Category: biosynthesis
Vitamin B6 has 6 vitamers; the active coenzyme is pyridoxal 5'-phosphate (PLP). Cofactor for ~150 enzymes — virtually all aminotransferases, decarboxylases (AADC → catecholamines + serotonin, GAD → GABA), heme synthesis (ALA synthase, see heme_biosynthesis), glycogen phosphorylase, cystathionine β-synthase (CBS, methionine cycle transsulfuration), kynureninase (tryptophan kynurenine path). Deficiency: peripheral neuropathy + sideroblastic anemia + seizures (especially in neonates with PNPO deficiency or ALDH7A1 pyridoxine-dependent epilepsy — IV pyridoxine bolus is the diagnostic test). ISONIAZID + cycloserine + hydralazine deplete PLP (isoniazid forms a hydrazone) — pyridoxine 25-50 mg/d co-prescribed with INH for TB to prevent neuropathy. High-dose pyridoxine (>200 mg/d chronic) causes a sensory neuronopathy via paradoxical mechanism.