Taurine: the pathway in the body · Biomarker & Blood Analysis

This page shows the biochemical pathway behind the laboratory value Taurine: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Taurine is a sulfur-containing amino sulfonic acid that the body makes from cysteine and also absorbs from food. It is attached to bile acids and, as a dissolved substance, regulates the water balance of cells. With little taurine, the liver binds glycine instead.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Methionine → Cysteine CBS and CSE · Vitamin B6 (PLP) Two enzymes with vitamin B6 as cofactor, cystathionine β-synthase and cystathionine γ-lyase, convert homocysteine via cystathionine into cysteine. When little cysteine is available, it is kept mainly for proteins and glutathione. Source 1↕ both, depending on amount Cysteine is the hub: it goes into proteins and glutathione or is broken down to taurine and sulfate. Free cysteine is highly reactive; through breakdown the cell keeps its amount low. established physiology Source 1
   ⚖ When the balance tips

   too much — If cysteine builds up, the cell makes more cysteine dioxygenase, which breaks it down faster; more then flows towards taurine and sulfate. This is described mainly in animal models.

   too little — If little cysteine is available, cysteine dioxygenase is broken down quickly; cysteine is then kept mainly for proteins and glutathione, and less goes to taurine.

   observed in studies · Source 1

2. Cysteine → Cysteine sulfinic acid Cysteine dioxygenase · Iron, oxygen Cysteine dioxygenase attaches two oxygen atoms to the sulfur. The enzyme contains iron and controls how much cysteine flows towards taurine. When cysteine builds up, the cell makes more of this enzyme. Source 1, 2↑ supplies Cysteine sulfinic acid is a branch point: the decarboxylase turns it into hypotaurine and thus taurine, a transaminase leads it to pyruvate and sulfite. The cysteine dioxygenase before it determines how much forms at all. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of cysteine sulfinic acid forms, part of it goes via the transaminase to pyruvate and sulfite instead of hypotaurine; how the amount is split depends on the activity of the decarboxylase.

   too little — If little cysteine sulfinic acid forms because cysteine dioxygenase is turned down, the cell makes hardly any taurine itself; the taurine then comes mainly from food.

   observed in studies · Source 1, 2

3. Cysteine sulfinic acid → Hypotaurine CSAD (decarboxylase) · Vitamin B6 (PLP) Cysteine sulfinic acid decarboxylase removes the carboxyl group; it too works with pyridoxal phosphate, the active form of vitamin B6. This step limits how fast taurine forms. Source 2, 3↑ supplies Hypotaurine is the last precursor; FMO1 oxidises it to taurine. The decarboxylase before it is regarded as the step that limits the speed of taurine formation. established physiology Source 2, 3
   ⚖ When the balance tips

   too much — If hypotaurine builds up, the cause lies in its oxidation to taurine: when FMO1 activity is absent in animal models, hypotaurine accumulates and less taurine forms.

   too little — If little PLP is available, the decarboxylase works more slowly and less hypotaurine forms; the enzymes on the way to cysteine depend on PLP in the same way.

   observed in studies · Source 3, 2

4. Hypotaurine → Taurine FMO1 · NADPH, oxygen The flavin monooxygenase FMO1 oxidises hypotaurine to taurine. In addition, a large share of the body’s taurine comes directly from meat, fish and seafood. Taurine itself is not built into proteins. Source 3↑ supplies Taurine is the end product of the pathway and is not built into proteins. The body distributes it via the blood to the tissues or attaches it to bile acids in the liver. established physiology Source 5, 4
   ⚖ When the balance tips

   too much — If a lot of taurine arrives, the kidney releases more of it into the urine, and cells make less of the transporter TauT; the store in the tissues thus stays the same.

   too little — If little taurine arrives with food, the store depends more on the body's own formation from cysteine and on how much the kidney retains.

   observed in studies · Source 2, 5

5. Taurine in the blood → In the liver cell In the liver, taurine is attached to bile acids. When little taurine is available, the enzyme BAAT attaches glycine instead. Source 4↑ supplies In the liver cell, taurine is available for binding to bile acids; the same enzyme BAAT can attach glycine instead of taurine. In humans both forms are made. established physiology Source 4
   ⚖ When the balance tips

   too much — If there is a lot of taurine in the liver cell, a larger share of taurine-conjugated bile salts forms compared with glycine-conjugated ones.

   too little — If little taurine is available, BAAT attaches more glycine; the bile acids are still conjugated, just with the other partner.

   observed in studies · Source 4

6. Taurine in the blood → In other cells In muscle, heart, retina and white blood cells, taurine is present in large amounts in free, dissolved form and is not built into proteins. The cell uses this store to balance its volume. Source 5↑ supplies Freely dissolved taurine forms a large store in muscle, heart and retina, which the cell uses to balance its volume. Because it is not built into proteins, the cell can release it quickly and absorb it again. established physiology Source 5
   ⚖ When the balance tips

   too much — If the store is full, the cell absorbs hardly any more via TauT; further taurine then stays in the blood.

   too little — If the store is small, the cell has less dissolved substance to release when it swells, and balances its volume more through other osmolytes and ions.

   observed in studies · Source 5

7. Cholyl-CoA → Taurocholic acid BAAT The enzyme BAAT attaches taurine to the bile acid. As a result the bile salt stays dissolved even in the acidic small intestine. It is therefore hardly absorbed in the upper small intestine. Source 4↑ supplies Binding to taurine turns the bile acid into a permanently charged bile salt: it dissolves well and is hardly absorbed passively in the upper small intestine, so it reaches the fats. established physiology Source 4
   ⚖ When the balance tips

   too much — If a lot of conjugated bile salt is present, BSEP exports it into the bile; what returns from the gut at the same time turns down new formation in the liver.

   too little — If little bile acid is conjugated, more of it stays uncharged; such bile acids are absorbed passively more easily in the gut and are less available for fat digestion.

   observed in studies · Source 4

8. Taurocholic acid → Bile · BSEP transporter The bile salts reach the gut with the bile. There they coat fat droplets so that the enzymes of the pancreas can act on them. Most of them are reclaimed in the lower small intestine. Source 4↑ supplies Bile salts form micelles with fats and thus make fats and fat-soluble vitamins accessible for absorption. In the lower small intestine most of them are reclaimed and return to the liver. established physiology Source 4
   ⚖ When the balance tips

   too much — If more bile salts reach the large intestine than are reclaimed, gut bacteria split off the taurine and convert the bile acids into secondary bile acids.

   too little — If few bile salts reach the gut, fats are dissolved less well in micelles, and more fat and fat-soluble vitamins pass on undigested.

   established physiology · Source 4

9. In other cells → Osmolyte in the cell Taurine is one of the osmolytes: when a cell swells, it releases taurine through dedicated channels and water follows. In this way the cell restores its volume. When it shrinks, it draws taurine in via TauT. Source 5↕ both, depending on amount Taurine acts as an osmolyte in both directions: when the cell swells, it releases taurine and water follows outwards; when it shrinks, it draws taurine in via TauT and retains water. established physiology Source 5
   ⚖ When the balance tips

   too much — If there is a lot of taurine in the cell, it holds water; if the cell then swells, it opens volume-sensitive channels and releases taurine until its volume is right again.

   too little — If there is little taurine in the cell, it has one osmolyte fewer that it can accumulate without disturbing its proteins; it then balances its volume more through ions such as potassium and chloride.

   observed in studies · Source 5

Further stations

• Methionine — sulfur-containing amino acid The sulfur in taurine comes from dietary amino acids. Methionine passes it on to cysteine via the transsulfuration pathway. It thus helps set how much sulfur is available for taurine. Source 1↑ supplies Methionine is the sulfur donor of the pathway: via homocysteine and transsulfuration it passes its sulfur atom on to cysteine. How much of it ends up in taurine depends on how much cysteine the cell needs for other purposes. established physiology Source 1
  ⚖ When the balance tips

  too much — If a lot of methionine arrives, more sulfur runs via transsulfuration to cysteine; the cell breaks the excess down to taurine and sulfate and releases it in this way.

  too little — If little methionine arrives, the cell tends to keep homocysteine in the methylation cycle, and less sulfur flows towards cysteine and taurine.

  established physiology · Source 1

• Taurine in the blood — uptake via TauT Cells actively import taurine with the transporter TauT, together with sodium and chloride. This is why cells contain far more taurine than the blood. When little taurine is present, cells make more TauT. Source 2, 5↑ supplies TauT pumps taurine into cells against a steep gradient; the energy for this comes from the sodium gradient. How much TauT a cell makes determines how much taurine it can hold. established physiology Source 2, 5
  ⚖ When the balance tips

  too much — If a lot of taurine is present, cells in cell culture make less TauT, and uptake is turned down.

  too little — If little taurine is present or the osmotic pressure outside the cell rises, cells make more TauT and draw in the available taurine more strongly.

  observed in studies · Source 2, 5

• Cholyl-CoA — activated bile acid The liver makes bile acids from cholesterol and activates them with coenzyme A. Only then can BAAT attach taurine or glycine. Source 4↑ supplies Binding to coenzyme A makes the bile acid ready to react; only then can BAAT attach taurine or glycine. Coenzyme A is released again in the process. established physiology Source 4
  ⚖ When the balance tips

  too much — If a lot of bile acid is made, the bile acids returning from the gut inhibit the first enzyme CYP7A1 via feedback, so that less is formed from cholesterol.

  too little — If few bile acids return from the gut, the feedback is weaker, and the liver makes more bile acid from cholesterol.

  established physiology · Source 4

Cofactors in this pathway

• Vitamin B6 (PLP) — Cofactor of CBS and CSE on the way to cysteine, and of cysteine sulfinate decarboxylase; without PLP the path stalls Source 1, 2In the ORY catalogue as a laboratory value: Vitamin B6
• Iron — Metal at the active site of cysteine dioxygenase; it sets how much cysteine flows to taurine Source 1In the ORY catalogue as a laboratory value: Eisen
• Methionine — Supplies, via transsulfuration, the sulfur contained in taurine; with little supply, less sulfur reaches cysteine Source 1In the ORY catalogue as a laboratory value: Methionin
• Vitamin B2 (FAD) — Component of the flavin monooxygenase FMO1, which oxidises hypotaurine to taurine; without FAD no oxidation Source 3
• Coenzyme A — Activates bile acids before BAAT attaches taurine; only then is the bile acid ready to react Source 4
• Sodium — The transporter TauT carries taurine into the cell together with sodium and chloride; sodium drives the uptake Source 2, 5In the ORY catalogue as a laboratory value: Natrium (intrazellulär)

Sources

1. Stipanuk MH, Ueki I. Dealing with methionine/homocysteine sulfur: cysteine metabolism to taurine and inorganic sulfur. J Inherit Metab Dis 2011 · PubMed 20162368
2. Tappaz ML. Taurine biosynthetic enzymes and taurine transporter: molecular identification and regulations. Neurochem Res 2004 · PubMed 14992266
3. Miyazaki T. Identification of a novel enzyme and the regulation of key enzymes in mammalian taurine synthesis. J Pharmacol Sci 2024 · PubMed 38081683
4. Russell DW. The enzymes, regulation, and genetics of bile acid synthesis. Annu Rev Biochem 2003 · PubMed 12543708
5. Pasantes-Morales H. Taurine Homeostasis and Volume Control. Adv Neurobiol 2017 · PubMed 28828605

Related pathways

• Methylation: methionine and homocysteine — methionine
• Bile acids — bile
• Glutathione — cysteine
• Vitamin B12 — methionine
• Carnitine — Vitamin B6, Eisen

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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