Vitamin B6: coenzyme forms, metabolic roles, and deficiency
THE CONTEXT
Vitamin B6 functions through pyridoxal phosphate in amino acid metabolism, glycogen metabolism, and regulation of steroid hormone action.
Forms and muscle reserve
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Six compounds have vitamin B6 activity: pyridoxine, pyridoxal, pyridoxamine, and their respective 5′-phosphates. The active coenzyme is pyridoxal 5′-phosphate.
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About 80% of total body vitamin B6 is muscle pyridoxal phosphate, mostly associated with glycogen phosphorylase.
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This muscle pool is unavailable during deficiency. During starvation, glycogen depletion releases it, making it available especially to the liver and kidney for increased gluconeogenesis from amino acids.
Metabolic and hormonal functions
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Pyridoxal phosphate serves as a coenzyme in amino acid metabolism, especially transamination and decarboxylation.
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It is also a cofactor of glycogen phosphorylase, where its phosphate group is catalytically important.
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Pyridoxal phosphate removes hormone–receptor complexes from DNA binding and terminates hormone action. Vitamin B6 deficiency increases sensitivity to low concentrations of estrogens, androgens, cortisol, and vitamin D.
Deficiency
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Clinical deficiency disease is rare, although a significant proportion of the population may have marginal vitamin B6 status.
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Moderate deficiency causes abnormalities of tryptophan and methionine metabolism.
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Increased sensitivity to steroid hormones may contribute to hormone-dependent cancers of the breast, uterus, and prostate; vitamin B6 status may affect prognosis.