Vitamin B6: the pathway in the body · Biomarker & Blood Analysis

This page shows the biochemical pathway behind the laboratory value Vitamin B6 (pyridoxine / P5P): 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

Vitamin B6 is a collective name for six related compounds; the active form is called pyridoxal phosphate (PLP). PLP sits in the active site of many enzymes that convert amino acids - transaminases, decarboxylases and cystathionine beta-synthase among them. Little PLP slows them.

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. B6 in food → Free B6 forms Alkaline phosphatase Enzymes on the gut wall remove the phosphate group. Only the free forms can pass through the wall of the intestinal cell. This is how B6 from food reaches the blood. Source 1↑ supplies Removing the phosphate group opens the way into the blood: only uncharged, free forms pass through the gut wall and on to the liver with the portal vein blood. established physiology Source 1
   ⚖ When the balance tips

   too much — If large amounts of free forms arrive, they are largely absorbed; uptake is hardly limited, and the excess reaches the liver with the portal vein blood.

   too little — If little B6 reaches the gut, fewer free forms pass through the gut wall, and the liver receives less supply.

   established physiology · Source 1

2. Free B6 forms → Uptake into the liver The free forms reach the liver via the portal vein. The liver is the central conversion site for vitamin B6. From here it supplies the other tissues. Source 1, 2↑ supplies The free forms converge in the liver; here most of it is converted to PLP and released into the blood, and part is broken down to 4-pyridoxic acid. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If more arrives than the liver converts into PLP, it increasingly breaks the excess down to 4-pyridoxic acid, which leaves with the urine.

   too little — If little arrives, the liver has less to convert, releases less PLP into the blood and also breaks down less.

   established physiology · Source 1, 2

3. Uptake into the liver → PLP, pyridoxal phosphate Pyridoxal kinase · ATP, zinc Pyridoxal kinase uses ATP to attach a phosphate group. A second reaction by the enzyme PNPO, which needs FMN derived from vitamin B2, leads to PLP, the active form. With little FMN, PLP forms more slowly. Source 1, 2↑ supplies PLP is the form that enzymes actually use. Pyridoxal kinase needs ATP and zinc, PNPO needs FMN from vitamin B2; only both steps together provide PLP from all dietary forms. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — Free PLP is reactive; the cell keeps it mostly bound to proteins. If PLP builds up, it slows PNPO, and phosphatases cleave excess PLP back to pyridoxal.

   too little — If little FMN from vitamin B2 is available, PNPO cannot keep up, and PLP forms more slowly from pyridoxine and pyridoxamine. If hydrazine compounds inhibit pyridoxal kinase, less is formed as well.

   established physiology · Source 1, 2

4. PLP, pyridoxal phosphate → PLP in the blood · albumin The liver releases PLP into the blood, where it circulates bound to albumin. Before entering a cell, a phosphatase removes the phosphate group; inside, it is attached again. This is how PLP reaches the other tissues. Source 1↑ supplies In the blood, PLP is bound to albumin and is thus broken down more slowly. At the cell, a phosphatase cleaves off the phosphate; pyridoxal enters and is turned back into PLP inside. established physiology Source 1
   ⚖ When the balance tips

   too much — If there is a lot of PLP in the blood, more binding sites on albumin are occupied; unbound PLP is cleaved more quickly to pyridoxal and goes into the cells or into breakdown.

   too little — If there is little PLP in the blood, the tissues receive less supply. During inflammatory processes, PLP in the blood has been described to fall because more of it is shifted into tissues.

   observed in studies · Source 1, 2

5. PLP in the blood → 4-pyridoxic acid Aldehyde oxidase Whatever is not needed is broken down by the liver via aldehyde oxidase to 4-pyridoxic acid. This form leaves the body in the urine. When a lot of B6 arrives, more of it appears in the urine. Source 2↓ depletes The formation of 4-pyridoxic acid removes B6 from circulation for good: aldehyde oxidase oxidises pyridoxal, and this form does not become PLP again. established physiology Source 2
   ⚖ When the balance tips

   too much — If a lot of B6 arrives, the amount of 4-pyridoxic acid in the urine rises; breakdown carries the excess out of the body.

   too little — If little B6 is available, less is broken down to 4-pyridoxic acid, and less of it appears in the urine.

   established physiology · Source 2

6. Alanine → Pyruvate ALT · alpha-ketoglutarate ALT transfers the amino group from alanine to alpha-ketoglutarate. What remains is pyruvate, a hub of sugar metabolism. Pyruvate serves as fuel or as a building block for glucose. Source 1↑ supplies Pyruvate is both fuel and building block: it goes into the mitochondria for energy production or serves in the liver as starting material for new glucose. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of pyruvate forms, more of it goes into glucose synthesis in the liver or into the energy pathway of the mitochondria.

   too little — If ALT runs more slowly, less pyruvate forms from alanine; the cell then obtains it mainly from sugar breakdown.

   established physiology · Source 1

7. L-DOPA → Dopamine AADC The decarboxylase AADC turns L-DOPA into dopamine and needs PLP to do so. By the same pattern, serotonin is formed from 5-HTP and GABA from glutamate. In this way PLP provides messengers of the nervous system. Source 1↑ supplies Dopamine, serotonin and GABA are messengers of the nervous system; all three are formed from their precursors by PLP-dependent decarboxylases. established physiology Source 1
   ⚖ When the balance tips

   too much — If plenty of PLP and precursor are available, the decarboxylations run up to the capacity of the enzymes; more PLP does not speed them up beyond that.

   too little — If little PLP is available, the decarboxylations to dopamine, serotonin and GABA run more slowly, and less of these messengers is formed.

   established physiology · Source 1, 2

8. Homocysteine → Cystathionine Cystathionine synthase · serine Cystathionine beta-synthase joins homocysteine to serine. This enzyme also works with PLP; the product leads on to cysteine. With little PLP, homocysteine is converted to cysteine more slowly. Source 4, 1↑ supplies Cystathionine is split into cysteine by cystathionine gamma-lyase, which is also PLP-dependent. Cysteine is a building block of proteins and of glutathione. established physiology Source 4
   ⚖ When the balance tips

   too much — If a lot of cystathionine is present, cystathionine gamma-lyase splits it into cysteine as far as PLP is available; cysteine is then available for proteins and glutathione.

   too little — If little PLP is available, both enzymes of this pathway work more slowly, and homocysteine is converted to cysteine more slowly.

   established physiology · Source 4

Further stations

• B6 in food — pyridoxine, pyridoxal etc. Vitamin B6 is a collective name for six related substances. Plant foods contain mostly pyridoxine, animal foods pyridoxal and pyridoxamine; they usually carry a phosphate group. All forms serve as starting material for PLP. Source 1↑ supplies All six forms are starting materials for PLP: pyridoxine, pyridoxal and pyridoxamine are brought into the active form in the liver via pyridoxal kinase and PNPO. established physiology Source 1
  ⚖ When the balance tips

  too much — If very large amounts of pyridoxine arrive, it must first become PLP via PNPO; free pyridoxine stays longer in the blood and, in cell experiments, competes with PLP for binding sites. Sensory nerves react to very large amounts.

  too little — If little B6 arrives, the liver receives less starting material, and the pool of PLP for the enzymes becomes smaller.

  observed in studies · Source 2, 1

• PLP in the cell — cofactor of many enzymes In the cell, PLP sits in the active site of more than a hundred enzymes. It binds the amino group of an amino acid and holds the intermediate in place – hence the large number of reactions. With little PLP, many of these reactions run more slowly. Source 1, 2↑ supplies PLP binds as a Schiff base to the amino group of an amino acid and holds electrons in place. This lets enzymes shift amino groups, split off carboxyl groups or rebuild side chains. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If more PLP is available than the enzymes bind, their activity does not rise further; unbound PLP is cleaved by phosphatases.

  too little — If little PLP is available, the pool is spread across many enzymes; transaminations and decarboxylations run more slowly.

  established physiology · Source 1, 2

• Alanine — amino acid Transaminases shift amino groups between amino acids and keto acids. Alanine is a common example; the enzymes ALT and AST work on this pattern. They need PLP to do so. Source 1↑ supplies Alanine supplies its amino group: ALT passes it via PLP to alpha-ketoglutarate, which becomes glutamate. Nitrogen and carbon skeleton thus move on separately. established physiology Source 1
  ⚖ When the balance tips

  too much — If a lot of alanine accumulates, ALT converts it as long as PLP and alpha-ketoglutarate are available; more amino groups then pass to glutamate.

  too little — If little PLP is available, ALT loses part of its activity, and alanine is converted more slowly.

  established physiology · Source 1

• L-DOPA — precursor of a messenger Decarboxylases remove the carboxyl group from an amino acid. L-DOPA is one example, as are the precursors of serotonin and GABA. With little PLP, these steps run more slowly. Source 1, 2↑ supplies L-DOPA is the precursor from which the PLP-dependent AADC makes dopamine. How fast this happens depends on how much PLP sits on the enzyme. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If a great deal of PLP is available, AADC decarboxylates L-DOPA already outside the brain; for L-DOPA as a drug it has been described that less of it then reaches the brain.

  too little — If little PLP is available, L-DOPA is converted more slowly and remains longer as a precursor.

  established physiology · Source 3, 1

• Homocysteine — amino acid Homocysteine arises when methionine is converted. It stands at a fork in the road: back to methionine or on towards cysteine. The path to cysteine needs PLP. Source 4↑ supplies Homocysteine is a transit station for sulphur: with vitamin B12 and folate it returns to methionine, via the PLP-dependent cystathionine beta-synthase it goes towards cysteine. established physiology Source 4
  ⚖ When the balance tips

  too much — If homocysteine builds up, more of it is passed on via cystathionine beta-synthase, provided PLP and serine are available; if that is not enough, more of it enters the blood.

  too little — If little homocysteine forms, for instance because little methionine is converted, less sulphur flows towards cysteine as well.

  established physiology · Source 4

Cofactors in this pathway

• ATP — Phosphate donor of pyridoxal kinase, which makes PLP out of pyridoxal; without ATP no PLP forms Source 1, 2
• Zinc — Metal partner of pyridoxal kinase in this step; this yields the phosphate form Source 1, 2In the ORY catalogue as a laboratory value: Zink
• Vitamin B2 (FMN) — Flavin cofactor of PNPO, which converts pyridoxine and pyridoxamine phosphate into PLP; slower with little FMN Source 1, 2
• Alpha-ketoglutarate — Takes up the amino group of alanine in the ALT reaction and becomes glutamate Source 1In the ORY catalogue as a laboratory value: Alpha-Ketoglutarat (2-Oxoglutarat)
• Serine — Partner of homocysteine in the PLP-dependent reaction to cystathionine; this leads on to cysteine Source 4, 1In the ORY catalogue as a laboratory value: Serin
• Albumin — Carrier protein that PLP is bound to as it travels through the blood to the tissues; free PLP is cleaved Source 1

What acts on this pathway

• Hydrazine compounds — For drugs with a hydrazine group, such as isoniazid, it has been described that they bind tightly to pyridoxal and inhibit pyridoxal kinase. Source 3

Sources

1. Parra M, Stahl S, Hellmann H. Vitamin B6 and Its Role in Cell Metabolism and Physiology. Cells 2018 · PubMed 30037155
2. Stach K, Stach W, Augoff K. Vitamin B6 in Health and Disease. Nutrients 2021 · PubMed 34579110
3. Ebadi M, Gessert CF, Al-Sayegh A. Drug-pyridoxal phosphate interactions. Q Rev Drug Metab Drug Interact 1982 · PubMed 6087425
4. Froese DS, Fowler B, Baumgartner MR. Vitamin B12, folate, and the methionine remethylation cycle - biochemistry, pathways, and regulation. J Inherit Metab Dis 2019 · PubMed 30693532

Related pathways

• Dopamine, noradrenaline, adrenaline — l-dopa
• Methylation: methionine and homocysteine — homocysteine
• Alanine — alanine
• Glutathione — homocysteine
• Liver enzymes — alanine

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