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.

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.

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.

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.

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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.

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) (PubChem, NCBI, 2026) · Public domain; Vitamin B6: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Pyridoxine-dependent epilepsy: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Homocysteine Test: MedlinePlus Medical 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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