From the Guidelines
Vitamin B6 metabolism is a complex process involving the conversion of dietary vitamin B6 into its active form, pyridoxal phosphate (PLP), which serves as a coenzyme for over 160 enzymatic reactions, including those involved in carbohydrate, protein, and lipid metabolism. The metabolism of vitamin B6 involves several key steps, including absorption in the small bowel, where a phosphate group must be removed from the dietary vitamin, allowing it to be absorbed into the blood by passive diffusion 1. The biologically active form of vitamin B6, PLP, is then tightly bound to serum albumin and secreted into the circulatory blood system for delivery to various tissues and organs.
The main functions of vitamin B6 include:
- Transaminations, racemizations, decarboxylations, and aldol cleavage reactions
- Biosynthesis and degradation of amino acids
- Gluconeogenesis
- Steroid receptor binding
- Neurotransmitter synthesis
- Heme biosynthesis
The recommended daily allowance for adults ranges from 1.3-1.7 mg, with slightly higher needs during pregnancy and lactation, and the upper limit for pyridoxine is 100 mg/day for adults 1. Vitamin B6 deficiency can lead to neurological symptoms, dermatitis, and anemia, while excessive supplementation may cause peripheral neuropathy. Regular dietary intake of vitamin B6 is necessary, as the body cannot store large amounts of this vitamin. Food sources of vitamin B6 include meat, whole grains, fortified cereals, and potatoes 1.
In terms of metabolism, vitamin B6 is converted into its active form, PLP, through a series of enzymatic reactions, and excess vitamin B6 is excreted in urine as 4-pyridoxic acid after being metabolized by pyridoxal oxidase. The active form of vitamin B6, PLP, plays a crucial role in various bodily functions, and its metabolism is essential for maintaining overall health.
From the Research
Metabolism of Vitamin B6
- Vitamin B6 is present in the diet in many forms, but only pyridoxal 5'-phosphate (PLP) can function as a cofactor for enzymes 2
- The intestine absorbs nonphosphorylated B6 vitamers, which are converted by specific enzymes to the active PLP form 2
- PLP plays a crucial role in various pathways, including amino acid and neurotransmitter metabolism, folate and 1-carbon metabolism, protein and polyamine synthesis, carbohydrate and lipid metabolism, mitochondrial function, and erythropoiesis 2, 3
Vitamin B6 Metabolism in the Liver
- The B6 vitamers (pyridoxine, pyridoxamine, and pyridoxal) are primarily metabolized in the liver to pyridoxal 5'-phosphate (PLP) and the dead-end catabolite 4-pyridoxic acid 4
- The human liver enzymes responsible for vitamin B6 metabolism have been described, and a model of the relative rates of these interconversions in vivo has been developed 4
Disorders Affecting Vitamin B6 Metabolism
- Various inborn errors can lead to PLP deficiency, manifesting as B6-responsive epilepsy, usually of early onset 2, 5
- These disorders can be detected and distinguished by their respective biomarkers and genetic analysis, but metabolic biomarkers for early detection and prognosis of some disorders, such as PLPBP deficiency, are currently lacking 5
Phylogenetic Analyses and Comparative Genomics
- Phylogenetic analyses and comparative genomics of vitamin B6 (pyridoxine) and pyridoxal phosphate biosynthesis pathways have been investigated in various organisms, including bacteria and animals 6
- The distribution of key enzymes involved in vitamin B6 biosynthesis has been analyzed, and two different vitamin B6 biosynthesis pathways have been identified 6