Pyridoxal phosphate

Vitamin B₆ is a water-soluble vitamin and is part of the vitamin B complex group. Several forms of the vitamin are known, but pyridoxal phosphate (PLP) is the active form and is a cofactor in many reactions of amino acid metabolism, including transamination, deamination, and decarboxylation. PLP also is necessary for the enzymatic reaction governing the release of glucose from glycogen.

History[link]

Vitamin B₆ is a water-soluble compound that was discovered in the 1930s during nutrition studies on rats. In 1934, a Hungarian physician, Paul György discovered a substance that was able to cure a skin disease in rats (dermititis acrodynia), this substance he named vitamin B₆.^[1][2] In 1938, Samuel Lepkovsky isolated vitamin B₆ from rice bran. Harris and Folkers in 1939 determined the structure of pyridoxine, and, in 1945, Snell was able to show the two forms of vitamin B₆, pyridoxal and pyridoxamine. Vitamin B₆ was named pyridoxine to indicate its structural homology to pyridine. All three forms of vitamin B₆ are precursors of an activated compound known as pyridoxal 5'-phosphate (PLP), which plays a vital role as the cofactor of a large number of essential enzymes in the human body.

Enzymes dependent on PLP focus a wide variety of chemical reactions mainly involving amino acids. The reactions carried out by the PLP-dependent enzymes that act on amino acids include transfer of the amino group, decarboxylation, racemization, and beta- or gamma-elimination or replacement. Such versatility arises from the ability of PLP to covalently bind the substrate, and then to act as an electrophilic catalyst, thereby stabilizing different types of carbanionic reaction intermediates.

Overall, the Enzyme Commission has catalogued more than 140 PLP-dependent activities, corresponding to ~4% of all classified activities.^[3]

Forms[link]

Pyridoxine

Seven forms of this vitamin are known:

• Pyridoxine (PN), the form that is most commonly given as vitamin B₆ supplement
• Pyridoxine 5'-phosphate (PNP)
• Pyridoxal (PL)
• Pyridoxal 5'-phosphate (PLP), the metabolically active form (sold as 'P-5-P' vitamin supplement)
• Pyridoxamine (PM)
• Pyridoxamine 5'-phosphate (PMP)
• 4-Pyridoxic acid (PA), the catabolite which is excreted in the urine

All forms except PA can be interconverted.

Functions[link]

Pyridoxal phosphate, the metabolically active form of vitamin B₆, is involved in many aspects of macronutrient metabolism, neurotransmitter synthesis, histamine synthesis, hemoglobin synthesis and function and gene expression. Pyridoxal phosphate generally serves as a coenzyme for many reactions and can help facilitate decarboxylation, transamination, racemization, elimination, replacement and beta-group interconversion reactions.^[4] The liver is the site for vitamin B₆ metabolism.

Amino acid metabolism[link]

Pyridoxal phosphate (PLP) is a cofactor in transaminases that can catabolize amino acids. PLP is also an essential component of two enzymes that convert methionine to cysteine via two reactions. Low vitamin B₆ status will result in decreased activity of these enzymes. PLP is also an essential cofactor for enzymes involved in the metabolism of selenomethionine to selenohomocysteine and then from selenohomocysteine to hydrogen selenide. Vitamin B₆ is also required for the conversion of tryptophan to niacin and low vitamin B₆ status will impair this conversion.^[4] PLP is also used to create physiologically active amines by decarboxylation of amino acids. Some notable examples of this include: histidine to histamine, tryptophan to serotonin, glutamate to gamma-aminobutyric acid (GABA), and dihydroxyphenylalanine to dopamine.

Gluconeogenesis[link]

Vitamin B₆ also plays a role in gluconeogenesis. Pyridoxal phosphate can catalyze transamination reactions that are essential for the providing amino acids as a substrate for gluconeogenesis. Also, vitamin B₆ is a required coenzyme of glycogen phosphorylase,^[4] the enzyme necessary for glycogenolysis to occur.

Lipid metabolism[link]

Vitamin B₆ is an essential component of enzymes that facilitate the biosynthesis of sphingolipids.^[4] Particularly, the synthesis of ceramide requires PLP. In this reaction serine is decarboxylated and combined with palmitoyl-CoA to form sphinganine which is combined with a fatty acyl CoA to form dihydroceramide. Dihydroceramide is then further desaturated to form ceramide. In addition, the breakdown of sphingolipids is also dependent on vitamin B₆ since S1P lyase, the enzyme responsible for breaking down sphingosine-1-phosphate, is also PLP dependent.

Metabolic functions[link]

The primary role of vitamin B₆ is to act as a coenzyme to many other enzymes in the body that are involved predominantly in metabolism. This role is performed by the active form, pyridoxal phosphate. This active form is converted from the other natural forms founds in food: pyridoxal, pyridoxine and pyridoxamine.^[5]

Vitamin B₆ is involved in the following metabolic processes:

• amino acid, glucose and lipid metabolism
• neurotransmitter synthesis
• histamine synthesis
• hemoglobin synthesis and function
• gene expression

Amino acid metabolism[link]

Pyridoxal phosphate is involved in almost all amino acid metabolism, from synthesis to breakdown.

1. Transamination: Transaminase enzymes needed to break down amino acids are dependent on the presence of pyridoxal phosphate. The proper activity of these enzymes is crucial for the process of moving amine groups from one amino acid to another.
2. Trans-sulfuration: Pyridoxal phosphate is a coenzyme needed for the proper function of the enzymes cystathionine synthase and cystathionase. These enzymes work to transform methionine into cysteine.
3. Selenoamino acid metabolism: Selenomethionine is the primary dietary form of selenium. Pyridoxal phosphate is needed as a cofactor for the enzymes that allow selenium to be used from the dietary form. Pyridoxal phosphate also plays a cofactor role in releasing selenium from selenohomocysteine to produce hydrogen selenide, which can then be used to incorporate selenium into selenoproteins.^[4]
4. Vitamin B₆ is also required for the conversion of tryptophan to niacin, so low vitamin B₆ status will impair this conversion.^[4]

Neurotransmitter synthesis[link]

Pyridoxal phosphate-dependent enzymes play a role in the biosynthesis of four important neurotransmitters: serotonin, epinephrine, norepinephrine and gamma-aminobutyric acid (GABA).^[4] Serine racemase, which synthesizes the neuromodulator D-serine, is also a pyridoxal phosphate-dependent enzyme.

Histamine synthesis[link]

Pyridoxal phosphate is involved in the metabolism of histamine.^[4]

Hemoglobin synthesis and function[link]

Pyridoxal phosphate aids in the synthesis of heme, by serving as a coenzyme for the enzyme ALA synthase.^[6] It also binds to two sites on hemoglobin to enhance the oxygen binding of hemoglobin.^[4]

Gene expression[link]

It transforms homocysteine into cistation then into cysteine. Pyridoxal phosphate has been implicated in increasing or decreasing the expression of certain genes. Increased intracellular levels of the vitamin will lead to a decrease in the transcription of glucocorticoid hormones. Also, vitamin B₆ deficiency will lead to the increased expression of albumin mRNA. Also, pyridoxal phosphate will influence gene expression of glycoprotein IIb by interacting with various transcription factors. The result is inhibition of platelet aggregation.^[4]

Dietary reference intakes[link]

The Institute of Medicine notes, "No adverse effects associated with vitamin B₆ from food have been reported. This does not mean that there is no potential for adverse effects resulting from high intakes. Because data on the adverse effects of vitamin B₆ are limited, caution may be warranted. Sensory neuropathy has occurred from high intakes of supplemental forms."^[7] See the full Dietary Reference Intake Table [1] from the Institute of Medicine.

Food sources[link]

Vitamin B₆ is widely distributed in foods in both its free and bound forms. Good sources include meats, whole grain products, vegetables, nuts and bananas. Cooking, storage and processing losses of vitamin B₆ vary and in some foods may be more than 50%,^[8] depending on the form of vitamin present in the food. Plant foods lose the least during processing, as they contain mostly pyridoxine, which is far more stable than the pyridoxal or pyridoxamine found in animal foods. For example, milk can lose 30-70% of its vitamin B₆ content when dried.^[4] Vitamin B₆ is found in the germ and aleurone layer of grains, and milling results in the reduction of this vitamin in white flour. Freezing and canning are other food processing methods that result in the loss of vitamin B₆ in foods.^[9]

Absorption and excretion[link]

Vitamin B₆ is absorbed in the jejunum and ileum via passive diffusion. With the capacity for absorption being so great, animals are able to absorb quantities much greater than what is needed for physiological demands. The absorption of pyridoxal phosphate and pyridoxamine phosphate involves their dephosphorylation catalyzed by a membrane-bound alkaline phosphatase. Those products and non-phosphorylated vitamers in the digestive tract are absorbed by diffusion, which is driven by trapping of the vitamin as 5'-phosphates through the action of phosphorylation (by a pyridoxal kinase) in the jejunal mucosa. The trapped pyridoxine and pyridoxamine are oxidized to pyridoxal phosphate in the tissue.^[4]

The products of vitamin B₆ metabolism are excreted in the urine; the major product of which is 4-pyridoxic acid. It has been estimated that 40-60% of ingested vitamin B₆ is oxidized to 4-pyridoxic acid. Several studies have shown that 4-pyridoxic acid is undetectable in the urine of vitamin B₆ deficient subjects, making it a useful clinical marker to assess the vitamin B₆ status of an individual.^[4] Other products of vitamin B₆ metabolism that are excreted in the urine when high doses of the vitamin have been given include pyridoxal, pyridoxamine, and pyridoxine and their phosphates. A small amount of vitamin B₆ is also excreted in the feces.

Deficiencies[link]

The classic clinical syndrome for B₆ deficiency is a seborrhoeic dermatitis-like eruption, atrophic glossitis with ulceration, angular cheilitis, conjunctivitis, intertrigo, and neurologic symptoms of somnolence, confusion, and neuropathy.^[10]

While severe vitamin B₆ deficiency results in dermatologic and neurologic changes, less severe cases present with metabolic lesions associated with insufficient activities of the coenzyme pyridoxal phosphate. The most prominent of the lesions is due to impaired tryptophan-niacin conversion. This can be detected based on urinary excretion of xanthurenic acid after an oral tryptophan load. Vitamin B₆ deficiency can also result from impaired transsulfuration of methionine to cysteine. The pyridoxal phosphate-dependent transaminases and glycogen phosphorylase provide the vitamin with its role in gluconeogenesis, so deprivation of vitamin B₆ results in impaired glucose tolerance.^[4]

A deficiency of vitamin B₆ alone is relatively uncommon and often occurs in association with other vitamins of the B complex. The elderly and alcoholics have an increased risk of vitamin B₆ deficiency, as well as other micronutrient deficiencies.^[11] Renal patients undergoing dialysis may experience vitamin B₆ deficiency. Also, patients with liver disease, rheumatoid arthritis, women with type 1 diabetes, and those infected with HIV also appear to be at risk, despite adequate dietary intakes.^[12][13] The availability of vitamin B₆ to the body can be affected by certain drugs such as anticonvulsants and corticosteroids.^[9] The drug isoniazid (used in the treatment of tuberculosis), and cycloserine, penicillamine, and hydrocortisone all interfere with vitamin B₆ metabolism. These drugs may form a complex with vitamin B₆ that is inhibitory for pyridoxal kinase, or they may positively displace PLP from binding sites.^[14]