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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Vitamin B6 (pyridoxal phosphate)

Vitamin B6, as pyridoxal phosphate, is the coenzyme of amino acid metabolism: transamination, neurotransmitters and heme.

Updated 2026-11-01

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## Key facts

- Active form: pyridoxal 5′-phosphate (PLP), which forms a Schiff base with the amino acid's amino group.
- PLP reactions: transamination (ALT, AST), decarboxylation (dopa, glutamate → GABA, histidine → histamine, 5-HTP → serotonin) and heme synthesis (ALA synthase).
- Also used by glycogen phosphorylase, cystathionine β-synthase (the homocysteine pathway) and kynureninase (tryptophan → niacin).
- Isoniazid binds pyridoxine and causes deficiency: peripheral neuropathy, seizures, sideroblastic anaemia.

## In detail

- Sources: meat, fish, poultry, potatoes, bananas, chickpeas, fortified cereals.
- Activation: in the liver pyridoxal kinase phosphorylates the three forms (pyridoxine, pyridoxal, pyridoxamine) and an FMN-dependent oxidase turns them into PLP.
- Mechanism: the PLP aldehyde binds the amino acid; the pyridine ring pulls electrons and helps break whichever bond the enzyme chooses (Cα–H, Cα–COOH or Cα–side chain).
- Without enough GABA (glutamate decarboxylase needs PLP) seizures occur, especially in infants.
- Without active ALA synthase less heme is made: microcytic anaemia with ring sideroblasts.
- Causes of deficiency: isoniazid, penicillamine, hydralazine, alcoholism, kidney disease on dialysis, oral contraceptives (mild).
- Excess (high-dose supplements for months): sensory neuropathy with ataxia, which improves when they are stopped.
- Lab: plasma PLP; homocysteine can rise because the transsulphuration pathway needs PLP.

**Also called**
Pyridoxine, Pyridoxal 5′-phosphate (PLP), Pyridoxamine

**Formula**
C₈H₁₀NO₆P

**PubChem**
[CID 1051](https://pubchem.ncbi.nlm.nih.gov/compound/1051)

## Structure

- A pyridine ring with a hydroxyl, a methyl, a phosphate on carbon 5′ and, on carbon 4, an aldehyde (pyridoxal), alcohol (pyridoxine) or amine (pyridoxamine).
- In transamination PLP becomes pyridoxamine phosphate and then returns to PLP: a coenzyme that goes in and out of the reaction.

## Clinical notes

- People receiving isoniazid for tuberculosis usually also receive pyridoxine to prevent neuropathy.
- Pyridoxine-dependent epilepsy (ALDH7A1 gene) causes neonatal seizures that stop only with B6.
- In isoniazid poisoning with seizures, intravenous pyridoxine is the antidote.

## Related conditions

- [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/)
- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)

## Related

- [Niacin (vitamin B3)](https://durgical.com/en/biochemistry/molecules/niacin/)
- [Riboflavin (vitamin B2)](https://durgical.com/en/biochemistry/molecules/riboflavin/)
- [Folate (vitamin B9)](https://durgical.com/en/biochemistry/molecules/folate/)
- [Cobalamin (vitamin B12)](https://durgical.com/en/biochemistry/topics/cobalamin/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)

## Test yourself

Think of the answer, then tap the question to see it.

## What is the active form of vitamin B6?

Pyridoxal 5′-phosphate (PLP), which forms a Schiff base with amino acids.

## Why can isoniazid cause seizures?

It depletes vitamin B6; without PLP glutamate decarboxylase makes less GABA, the main inhibitory neurotransmitter.

## Which anaemia does B6 deficiency cause, and why?

Microcytic sideroblastic anaemia: ALA synthase, the first step of heme synthesis, needs PLP, and iron builds up in mitochondria.

## What harm does excess vitamin B6 cause?

A sensory neuropathy with ataxia, from high-dose supplements over months; it improves when they are stopped.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Pyridoxal 5'-phosphate (CID 1051)](https://pubchem.ncbi.nlm.nih.gov/compound/1051) (PubChem, NCBI, 2026) · Public domain; [Vitamin B6: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002402.htm) (MedlinePlus, US National Library of Medicine, 2026); [Pyridoxine-dependent epilepsy: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/pyridoxine-dependent-epilepsy/) (MedlinePlus, US National Library of Medicine, 2026); [Homocysteine Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/homocysteine-test/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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