Vitamin B6: the pathway in the body

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.

The pathway step by step

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. 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. 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. 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. Source 1
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. 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. 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. Source 1
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. Source 4, 1

Cofactors in this pathway

• ATP — Phosphate donor of pyridoxal kinase, which makes PLP out of pyridoxal Source 1, 2
• Zinc — Metal partner of pyridoxal kinase in this step 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 Source 1, 2
• Alpha-ketoglutarate — Takes up the amino group of alanine in the ALT reaction Source 1In the ORY catalogue as a laboratory value: Alpha-Ketoglutarat (2-Oxoglutarat)
• Serine — Partner of homocysteine in the PLP-dependent reaction to cystathionine 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 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
• Vitamin B12 — homocysteine

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