A high level of homocysteine in the blood (hyperhomocysteinemia) makes a person more prone to endothelial cell injury, which leads to inflammation in the blood vessels, which in turn may lead to atherogenesis, which can result in ischemic injury. Hyperhomocysteinemia is therefore a possible risk factor for coronary artery disease. Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries, which supply the heart with oxygenated blood.
Hyperhomocysteinemia has been correlated with the occurrence of
blood clots, heart attacks and strokes, though it is unclear whether
hyperhomocysteinemia is an independent risk factor for these conditions. Hyperhomocysteinemia has also been associated with early pregnancy loss and with neural tube defects.
Structure
Zwitterionic form of (S)-homocysteine (left) and (R)-homocysteine (right)
Homocysteine exists at neutral pH values as a zwitterion.
Biosynthesis and biochemical roles
Two
of homocysteine's main biochemical roles. (Homocysteine is seen in the
left middle of the image.) It can be synthesized from methionine and
then converted back to methionine via the SAM cycle or used to create
cysteine and alpha-ketobutyrate.
Mammals biosynthesize the amino acid cysteine via homocysteine. Cystathionine β-synthase catalyses the condensation of homocysteine and serine to give cystathionine. This reaction uses pyridoxine (vitamin B6) as a cofactor. Cystathionine γ-lyase
then converts this double amino acid to cysteine, ammonia, and
α-ketobutyrate. Bacteria and plants rely on a different pathway to
produce cysteine, relying on O-acetylserine.
Homocysteine can be recycled into methionine. This process uses N5-methyl tetrahydrofolate as the methyl donor and cobalamin (vitamin B12)-related enzymes. More detail on these enzymes can be found in the article for methionine synthase.
Homocysteine also acts as an allosteric antagonist at Dopamine D2 receptors. It has been proposed that both homocysteine and its thiolactone may have played a significant role in the appearance of life on the early Earth.
Homocysteine levels
Total plasma homocysteine
Homocysteine levels are typically higher in men than women, and increase with age.
Common levels in Western populations are 10 to 12 μmol/L, and
levels of 20 μmol/L are found in populations with low B-vitamin intakes
or in the elderly (e.g., Rotterdam, Framingham).
It is decreased with methyl folate trapping, where it is
accompanied by decreased methylmalonic acid, increased folate and a
decrease in formiminoglutamic acid. This is the opposite of MTHFR C677T mutations, which result in an increase in homocysteine.
The ranges above are provided as examples only; test results should
always be interpreted using the range provided by the laboratory that
produced the result.
Elevated homocysteine
Abnormally high levels of homocysteine in the serum, above 15 µmol/L, are a medical condition called hyperhomocysteinemia. This has been claimed to be a significant risk factor for the development of a wide range of diseases, including thrombosis, neuropsychiatric illness, and fractures.
It is also found to be associated with microalbuminuria which is a
strong indicator of the risk of future cardiovascular disease and renal
dysfunction. Vitamin B12 deficiency, when coupled with high serum folate levels, has been found to increase overall homocysteine concentrations as well.
Choline chloride can be made by treating TMA with 2-chloroethanol:
N(CH3)3 + ClCH2CH2OH → (CH3)3N+CH2CH2OH · Cl–
The 2-chloroethanol can be generated from ethylene oxide. Choline has
historically been produced from natural sources, e.g., via hydrolysis of lecithin.
In humans, certain PEMT-enzyme mutations and estrogen deficiency (often due to menopause)
increase the dietary need for choline. In rodents, 70% of
phosphatidylcholines are formed via the PEMT route and only 30% via the
CDP-choline route. In knockout mice, PEMT inactivation makes them completely dependent on dietary choline.
Phosphocholine and glycerophosphocholines are hydrolyzed via phospholipases
to choline, which enters the portal vein. Due to their water
solubility, some of them escape unchanged to the portal vein.
Fat-soluble choline-containing compounds (phosphatidylcholines and
sphingomyelins) are either hydrolyzed by phospholipases or enter the lymph incorporated into chylomicrons.
Transport
In humans, choline is transported as a free molecule in blood. Choline–containing phospholipids and other substances, like glycerophosphocholines, are transported in blood lipoproteins. Blood plasma choline levels in healthy fasting adults is 7–20 micromoles
per liter (µmol/l) and 10 µmol/l on average. Levels are regulated, but
choline intake and deficiency alters these levels. Levels are elevated
for about 3 hours after choline consumption. Phosphatidylcholine levels
in the plasma of fasting adults is 1.5–2.5 mmol/l. Its consumption
elevates the free choline levels for about 8–12 hours, but does not
affect phosphatidylcholine levels significantly.
Choline is a water soluble ion and thus requires transporters to pass through fat-soluble cell membranes. Three types of choline transporters are known:
SLC5A7s are sodium (Na+) and ATP dependent transporters. They have high binding affinity for choline, transport it primarily to neurons and are indirectly associated with the acetylcholine production. Their deficient function causes hereditary weakness in the pulmonary and other muscles in humans via acetylcholine deficiency. In knockout mice, their dysfunction results easily in death with cyanosis and paralysis.
CTL1s have moderate affinity for choline and transport it in almost all tissues: e.g., intestines, the liver, kidneys, the placenta and mitochondria. CTL1s supply choline for phosphatidylcholine and trimethylglycine production.
CTL2s occur especially in the mitochondria in the tongue, kidneys,
muscles and heart. They are associated with the mitochondrial oxidation
of choline to trimethylglycine. CTL1s and CTL2s are not associated with
the acetylcholine production, but transport choline together via the blood–brain barrier.
Only CTL2s occur on the brain side of the barrier. They also remove
excess choline from the neurons back to blood. CTL1s occur only on the
blood side of the barrier, but also on the membranes of astrocytes and neurons.
OCT1s and OCT2s are not associated with the acetylcholine production.
They transport choline with low affinity. OCT1s transport choline
primarily in the liver and kidneys; OCT2s in kidneys and the brain.
Storage
Choline is stored in the cell membranes and organelles as phospholipids, and inside cells as phosphatidylcholines and glycerophosphocholines.
Excretion
Even
at choline doses of 2–8 g, little choline is excreted to urine in
humans. Excretion happens via transporters that occur within kidneys
(see transport). Trimethylglycine is demethylated in the liver and kidneys to dimethylglycine (tetrahydrofolate receives one of the methyl groups). Methylglycine forms, is excreted to urine, or is demethylated to glycine.
Function
Choline
and its derivatives have many functions in humans and in other
organisms. The most notable function is that choline serves as a
synthetic precursor for other essential cell components and signalling
molecules, such as phospholipids that form cell membranes, the neurotransmitter acetylcholine, and the osmoregulatortrimethylglycine (betaine). Trimethylglycine in turn serves as a source of methyl groups by participating in the biosynthesis of S-adenosylmethionine.
Phospholipid precursor
Choline is transformed to different phospholipids, like phosphatidylcholines and sphingomyelins. These are found in all cell membranes and from the membranes of most cell organelles.
Phosphatidylcholines are structurally important part of the cell
membranes. In humans 40–50% of their phospholipids are
phosphatidylcholines.
Phosphatidylcholines are needed for the synthesis of VLDLs: 70–95% of their phospholipids are phosphatidylcholines in humans.
Choline is also needed for the synthesis of pulmonary surfactant,
which is a mixture consisting mostly of phosphatidylcholines. The
surfactant is responsible for lung elasticity, i.e., for its ability to
contract and expand. For example, deficiency of phosphatidylcholines in
the lung tissues has been linked to acute respiratory distress syndrome.
Phosphatidylcholines are excreted into bile and work together with bile acid salts as surfactants in it, thus helping with the intestinal absorption of lipids.
Acetylcholine synthesis
Choline is needed to produce acetylcholine. This is a neurotransmitter which plays a necessary role in muscle contraction, memory and neural development, for example. Nonetheless, there is little acetylcholine in the human body relative to other forms of choline. Neurons also store choline in the form of phospholipids to their cell membranes for the production of acetylcholine.
Choline occurs in foods as a free molecule and in the form of phospholipids, especially as phosphatidylcholines. The total choline content accounting for all of these forms is one of the highest of all foods in hen egg yolk. It has about 670 milligrams of total choline per 100 grams of yolk (mg/100 g). After eggs, content decreases in general and respectively in meats, grains, vegetables, fruits and fats. Cooking oils and other food fats have about 5 mg/100 g of total choline. In the United States, food labels express the amount of choline in a serving as a percentage of daily value (%DV) based on the adequate intake of 550 mg/d. 100% of the daily value means that a serving of food has 550 mg of choline.
Human breast milk is rich in choline. Exclusive breastfeeding
corresponds to about 120 mg of choline per day for the baby. Increase
in a mother's choline intake raises the choline content of breast milk
and low intake decreases it. Infant formulas may or may not contain enough choline. In the EU and the US, it is mandatory to add at least 7 mg of choline per 100 kilocalories (kcal) to every infant formula. In the EU, levels above 50 mg/100 kcal are not allowed.
Trimethylglycine is a functional metabolite of choline. It substitutes for choline nutritionally, but only partially. High amounts of trimethylglycine occur in wheat bran (1339 mg/100 g), toasted wheat germ (1240 mg/100 g) and spinach (600–645 mg/100 g), for example.
Twelve surveys undertaken in 9 EU countries between 2000 and 2011 estimated choline intake of adults in these countries to be 269–468 milligrams
per day (mg/d). Intake was 269–444 mg/d in adult women and 332–468 mg/d
in adult men. Intake was 75–127 mg/d in infants, 151–210 mg/d in 1–3
year olds, 177–304 mg/d in 3–10 year olds and 244–373 mg/d in 10–18
years. The total choline intake mean estimate was 336 mg/day in pregnant
adolescents and 356 mg/day in pregnant women.
A study based on the NHANES 2009–2012 survey estimated the choline intake to be too low in some US
subpopulations. Intake was 315.2–318.8 mg/d in ≥2 year olds between
this time period. Out of ≥2 year olds, only 15.6 ± 0.8% of males and 6.1
± 0.6% of females exceeded the adequate intake
(AI). AI was exceeded by 62.9 ± 3.1% of 2–3 year olds, 45.4 ± 1.6% of
4–8 year olds, 9.0 ± 1.0% of 9–13 year olds, 1.8 ± 0.4% of 14–18 and 6.6
± 0.5% of over 19 year olds. Upper intake level was not exceeded in any
subpopulations.
A 2013–2014 NHANES study of the US population found the choline
intake of 2–19 year olds to be 256 ± 3.8 mg/d and 339 ± 3.9 mg/d in ≥20
year olds. Intake was 402 ± 6.1 mg/d in ≥20-year-old men and 278 mg/d in
≥20-year-old women.
Deficiency
Signs and symptoms
Symptomatic
choline deficiency is rare in humans. Most obtain sufficient amounts of
it from the diet and are able to biosynthesize limited amounts of it.
Symptomatic deficiency is often caused by certain diseases or by other
indirect causes. Severe deficiency causes muscle damage and non-alcoholic fatty liver disease, which may develop into cirrhosis.
Besides humans, fatty liver is also a typical sign of choline
deficiency in other animals. Bleeding in the kidneys can also occur in
some species. This is suspected to be due to deficiency of choline
derived trimethylglycine, which functions as an osmoregulator.
Causes and mechanisms
Estrogen
production is a relevant factor which predisposes individuals to
deficiency along with low dietary choline intake. Estrogens activate phosphatidylcholine producing PEMT-enzymes. Women before menopause have lower dietary need for choline than men due to women's higher estrogen production. Without estrogen therapy, the choline needs of post-menopausal women are similar to men's. Some single-nucleotide polymorphisms (i.e., genetic factors) affecting choline and folate metabolism are also relevant. Certain gut microbes also degrade choline more efficiently than others, so they are also relevant.
In deficiency, availability of phosphatidylcholines in the liver are decreased – these are needed for formation of VLDLs. Thus VLDL-mediated fatty acid transport out of the liver decreases leading to fat accumulation in the liver.
Other simultaneously occurring mechanisms explaining the observed liver
damage have also been suggested. For example, choline phospholipids are
also needed in mitochondrial membranes. Their inavailability leads to the inability of mitochondrial membranes to maintain proper electrochemical gradient, which, among other things, is needed for degrading fatty acids via β-oxidation. Fat metabolism within liver therefore decreases.
Excess intake
Excessive doses of choline can have adverse effects. Daily 8–20 gram doses of choline, for example, have been found to cause low blood pressure, nausea, diarrhea and fish-like body odor. The odor is due to trimethylamine (TMA) formed by the gut microbes from the unabsorbed choline (see trimethylaminuria).
The liver oxidizes TMA to trimethylamine N-oxide (TMAO). Elevated levels of TMA and TMAO in the body have been linked to increased risk of atherosclerosis and mortality. Thus, excessive choline intake has been suggested to increase these risks in addition to carnitine, which also forms TMA and TMAO.
However, it is plausible that elevated TMA and TMAO levels are just a
symptom of other underlying illnesses or genetic factors that predispose
individuals for increased mortality. Such factors may have not been
properly accounted for in certain studies observing TMA and TMAO level
related mortality. Causality may be reverse or confounding and large
choline intake might not increase mortality in humans. For example, kidney dysfunction predisposes for cardiovascular diseases, but can also decrease TMA and TMAO excretion.
Health effects
Neural tube closure
Some human studies showed low maternal intake of choline to significantly increase the risk of neural tube defects (NTDs) in newborns. Folate deficiency also causes NTDs. Choline and folate, interacting with vitamin B12, act as methyl donors to homocysteine to form methionine, which can then go on to form SAM (S-adenosyl methionine).
SAM is the substrate for almost all methylation reactions in mammals.
It has been suggested that disturbed methylation via SAM could be
responsible for the relation between folate and NTDs. This may also apply to choline. Certain mutations
that disturb choline metabolism increase the prevalence of NTDs in
newborns, but the role of dietary choline deficiency remains unclear, as
of 2015.
Cardiovascular diseases and cancer
Choline deficiency can cause fatty liver, which increases cancer and cardiovascular disease risk. Choline deficiency also decreases SAM production, which partakes in DNA methylation – this decrease may also contribute to carcinogenesis. Thus, deficiency and its association with such diseases has been studied. However, observational studies
of free populations have not convincingly shown an association between
low choline intake and cardiovascular diseases or most cancers. Studies on prostate cancer have been contradictory.
Cognition
Studies observing the effect between higher choline intake and cognition have been conducted human adults, with contradictory results. Similar studies on human infants and children have been contradictory and also limited.
Pregnancy and brain development
Both pregnancy and lactation increase demand for choline dramatically. This demand may be met by upregulation of PEMT via increasing estrogen levels to produce more choline de novo,
but even with increased PEMT activity, the demand for choline is still
so high that bodily stores are generally depleted. This is exemplified
by the observation that Pemt -/- mice (mice lacking functional PEMT) will abort at 9–10 days unless fed supplemental choline.
While maternal stores of choline are depleted during pregnancy
and lactation, the placenta accumulates choline by pumping choline
against the concentration gradient into the tissue, where it is then
stored in various forms, mostly as acetylcholine. Choline concentrations
in amniotic fluid can be ten times higher than in maternal blood.
Functions in the fetus
Choline
is in high demand during pregnancy as a substrate for building cellular
membranes, (rapid fetal and mother tissue expansion), increased need
for one-carbon moieties (a substrate for addition of methylation to DNA
and other functions), raising choline stores in fetal and placental
tissues, and for increased production of lipoproteins (proteins
containing "fat" portions).
In particular, there is interest in the impact of choline consumption
on the brain. This stems from choline's use as a material for making
cellular membranes, (particularly in making phosphatidylcholine). Human
brain growth is most rapid during the third trimester of pregnancy and
continues to be rapid to approximately five years of age.
During this time, the demand is high for sphingomyelin, which is made
from phosphatidyl choline (and thus from choline), because this material
is used to myelinate (insulate) nerve fibers.
Choline is also in demand for the production of the neurotransmitter
acetylcholine, which can influence the structure and organization of
brain regions, neurogenesis, myelination, and synapse formation.
Acetylcholine is even present in the placenta and may help control cell
proliferation/differentiation (increases in cell number and changes of
multiuse cells into dedicated cellular functions) and parturition.
Choline uptake into the brain is controlled by a low-affinity transporter located at the blood-brain barrier.
Transport occurs when arterial plasma choline concentrations increase
above 14 μmol/l, which can occur during a spike in choline concentration
after consuming choline-rich foods. Neurons, conversely, acquire
choline by both high- and low-affinity transporters. Choline is stored
as membrane-bound phosphatidylcholine, which can then be used for
acetylcholine neurotransmitter synthesis later. Acetylcholine is formed
as needed, travels across the synapse, and transmits the signal to the
following neuron. Afterwards, acetylcholinesterase degrades it, and the
free choline is taken up by a high-affinity transporter into the neuron
again.
Hundreds of choline antagonists and enzyme inhibitors have been developed for research purposes. Aminomethyl propanol
is among the first ones used as a research tool. It inhibits choline
and trimethylglycine synthesis. It is able to induce choline deficiency
that in turn results in fatty liver in rodents. Diethanolamine is another such compound, but also an environmental pollutant. N-cyclohexylcholine inhibits choline uptake primarily in brains. Hemicholinium-3 is a more general inhibitor, but also moderately inhibits choline kinases. More specific choline kinase inhibitors have also been developed. Trimethylglycine synthesis inhibitors also exists: carboxybutylhomocysteine is an example of a specific BHMT inhibitor.
In 1849, Adolph Strecker was the first to isolate choline from pig bile. In 1852, L. Babo and M. Hirschbrunn extracted choline from white mustard seeds and named it sinkaline. In 1862, Strecker repeated his experiment with pig and ox bile, calling the substance choline for the first time after the Greek word for bile, chole, and identifying it with the chemical formula C5H13NO. In 1850, Theodore Nicolas Gobley extracted from the brains and roe of carps a substance he named lecithin after the Greek word for egg yolk, lekithos, showing in 1874 that it was a mixture of phosphatidylcholines.
In 1865, Oscar Liebreich isolated "neurine" from animal brains. The structural formulas of acetylcholine and Liereich's "neurine" were resolved by Adolf von Baeyer in 1867. Later that year "neurine" and sinkaline were shown to be the same substances as Strecker's choline. The compound now known as neurine is unrelated to choline.
Discovery as a nutrient
In the early 1930s, Charles Best and colleagues noted that fatty liver in rats on a special diet and diabetic dogs could be prevented by feeding them lecithin, proving in 1932 that choline in lecithin was solely responsible for this preventive effect. In 1998, the US National Academy of Medicine reported their first recommendations for choline in the human diet.
