetic properties of a myrosinase from Raphanus sativus seedlings |journal=Journal of Biochemistry |volume=341 |pages=725–732 |pmc=1220411 |pmid=10417337}}</ref>
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| ==Myrosinase activity==
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| Myrosinase is regarded as a defence-related enzyme and is capable of hydrolyzing [[glucosinolate]]s into various compounds, some of which are toxic.<ref>[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=157743 A wound- and methyl jasmonate-inducible transcript coding for a myrosinase-associated protein with similarities to an early nodulin<!-- Bot generated title -->]</ref>
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| ===Mechanism===
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| Myrosinase [[catalysis|catalyzes]] the [[chemical reaction]]
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| :a thioglucoside + H<sub>2</sub>O <math>\rightleftharpoons</math> a sugar + a thiol
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| Thus, the two [[substrate (biochemistry)|substrates]] of this enzyme are [[thioglucoside]] and [[water|H<sub>2</sub>O]], whereas its two [[product (chemistry)|products]] are [[sugar]] and [[thiol]].
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| In the presence of [[water]], myrosinase cleaves off the [[glucose]] group from a [[glucosinolate]]. The remaining molecule then quickly converts to a [[thiocyanate]], an [[isothiocyanate]], or a [[nitrile]]; these are the active substances that serve as defense for the plant.
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| The hydrolysis of [[glucosinolates]] by myrosinase can yield a variety of products, depending on various physiological conditions such as [[pH]] and the presence of certain [[cofactor (biochemistry)|cofactor]]s. All known reactions have been observed to share the same initial steps. ''(See Figure 2.)'' First, the β-thioglucoside linkage is cleaved by myrosinase, releasing [[D-glucose]].
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| The resulting [[aglycone]] undergoes a spontaneous [[lossen rearrangement|Lossen]]-like rearrangement, releasing a [[sulfate]]. The last step in the [[Reaction mechanism|mechanism]] is subject to the greatest variety depending on the physiological conditions under which the reaction takes place. At neutral [[pH]], the primary product is the [[isothiocyanate]]. Under acidic conditions (pH < 3), and in the presence of [[ferrous]] [[ions]] or epithiospecifer proteins, the formation of [[nitriles]] is favored instead.
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| <ref name=Halkier /><ref name=ESP>{{cite journal |author=Lambrix, V. ''et al.'' |year=2001 |title=The Arabidopsis Epithiospecifier Protein Promotes the Hydrolysis of Glucosinolates to Nitriles and Influences Trichoplusia ni Herbivory |journal=The Plant Cell |volume=13 |pages=2793–2807 |doi=10.1105/tpc.010261 |url=http://dx.doi.org/10.1105/tpc.010261}}</ref>
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| [[File:Myrosinase general mechanism.png|thumb|center|760px|alt text=mechanism of glucosinolate hydrolysis by myrosinase|'''Figure 2:''' Mechanism of glucosinolate hydrolysis by myrosinase.<ref name=Halkier />]]
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| ===Cofactors and inhibitors===
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| [[Ascorbate]] is a known [[cofactor (biochemistry)|cofactor]] of myrosinase, serving as a [[Base_(chemistry)|base]] [[catalyst]] in [[glucosinolate]] hydrolysis.<ref>{{cite journal |author=Burmeister, W. P., Cottaz, S., Driguez, H., Iori, R., Palmieri, S. and Henrissat |year=1997 |journal=Structure |volume=5 |pages=663–675}}</ref>
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| <ref>{{cite journal |author=Burmeister, W.P., Cottaz, S., Rollin, P., Vasella, A., Henrissat, B. |year=2000 |title=High resolution X-ray crystallography shows that ascorbate is a cofactor for myrosinase and substitutes for the function of the catalytic base. |journal=Journal of Biological Chemistry |volume=275 |pages=39385–39393 |PMID= 9195886 |doi=10.1074/jbc.M006796200}}</ref>
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| For example, myrosinase isolated from [[daikon]] (''Raphanus sativus'') demonstrated an increase in [[V max]] from 2.06 µmol/min per mg of protein to 280 µmol/min per mg of protein on the substrate, [[sinigrin|allyl glucosinolate (sinigrin)]] when in the presence of 500 µM ascorbate.<ref name=ascorbate />
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| [[Sulfate]], a byproduct of [[glucosinolate]] hydrolysis, has been identified as a [[competitive inhibitor]] of myrosinase.<ref name=ascorbate />
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| In addition, 2-F-2-deoxybenzylglucosinolate, which was synthesized specifically to study the mechanism of myrosinase, inhibits the enzyme by trapping one of the [[glutamic acid]] residues in the [[active site]], Glu 409.<ref name=bones /><ref>{{cite journal |author=Cottaz, S., Rollin, P., Driguez, H. |year=1997 |title=Synthesis of 2-deoxy-2-fluoroglucotropaeolin, a thioglucosidase inhibitor |journal=Carbohydrate Research |volume=298 |pages=127–130}}</ref>
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| ===Structure===
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| Myrosinase exists as a [[Protein dimer|dimer]] with subunits of 60-70 kDa each.<ref name=white>{{cite journal |author=Björkman, R. and Janson, J.-C. |year=1972 |journal=Biochim. Biophys. Acta |volume=276 |pages=508–518 |doi= 10.1016/0005-2744(72)91011-X |url=http://dx.doi.org/10.1016/0005-2744(72)91011-X}}</ref>
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| <ref>{{cite journal |author=Pessina, A., Thomas, R. M., Palmieri, S. and Luisi, P. L. |year=1990 |title=An improved method for the purification of myrosinase and its physicochemical characterization |journal=Arch. Biochem. Biophys. |volume=280 |pages=383–389 |doi= 10.1016/0003-9861(90)90346-Z |url=http://dx.doi.org/10.1016/0003-9861(90)90346-Z}}</ref>
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| X-ray crystallography of myrosinase isolated from ''[[White mustard|Sinapis alba]]'' revealed the two subunits are linked by a zinc atom.<ref>{{cite journal |author=Burmeister, W. P., Cottaz, S., Driguez, H., Iori, R., Palmieri, S. and Henrissat, B. |year=1997 |journal=Structure |volume=5 |pages=663–675}}</ref>
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| The prominence of [[Salt bridge (protein and supramolecular)|salt bridges]], [[disulfide bridges]], [[hydrogen bonding]], and [[glycosylation]] are thought to contribute to the [[enzyme]]’s stability, especially when the plant is under attack and experiences severe tissue damage.<ref name=Halkier />
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| A feature of many ß-[[glucosidases]] are catalytic [[glutamate]] [[Residue (chemistry)|residues]] at their [[active site]]s, but two of these have been replaced by a single [[glutamine]] residue in myrosinase.<ref name=bones /><ref>{{cite journal |author= Henrissat, B. and Davies, J. G. |year=2000 |title=Glycoside Hydrolases and Glycosyltransferases: Families, Modules, and Implications for Genomics |journal=Plant Physiology |volume=124 |issue=4 |pages=1515–1519 |doi=10.1104/pp.124.4.1515 |url=http://dx.doi.org/10.1104/pp.124.4.1515}}</ref> Ascorbate has been shown to substitute for the activity of the glutamate residues.<ref>{{cite journal |author=Burmeister, W.P., Cottaz, S., Rollin, P., Vasella, A., Henrissat, B. |year=2000 |title=High resolution X-ray crystallography shows that ascorbate is a [[cofactor (biochemistry)|cofactor]] for myrosinase and substitutes for the function of the catalytic base. |journal=Journal of Biological Chemistry |volume=275 |pages=39385–39393 |doi=10.1074/jbc.M006796200 |pmid=9195886}}</ref> ''(See Figure 3 for mechanism.)''
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| [[File:Glucosinolate hydrolysis with ascorbate cofactor at active site of myrosinase.png|thumb|center|760px|alt text=First step of glucosinolate hydrolysis by myrosinase showing active site and ascorbate cofactor|'''Figure 3:''' Active site of myrosinase during the first step of [[glucosinolate]] [[hydrolysis]]. Here, [[ascorbate]] is used as a [[cofactor (biochemistry)|cofactor]] to substitute for the missing second [[catalytic]] [[glutamate]] in order to cleave the [[Thio-|thio]]-linked [[glucose]].<ref name=bones />]]
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| ==Biological function==
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| Myrosinase and its natural [[Substrate (biochemistry)|substrate]], [[glucosinolate]], are known to be part of the [[Plant defense against herbivory|plant’s defense response]]. When the plant is attacked by [[pathogens]], [[insects]], or other [[herbivores|herbivore]], the plant uses myrosinase to convert [[glucosinolates]], which are otherwise-benign, into toxic products like [[isothiocyanates]], [[thiocyanates]], and [[nitriles]].<ref name=Halkier />
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| ===Compartmentalization in plants===
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| The glucosinolate-myrosinase defensive system is packaged in the plant in a unique manner. Plants store myrosinase [[glucosinolates]] by compartimentalization, such that the latter is released and activated only when the plant is under attack.
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| Myrosinase is stored largely as myrosin grains in the [[vacuoles]] of particular [[idioblasts]] called [[myrosin cells]], but have also been reported in protein bodies or [[vacuole]]s, and as cytosolic enzymes that tend to bind to membranes.<ref>Luthy B, Matile P. The mustard oil bomb - rectified analysis of the subcellular organization of the myrosinase system. Biochem Physiol Pfl 179 (1-2): 5-12 (1984).</ref><ref name="myrosin cells">{{cite journal |author=Andréasson, E. ‘’et al.’’ |year=2001 |title=Different Myrosinase and Idioblast Distribution in Arabidopsis and Brassica napus |journal=Plant Physiology |volume=127 |pages=1750–1763 |doi= 10.1104/pp.010334 |url=http://dx.doi.org/10.1104/pp.010334}}</ref> [[Glucosinolates]] are stored in adjacent but separate “S-cells.”
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| <ref>{{cite journal |author=Koroleva, O. A. ‘’et al.’’ |year=2000 |title=Identification of a new glucosinolate-rich cell type in Arabidopsis flower stalk |journal=Plant Physiol. |journal=124 |pages=599–608 |doi= 10.1104/pp.124.2.599 |url=http://dx.doi.org/10.1104/pp.124.2.599}}</ref> When the plant experiences tissue damage, the myrosinase comes into contact with [[glucosinolates]], quickly activating them into their potent, antibacterial form.<ref name=Halkier /> The most potent of such products are [[isothiocyanates]], followed by [[thiocyanates]] and [[nitriles]].<ref name=biofumigation>{{cite journal |author=Gimsing, A. L. and Kirkegaard, J. A. |year=2009 |title=Glucosinolates and biofumigation: fate of glucosinolates and their hydrolysis products in soil |journal=Phytochem Rev |volume=8 |pages=299–310 |doi=10.1007/s11101-008-9105-5 |url=http://www.springerlink.com/content/f571512l3211q6t3/}}</ref>
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| ===Evolution===
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| Plants known to have evolved a myrosinase-glucosinolate defense system include: [[white mustard]] (''Sinapis alba''),
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| <ref name=white />
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| [[garden cress]] (''Lepidium sativum''),<ref name=cress>{{cite journal |author=Durham, P. and Poulton, J. E. |year=1989 |journal=Plant Physiol. |volume=90 |pages=48–52}}</ref>
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| [[wasabi]] (''Wasabia japonica''),<ref name="wasabi">{{cite journal |author=Ohtsuru, M. and Kawatani, H. |year=1979 |journal=Agric. Biol. Chem. |volume=43 |pages=2249–2255}}</ref> [[daikon]] (''Raphanus sativus''),<ref name="daikon1">{{cite journal |author=Iversen, T.-H. and Baggerud, C. |year=1980 |journal=Z. Pflanzenphysiol. |volume=97 |pages=399–407}}</ref><ref name="daikon2">{{cite journal |author=El-Sayed, S. T., Jwanny, E. W., Rashad, M. M., Mahmoud, A. E. and Abdallah, N. M. |year=1995 |journal=Appl. Biochem. Biotechnol. |volume=55 |pages=219–230}}</ref>
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| as well as several members of the [[Brassicaceae]] family, including
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| [[yellow mustard]] (''Brassica juncea''),<ref name="yellow mustard">{{cite journal |author=Ohtsuru, M. and Hata, T. |year=1972 |journal=Agric. Biol. Chem. |volume=36 |pages=2495–2503}}</ref>
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| rape seed (''Brassica napus''),<ref name="rape seed">{{cite journal |author=Lonnerdal, B. and Janson, J.-C. |year=1973 |journal=Biochim. Biophys. Acta |volume=315 |pages=421–429}}</ref> and common dietary brassicas like [[broccoli]], [[cauliflower]], [[cabbage]], [[bok choy]], and [[kale]].
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| <ref name=Halkier /> The bitter aftertaste of many of these vegetables can often be attributed to the [[hydrolysis]] of [[glucosinolates]] upon tissue damage during food preparation or when consuming these vegetables raw.<ref name=Halkier />
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| Myrosinase has also been isolated from the [[Brevicoryne brassicae|cabbage aphid]].<ref name="aphid myrosinase">{{cite journal |author=Husebye, H. ‘’et al’’ |year=2005 |title=Crystal structure at 1.1 Å resolution of an insect myrosinase from Brevicoryne brassicae shows its close relationship to β-glucosidases |journal=Insect Bochemistry and Molecular Biology |volume=35 |issue=12 |pages=1311–1320 |doi=10.1016/j.ibmb.2005.07.004 |url=http://dx.doi.org/10.1016/j.ibmb.2005.07.004}}</ref> This suggests [[coevolution]] of the cabbage aphid with its main food source. The aphid employs a similar defense strategy to plants. Like its main food source, the cabbage aphid compartmentalizes its native myrosinase and the glucosinolates it ingests. When the cabbage aphid is attacked and its tissues are damaged, its stored glucosinolates are activated, producing isothiocyanates and deterring predators from attacking other aphids.<ref name="aphid myrosinase2">{{cite journal |author=Bridges, M. “et al.” |year=2002 |title=Spatial organization of the glucosinolate–myrosinase system in brassica specialist aphids is similar to that of the host plant |journal=Proceedings of the Royal Society |volume=269 |pages=187–191 |doi=10.1098/rspb.2001.1861 |url=http://rspb.royalsocietypublishing.org/content/269/1487/187.short |issue=1487}}</ref>
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| ==Historical relevance and modern applications==
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| ===Agriculture===
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| Historically, crops like [[rapeseed]] that contained the glucosinolate-myrosinase system were deliberately [[bred]] to minimize glucosinolate content, since rapeseed in animal feed was proving toxic to [[livestock]].<ref name=rapemeal>{{cite journal |author=Brabban, A. D. and Edwards, C. |year=1994 |title=Isolation of glucosinolate degrading microorganisms and their potential for reducing the glucosinolate content of rapemeal |journal=FEMS Microbiology Letters |volume=119 |issue=1-2 |pages=83–88 |url=http://www.sciencedirect.com/science/article/pii/0378109794903964}}</ref>
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| The glucosinolate-myrosinase system has been investigated as a possible biofumigant to protect crops against pests. The potent glucosinolate hydrolysis products (GHPs) could be sprayed onto crops to deter herbivory. Another option would be to use techniques in [[genetic engineering]] to introduce the glucosinolate-myrosinase system in crops as a means of fortifying their resistance against pests.<ref name=biofumigation />
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| ===Human health===
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| [[Isothiocyanates]], the primary product of glucosinolate hydrolysis, has been known to prevent [[iodine]] uptake in the [[thyroid]], causing [[goiters]].
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| <ref name="goiter review">{{cite journal |author=Bones, A. M. and Rossiter, J. T. |year=1996 |title=The myrosinase-glucosinolate system, its organisation and biochemistry |journal=Physiologia Plantarum |volume=97 |pages=194–208 |doi=10.1111/j.1399-3054.1996.tb00497.x |url=http://onlinelibrary.wiley.com/doi/10.1111/j.1399-3054.1996.tb00497.x/abstract}}</ref>
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| [[Isothiocyanates]] in high concentrations have also been known to cause hepatotoxicity, or liver damage.<ref name=ascorbate /> However, more recent studies have shown that diets high in glucosinolate-containing vegetables such as dietary brassicas have been associated with lower risks of heart disease, diabetes, and cancer.<ref name=Halkier />
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| <ref>{{cite journal |author=Hayes, J., Kelleher, M. O. and Eggleston, I. M. |year=2008 |title=The Cancer Chemopreventetive Actions of Phytochemicals Derived From Glucosinolates |journal=European Journal of Nutrition |volume=47 |supplement=2 |pages=73–88}}</ref>
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| Isothiocyanates have been shown to induce phase II [[detoxification]] [[enzymes]] involved in the [[xenobiotic]] [[metabolism]] of [[carcinogens]].<ref>{{cite journal |author=Ahn, Y.-H. ‘’et al.’’ |year=2010 |title=Electrophilic tuning of the chemoprotective natural product sulforaphane |journal=PNAS |volume=107 |pages=9590–9595 |doi=10.1073/pnas.1004104107 |url=http://www.pnas.org/content/107/21/9590.short |issue=21}}</ref>
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| There has been increasing evidence to suggest that a myrosinase-like enzyme may also be present in members of the human gut [[microbiome]]. Although myrosinase, like many [[enzymes]], will be denatured at high temperatures and thus lose its activity when cooked, a gut [[microbe]] capable of catalyzing the same [[hydrolysis]] of [[glucosinolates]] would therefore be able to activate ingested [[glucosinolates]] into their more potent forms, e.g. [[isothiocyanates]].
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| <ref name=Bifidobacterium>{{cite journal |author=Cheng, D.-L., Hashimoto, K. and Uda, Y. |year=2004 |title=In vitro digestion of sinigrin and glucotropaeolin by single strains of Bifidobacterium and identification of the digestive products |journal=[[Food and Chemical Toxicology]] |volume=42 |pages=351–357 |url=http://www.sciencedirect.com/science/article/pii/S0278691503002953 |doi=10.1016/j.fct.2003.09.008}}</ref>
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| <ref name=Bacteroides>{{cite journal |author=Elfoul, L. ‘’et al.’’ |year=2001 |title=Formation of allyl isothiocyanate from sinigrin in the digestive tract of rats monoassociated with a human colonic strain of Bacteroides thetaiotaomicron |journal=FEMS Microbiology Letters |volume=197 |pages=99–103 |doi=10.1111/j.1574-6968.2001.tb10589.x |url=http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6968.2001.tb10589.x/full}}</ref>
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| According to one news report, a woman ate so much [[bok choy]] that she was driven into severe [[hypothyroidism]] by the excessive myrosinase.<ref>{{cite news | last = Aleccia | first = JoNel | title = Back away from the bok choy, ma'am | publisher = [[NBC News]] Health`| date = 29 May 2010 | url = http://www.nbcnews.com/health/back-away-bok-choy-maam-1C9926279 | accessdate = 2013-08-18 }}</ref>
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| ==References==
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| {{reflist}}
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| [[Category:Enzymes]]
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