Pyridoxine (Vitamin B6)

PYRIDOXINE (VITAMIN B6)

Vitamin B₆ is a collective term for pyridoxine, pyridoxal, and pyridoxamine, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring (Figure 28.10). Pyridoxine occurs primarily in plants, whereas pyridoxal and pyridoxamine are found in foods obtained from animals. All three compounds can serve as precursors of the biologically active coenzyme, pyridoxal phosphate (PLP). PLP functions as a coenzyme for a large number of enzymes, particularly those that catalyze reactions involving amino acids, for example, in the synthesis of cysteine from Hcy. [Note: PLP is also required by glycogen phosphorylase.]

Figure 28.10 Structures of vitamin B6 and the antituberculosis drug isoniazid.

Reaction type - Example

Transamination : Oxaloacetate + glutamate ↔ aspartate + α- ketoglutarate

Deamination : Serine → pyruvate + NH₃

Decarboxylation : Histidine → histamine + CO₂

Condensation : Glycine + succinyl CoA → δ-aminolevulinic acid

A. Clinical indications for pyridoxine

Isoniazid, a drug commonly used to treat tuberculosis, can induce a vitamin B₆ deficiency by forming an inactive derivative with PLP. Dietary supplementation with B 6 is, thus, an adjunct to isoniazid treatment. Otherwise, dietary deficiencies in pyridoxine are rare but have been observed in newborn infants fed formulas low in B6, in women taking oral contraceptives, and in alcoholics.

B. Toxicity of pyridoxine

Pyridoxine is the only water-soluble vitamin with significant toxicity. Neurologic symptoms (sensory neuropathy) occur at intakes above 500 mg/day, an amount nearly 400 times the Recommended Dietary Allowance (RDA) and over 5 times the Tolerable Upper Limit (UL). Substantial improvement, but not complete recovery, occurs when the vitamin is discontinued.