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維生素B6 (Vitamin B6)
      維生素B6是水溶性的維他命，B群的成員之一，主要的功能是擔任輔酶，配合酵素的作用，這些酵素負責蛋白質與胺基酸代謝的相關反應。由於許多神經傳導物質與荷爾蒙的原料都是胺基酸，充足的維生素B6可以維護神經與內分泌系統的作用，有效調節全身機能，達到維護健康的效益。
 

維生素B6的生理功能
      維生素B6作為酵素輔酶，使100多種酵素有正常的作用。主要參與胺基酸的代謝和利用，可以使血紅素合成正常而避免貧血，維護正常的神經機能，保護淋巴組織，促進免疫細胞增生，而能避免感染。
      維生素B6協助合成多種胺基酸類荷爾蒙和神經傳導物質，諸如：血清刺激素、正腎上腺素，GABA，血紅素的前驅物質等等。色胺酸代謝生成菸鹼素也需要維生素B6。維生素B6也參與肝醣和脂肪酸的代謝反應，幫助維持正常的血糖濃度。

維生素B6的缺乏
      維生素B6缺乏的症狀有皮膚炎，舌炎，憂鬱，神智不清，抽筋等。營養調查指出，民眾有血漿維生素B6濃度偏低，維生素B6潛伏性缺乏的問題。
      原發性缺乏很少見，導因於飲食品質惡劣，長期營養不良，通常都伴有其他B群營養素之欠缺。1950年代曾經發生嬰兒嚴重缺乏維生素B6的事件，廠商將嬰兒奶粉過度加熱，以至於維生素B6與胺基酸產生化學反應。嬰兒有代謝異常以及抽筋的現象。
      繼發性缺乏的原因有慢性酗酒，還有藥物干擾，因為治療肺結核或關節炎的藥物會與維生素B6結合而干擾其利用。

維生素B6豐富的食物來源
      動物食品是維生素B6的良好飲食來源。植物中，五穀根莖類、豆類及堅果類也是維生素B6的良好飲食來源，蔬菜中的菠菜、青花菜、白花椰菜、蒜頭，水果中的香蕉、芒果、奇異果、小番茄等也含有豐富的維生素B6。

Pyridoxine 
     Pyridoxine is a vitamin. It can be found in certain foods such as cereals, beans, vegetables, liver, meat, and eggs. It can also be made in a laboratory.
      Pyridoxine is used for preventing and treating low levels of pyridoxine (pyridoxine deficiency) and the “tired blood” (anemia) that may result. It is also used for heart disease; high cholesterol; reducing blood levels of homocysteine, a chemical that might be linked to heart disease; and helping clogged arteries stay open after a balloon procedure to unblock them (angioplasty).
      Women use pyridoxine for premenstrual syndrome (PMS) and other menstruation problems, "morning sickness" (nausea and vomiting) in early pregnancy, stopping milk flow after childbirth, depression related to pregnancy or using birth control pills, and symptoms of menopause.
      Pyridoxine is also used for Alzheimer's disease, attention deficit-hyperactivity disorder (ADHD), Down syndrome, autism, diabetes and related nerve pain, sickle cell anemia, migraine headaches, asthma, carpal tunnel syndrome, night leg cramps, muscle cramps, arthritis, allergies, acne and various other skin conditions, and infertility. It is also used for dizziness, motion sickness, preventing the eye disease age-related macular degeneration (AMD), seizures, convulsions due to fever, and movement disorders (tardive dyskinesia, hyperkinesis, chorea), as well as for increasing appetite and helping people remember dreams.
      Some people use pyridoxine for boosting the immune system, eye infections, bladder infections, and preventing cancer and kidney stones.
      Pyridoxine is also used to overcome certain harmful side effects related to radiation treatment and treatment with medications such as mitomycin, procarbazine, cycloserine, fluorouracil, hydrazine, isoniazid, penicillamine, and vincristine.
      Pyridoxine is frequently used in combination with other B vitamins in vitamin B complex products.
      You may remember a prescription medication called Bendectin that was used for morning sickness in pregnancy. Bendectin contained pyridoxine and a sleep-inducing antihistamine called doxylamine. The makers of Bendectin took it off the market in 1983 because they were running up expensive legal bills in defense of their product. Opponents charged it might be responsible for birth defects. Meanwhile, a product called Diclectin that is similar to Bendectin remained available in Canada, and there was research showing that neither pyridoxine nor Bendectin seems to cause birth defects in animals. After Bendectin was removed from the market, there was no reduction in birth defects, but hospitalization rates for pregnancy-related nausea and vomiting doubled.
How does it work?
     Pyridoxine is required for the proper function of sugars, fats, and proteins in the body. It is also required for the proper growth and development of the brain, nerves, skin, and many other parts of the body.

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Vitamin B6
      Vitamin B6 refers to a group of chemically very similar compounds which can be interconverted in biological systems. Vitamin B6 is part of the vitamin B group, and its active form, pyridoxal 5'-phosphate (PLP) serves as a coenzyme in many enzyme reactions in amino acid, glucose, and lipid metabolism.
Forms
Several forms (vitamers) of vitamin B6 are known:
Pyridoxine (PN), the form 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 urine
Pyritinol, a semi-synthetic derivative of pyridoxine, where two pyridoxine moieties are bound by a disulfide bridge.

       All forms except pyridoxic acid and pyritinol can be interconverted. Absorbed pyridoxamine is converted to PMP by pyridoxal kinase, which is further converted to PLP by pyridoxamine-phosphate transaminase or pyridoxine 5'-phosphate oxidase which also catalyzes the conversion of PNP to PLP. Pyridoxine 5'-phosphate oxidase is dependent on flavin mononucleotide (FMN) as a cofactor which is produced from riboflavin (vitamin B2) i.e. in this biochemical pathway, dietary vitamin B6 cannot be used without vitamin B2.

Functions
       PLP, 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. PLP generally serves as a coenzyme (cofactor) for many reactions including decarboxylation, transamination, racemization, elimination, replacement, and beta-group interconversion. The liver is the site for vitamin B6 metabolism.

Amino acid metabolism
1.PLP is a cofactor in the biosynthesis of five important neurotransmitters: serotonin, dopamine, epinephrine, norepinephrine, and gamma-aminobutyric acid (GABA). PLP is also involved in the synthesis of histamine.
2.Transaminases break down amino acids with PLP as a cofactor. The proper activity of these enzymes is crucial for the process of moving amine groups from one amino acid to another.
3.Serine racemase which synthesizes the neuromodulator serine is a PLP-dependent enzyme.
4.PLP is a coenzyme needed for the proper function of the enzymes cystathionine synthase and cystathionase. These enzymes catalyze reactions in the catabolism of methionine. Part of this pathway (the reaction catalyzed by cystathionase) also produces cysteine.
5.Selenomethionine is the primary dietary form of selenium. PLP is needed as a cofactor for the enzymes that allow selenium to be used from the dietary form. PLP also plays a cofactor role in releasing selenium from selenohomocysteine to produce hydrogen selenide, which can then be used to incorporate selenium into selenoproteins.
6.PLP is required for the conversion of tryptophan to niacin, so low vitamin B6 status impairs this conversion.

Glucose Metabolism
       PLP is a required coenzyme of glycogen phosphorylase, the enzyme necessary for glycogenolysis to occur. PLP can catalyze transamination reactions that are essential for providing amino acids as a substrate for gluconeogenesis.

Lipid Metabolism
       PLP is an essential component of enzymes that facilitate the biosynthesis of sphingolipids. 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 because sphingosine-1-phosphate lyase, the enzyme responsible for breaking down sphingosine-1-phosphate, is also PLP-dependent.

Hemoglobin synthesis and function
       PLP aids in the synthesis of hemoglobin, by serving as a coenzyme for the enzyme ALA synthase. It also binds to two sites on hemoglobin to enhance the oxygen binding of hemoglobin.

Gene expression
       PLP has been implicated in increasing or decreasing the expression of certain genes. Increased intracellular levels of the vitamin lead to a decrease in the transcription of glucocorticoids. Also, vitamin B6 deficiency leads to the increased gene expression of albumin mRNA. Also, PLP influences expression of glycoprotein IIb by interacting with various transcription factors. The result is inhibition of platelet aggregation.

Nutrition
Food sources
       Vitamin B6 is widely distributed in foods in both its free and bound forms. Cooking, storage, and processing losses of vitamin B6 vary and in some foods may be more than 50%, 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 to 70% of its vitamin B6 content when dried. Vitamin B6 is found in the germ and aleurone layer of grains, and milling results in the reduction of this vitamin in white flour. The heating that occurs before most freezing and canning processes are other methods that may result in the loss of vitamin B6 in foods.
       Foods that contain large amounts of vitamin B6 include pork, turkey, beef, bananas, chickpeas, potatoes, and pistachios.
Dietary reference intakes
       The Recommended Daily Allowance of the Institute of Medicine is 1.3 mg/d for a 19- to 50-year-old adult.

Absorption and excretion
       Vitamin B6 is absorbed in the jejunum and ileum by passive diffusion. With the capacity for absorption being so great, animals are able to absorb quantities much greater than necessary for physiological demands. The absorption of pyridoxal phosphate and pyridoxamine phosphate involves their dephosphorylation catalyzed by a membrane-bound alkaline phosphatase. Those products and nonphosphorylated forms 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.
       The products of vitamin B6 metabolism are excreted in the urine, the major product of which is 4-pyridoxic acid. An estimated 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. Other products of vitamin B6 metabolism 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.

Deficiency
Signs and symptoms
       The classic clinical syndrome for vitamin B6 deficiency is a seborrhoeic dermatitis-like eruption, atrophic glossitis with ulceration, angular cheilitis, conjunctivitis, intertrigo, and neurologic symptoms of somnolence, confusion, and neuropathy[16] (due to impaired sphingosin synthesis) and sideroblastic anemia (due to impaired heme synthesis).
       Less severe cases present with metabolic lesions associated with insufficient activities of the coenzyme PLP. 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 in impaired transsulfuration of methionine to cysteine. The PLP-dependent transaminases and glycogen phosphorylase provide the vitamin with its role in gluconeogenesis, so deprivation of vitamin B6 results in impaired glucose tolerance.

Diagnosis
       The assessment of vitamin B6 status is essential, as the clinical signs and symptoms in less severe cases are not specific. The three biochemical tests most widely used are the activation coefficient for the erythrocyte enzyme aspartate aminotransferase, plasma PLP concentrations, and the urinary excretion of vitamin B6 degradation products, specifically urinary PA. Of these, plasma PLP is probably the best single measure, because it reflects tissue stores. Plasma PLP less than 10nmol/l is indicative of vitamin B6 deficiency. A PLP concentration greater than 20nmol/l has been chosen as a level of adequacy for establishing Estimated Average Requirements and Recommended Daily Allowances in the USA. Urinary PA is also an indicator of vitamin B6 deficiency; levels of less than 3.0 mmol/day is suggestive of vitamin B6 deficiency.

Also liver test AST ALT can be low due to pyridoxine deficiency
      The classic syndrome for vitamin B6 deficiency is rare, even in developing countries. A handful of cases were seen between 1952 and 1953, particularly in the United States, and occurred in a small percentage of infants who were fed a formula lacking in pyridoxine.

Causes
      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.[22] Evidence exists for decreased levels of vitamin B6 in women with type 1 diabetes and in patients with systemic inflammation, liver disease, rheumatoid arthritis, and those infected with HIV. Use of oral contraceptives and treatment with certain anticonvulsants, isoniazid, cycloserine, penicillamine, and hydrocortisone negatively impact vitamin B6 status. Hemodialysis reduces vitamin B6 plasma levels.
      There are indications that B6 deficiency can be influenced by theophylline in medication or malnutrition.
Treatment
       Treatment of vitamin B6 deficiency lies with replacement, usually in the form of pyridoxine hydrochloride, orally, as a nasal spray, or for injection when in its solution form.

Toxicity
       Adverse effects have been documented from vitamin B6 supplements, but never from food sources. Toxicologic animal studies identify specific destruction of the dorsal root ganglia[30] which is documented in human cases of overdose of pyridoxine. Although it is a water-soluble vitamin and is excreted in the urine, doses of pyridoxine in excess of the RDI over long periods of time result in painful and ultimately irreversible neurological problems.
      The primary symptoms are pain and numbness of the extremities. In severe cases, motor neuropathy may occur with "slowing of motor conduction velocities, prolonged F wave latencies, and prolonged sensory latencies in both lower extremities", causing difficulty in walking. Sensory neuropathy typically develops at doses of pyridoxine in excess of 1,000 mg per day, but adverse effects can occur with much less, so doses over 200 mg are not considered safe. Symptoms among women taking lower doses have been reported. Two reported cases of neuropathy with pyridoxine treatment of 24 and 40 mg/day may have been coincidental.
       Existing authorizations and valuations vary considerably worldwide. In 1993, the European Community Scientific Committee on Food defined intakes of 50 mg of vitamin B6 per day as harmful and established a tolerable upper intake level of 25 mg/day for adults in 2000. The Expert Group on Vitamins and Minerals of the Food Standard Agency UK (UK EVM) derived a safe upper level (SUL) of 10 mg/day for a 60-kg adult in 2003. The tolerable upper limit has been set by the US FDA at 100 mg/day in 2000.
       The nutrient reference values in Australia and New Zealand recommend an upper limit of 50 mg a day in adults. "The same figure was set for pregnancy and lactation as there is no evidence of teratogenicity at this level. The UL was set based on metabolic body size and growth considerations for all other ages and life stages except infancy. It was not possible to set a UL for infants, so intake is recommended in the form of food, milk or formula." "The ULs were set using results of studies involving long-term oral administration of pyridoxine at doses of less than 1 g/day. A no-observed-adverse-effect level (NOAEL) of 200 mg/day was identified from the studies of Bernstein & Lobitz (1988) and Del Tredici et al (1985). These studies involved subjects who had generally been on the supplements for five to six months or less. The study of Dalton and Dalton (1987), however, suggested the symptoms might take substantially longer than this to appear. In this latter retrospective survey, subjects who reported symptoms had been on supplements for 2.9 years, on average. Those reporting no symptoms had taken supplements for 1.9 years."
       Because no placebo-controlled studies show therapeutic benefits of high doses of pyridoxine, and the well-documented occurrence of significant toxic effects, little reason exists to exceed the RDI using supplements unless under medical supervision e.g. in treatment of primary hyperoxaluria.

Medical Uses
       Vitamin B6 has been used to treat nausea and vomiting in early pregnancy for decades, commonly in conjunction with other medications such as metoclopramide or doxylamine. Alone, it has been found safe and effective, though any woman's prenatal caregiver must help guide treatment for these symptoms.
       At least one preliminary study has found this vitamin may increase dream vividness or the ability to recall dreams. This effect is possibly due to the role this vitamin plays in the conversion of tryptophan to serotonin. Anecdotal evidence suggests supplemental vitamin B6 may be associated with lucid dreaming.
       The intake of vitamin B6, from either diet or supplements, could cut the risk of Parkinson's disease in smokers by half, according to a prospective study from the Netherlands.

Nutritional supplementation with high dose vitamin B6 and magnesium is one of the most popular alternative medicine choices for autism, but randomised control trials have had mixed results and small sample sizes mean no conclusions can be drawn as to the efficacy of this treatment.
       Some studies suggest the vitamin B6-magnesium combination can also help attention deficit disorder, citing improvements in hyperactivity, hyperemotivity/aggressiveness, and improved school attention.
       Vitamin B6 has been shown in at least two small-scale clinical studies to have a beneficial effect on the carpal tunnel syndrome, particularly in cases where no trauma or overuse etiology is known.
       Pyridoxine may help balance hormonal changes in women and aid the immune system.
       The effectiveness as treatment for PMS, PMDD, and clinical depression is debatable, although a recent study has pointed to the increase in efficacy of citalopram when given with magnesium and Vitamin B6 in adults with severe depression.

       Ingestion of vitamin B6 possibly can alleviate some of the many symptoms of an alcoholic hangover and morning sickness from pregnancy. This might result from its mild diuretic effect. Though the mechanism is not known, results show pyridoxamine has therapeutic effects in clinical trials for diabetic nephropathy.
       Vitamin B6 intake and PLP levels are inversely related to the risk of colon cancer. While the correlation with B6 intake is moderate, it was quite dramatic with PLP levels, where the risk of colon cancer was decreased by nearly half.
       Vitamin B6 is also known to increase the metabolism of Parkinson's medications, such as levodopa, and should be used cautiously.
       Pyridoxine-dependent epilepsy is an extremely rare genetic disorder characterized by intractable seizures in the prenatal and neonatal period that can be treated with pharmacological doses of vitamin B6.
       Pyridoxine is given to patients taking isoniazid (INH) to combat the toxic side effects of the drug.
       In one form of homocystinuria, activity of the deficient enzyme can be enhanced by the administration of large doses of pyridoxine (100–1000 mg/day).

History
       In 1934, the Hungarian physician Paul Gyorgy discovered a substance that was able to cure a skin disease in rats (dermititis acrodynia). He named this substance vitamin B6.[60][61] 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.