vitamin B6
Summary
Vitamin B6 (pyridoxine) is a water-soluble vitamin that serves as a cofactor (as pyridoxal 5'-phosphate, PLP) for transamination, decarboxylation, and glycogen phosphorylase reactions. Deficiency classically causes sideroblastic anemia, peripheral neuropathy, seizures, and dermatitis/cheilosis/glossitis. It is notably associated with isoniazid therapy (INH depletes B6) and can also cause deficiency during pregnancy or with certain medications.
Detail
Pyridoxal 5'-phosphate (PLP), the active form of vitamin B6, is a critical cofactor for enzymes involved in amino acid metabolism, including transaminases (AST/ALT), decarboxylases (e.g., in neurotransmitter synthesis: GABA, serotonin, dopamine, histamine, epinephrine), and heme synthesis (ALA synthase). It is also a cofactor for glycogen phosphorylase and cystathionine synthase in homocysteine metabolism.
Deficiency: Causes include isoniazid (INH) therapy (inhibits pyridoxine phosphokinase and forms inactive hydrazones with PLP, necessitating prophylactic B6 supplementation), oral contraceptive use, alcoholism, and pregnancy. Deficiency leads to: - Sideroblastic anemia (impaired heme synthesis due to decreased ALA synthase activity) - Peripheral neuropathy (impaired neurotransmitter/myelin metabolism) - Seizures (decreased GABA synthesis) - Cheilosis, glossitis, stomatitis - Dermatitis (seborrheic-like rash) - Elevated homocysteine and increased risk of hyperhomocysteinemia-related complications
Toxicity: Excessive vitamin B6 supplementation can cause a reversible peripheral sensory neuropathy, distinct from the neuropathy seen in deficiency.
Clinical correlations for boards: Classic vignette is a patient on isoniazid for tuberculosis presenting with peripheral neuropathy or seizures, prevented/treated by pyridoxine supplementation. Also tested in the vitamin B6-dependent enzymes for neurotransmitter synthesis and its role as a cofactor in transamination reactions used to diagnose liver injury (AST/ALT).
Sources
- First Aid for the USMLE Step 1
- Lippincott Illustrated Reviews: Biochemistry
- Robbins Basic Pathology
- Katzung's Basic and Clinical Pharmacology
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