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As a consequence of the biochemical reactions in which homocysteine is involved, deficiencies of
[[vitamin B6|vitamin B<sub>6</sub>]], [[folic acid]] (vitamin B<sub>9</sub>), and [[Cobalamin|vitamin B<sub>12</sub>]] can lead to high homocysteine levels.<ref name="Miller-p1033-9">{{cite journal |pmid=8172087 |year=1994 |last1=Miller |first1=J. W. |title=Vitamin B-6 deficiency vs folate deficiency: Comparison of responses to methionine loading in rats |journal=The American Journal of Clinical Nutrition |volume=59 |issue=5 |pages=1033–9 |last2=Nadeau |first2=M. R. |last3=Smith |first3=D |last4=Selhub |first4=J |doi=10.1093/ajcn/59.5.1033}}</ref> Other possible causes of hyperhomocysteinemia include genetics, excessive [[methionine]] intake, and other diseases.
Hyperhomocysteinemia is typically managed with vitamin B<sub>6</sub>, vitamin B<sub>9</sub> and vitamin B<sub>12</sub> supplementation.<ref name="ExpertOpPharm2001-Coen">{{cite journal |doi=10.1517/14656566.2.9.1449 |pmid=11585023 |title=Homocysteine-lowering treatment: An overview |journal=Expert Opinion on Pharmacotherapy |volume=2 |issue=9 |pages=1449–60 |year=2005 |last1=Stehouwer |first1=Coen DA |last2=Guldener |first2=Coen van |s2cid=45945199 }}</ref> Hyperhomocysteinemia is a risk factor for cardiovascular disease;
== Signs and symptoms ==
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=== Bone health ===
Elevated levels of homocysteine have also been linked to increased [[Fracture (bone)|fractures]] in elderly persons. Homocysteine auto-oxidizes and reacts with reactive oxygen intermediates, damaging endothelial cells and increasing the risk of [[thrombus]] formation.<ref>{{cite journal |doi=10.1056/NEJMoa032739 |pmid=15141042 |title=Homocysteine as a Predictive Factor for Hip Fracture in Older Persons |journal=New England Journal of Medicine |volume=350 |issue=20 |pages=2042–9 |year=2004 |last1=McLean |first1=Robert R. |last2=Jacques |first2=Paul F. |last3=Selhub |first3=Jacob |last4=Tucker |first4=Katherine L. |last5=Samelson |first5=Elizabeth J. |last6=Broe |first6=Kerry E. |last7=Hannan |first7=Marian T. |last8=Cupples |first8=L. Adrienne |last9=Kiel |first9=Douglas P. |s2cid=22853996 |doi-access=free }}</ref><ref>{{cite journal |doi=10.1056/NEJMoa032546 |pmid=15141041 |title=Homocysteine Levels and the Risk of Osteoporotic Fracture |journal=New England Journal of Medicine |volume=350 |issue=20 |pages=2033–41 |year=2004 |last1=Van Meurs |first1=Joyce B.J. |last2=Dhonukshe-Rutten |first2=Rosalie A.M. |last3=Pluijm |first3=Saskia M.F. |last4=Van Der Klift |first4=Marjolein |last5=De Jonge |first5=Robert |last6=Lindemans |first6=Jan |last7=De Groot |first7=Lisette C.P.G.M. |last8=Hofman |first8=Albert |last9=Witteman |first9=Jacqueline C.M. |last10=Van Leeuwen |first10=Johannes P.T.M. |last11=Breteler |first11=Monique M.B. |last12=Lips |first12=Paul |last13=Pols |first13=Huibert A.P. |last14=Uitterlinden |first14=André G. |hdl=1765/8452 |url=http://repub.eur.nl/pub/8452 |hdl-access=free }}</ref>
=== Ectopia lentis ===
[[Homocystinuria]] is the second most common cause of heritable [[ectopia lentis]]. Homocystinuria is an autosomal recessive metabolic disorder most often caused by a near absence of cystathionine b-synthetase. It is associated with intellectual disability, osteoporosis, chest deformities, and increased risk of thrombotic episodes. Lens dislocation occurs in 90% of patients, and is thought to be due to decreased zonular integrity due to the enzymatic defect. Lens dislocation in homocystinuria is usually bilateral and in 60% of cases occurs in the inferior or nasal direction.{{
== Causes ==
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=== Genetic ===
Homocysteine is a non-protein amino acid, synthesized from [[methionine]] and either recycled back into methionine or converted into [[cysteine]] with the aid of the B-group vitamins.
* About 50% of homocysteine {{Citation needed|date=May 2014}} is converted back to methionine by [[remethylation]] via the [[methionine synthase]] major pathway. This requires [[5-methyltetrahydrofolate|active folate]] and vitamin B<sub>12</sub>, in order to donate a methyl group. Active folate is known as 5-methyltetrahydrofolate (5-MTHF).
* Another pathway for the conversion of homocysteine back to methionine also exists, involving methylation with [[trimethylglycine]] (also called betaine or abbreviated to TMG) as a methyl donor.
* The remaining homocysteine is transsulfurated to cysteine, with vitamin B6 as the co-factor.
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== Diagnosis ==
A blood test can be performed to quantify total homocysteine concentration in the plasma, of which approximately 80% is generally protein-bound. Classification of hyperhomocysteinemia is defined with respect to serum concentration as follows:{{
* Moderate: 15–30 nmol/mL (or μmol/L)
* Intermediate: 30–100 nmol/mL
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== Treatment ==
Vitamins B<sub>6</sub>, B<sub>9</sub>, or B<sub>12</sub> supplements (alone or combined)
== See also ==
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| SNOMED CT = 419503008
}}
{{Amino acid metabolic pathology}}
{{Authority control}}
[[Category:Amino acid metabolism disorders]]
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