Vitamin B6 (Pyridoxine)

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Subject: Vitamin B6 (Pyridoxine)

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Definition

  • Vitamin B6 is a complex of six vitamers: pyridoxal, pyridoxol, pyridoxamine (pyridoxine), and their 5′-phosphate esters. Because of its role as a cofactor in a number of enzymatic reactions, pyridoxal phosphate (PLP) has been determined to be the biologically active form of vitamin B6. Vitamin B6 is important in heme synthesis and functions as a coenzyme in amino acid metabolism and glycogenolysis. Vitamin B6 deficiency is associated with symptoms of irritability, weakness, depression, dizziness, peripheral neuropathy, and seizures. In the pediatric population, deficiencies have been characterized by diarrhea, anemia, and seizures.

  • Normal range: 5–50 μg/L.

Use

  • Determining vitamin B6 status

  • Investigating suspected malabsorption or malnutrition

  • Determining the overall success of a vitamin B6 supplementation program

  • Diagnosis and evaluation of hypophosphatasia

Interpretation

Increased In

  • Hypophosphatasia

Decreased In

  • Alcoholism

  • Asthma

  • Carpal tunnel syndrome

  • Gestational diabetes

  • Lactation

  • Malabsorption

  • Malnutrition

  • Neonatal seizures

  • Normal pregnancies

  • Occupational exposure to hydrazine compounds

  • Pellagra

  • Preeclamptic edema

  • Renal dialysis

  • Uremia

Limitations

  • In addition to PLP, the following methods can be used to assess for vitamin B6 deficiency:

    • Erythrocyte transaminase activity with and without PLP added has been used as a functional test of pyridoxine status.

    • Urinary 4-pyridoxic acid excretion >3.0 mmol/day can be used as an indicator of adequate short-term vitamin B6 status.

    • Urinary excretion of xanthurenic acid is normally <65 mmol/day following a 2-g tryptophan load.

  • Drugs that may decrease vitamin B6 levels include amiodarone, anticonvulsants, cycloserine, disulfiram, ethanol, hydralazine, isoniazid, levodopa, oral contraceptives, penicillamine, pyrazinoic acid, and theophylline.

  • B6 may be decreased with pregnancy, lactation, alcoholism, DM, and in an uncommon B6 dependency state, vitamin B6–responsive neonatal convulsions. There is evidence of significant neurotoxicity associated with pyridoxine megavitaminosis; tingling, numbness, clumsiness, gait disturbances, and pseudoathetosis, with doses >2 g/day.

 
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