Vitamin B6

Vitamin B-6 is a water-soluble vitamin.

The three major forms of vitamin B-6 are pyridoxine, pyridoxal, and pyridoxamine, which, in the liver, are converted to pyridoxal 5'-phosphate (PLP) — a cofactor in many reactions of amino acid metabolism. PLP also is necessary for the enzymatic reaction governing the release of glucose from glycogen.

History

Vitamin B-6 is a water-soluble compound that was discovered in 1930s during nutrition studies on rats. The vitamin was named pyridoxine to indicate its structural homology to pyridine. Later it was shown that vitamin B-6 could exist in two other, slightly different, chemical forms, termed pyridoxal and pyridoxamine. All three forms of vitamin B-6 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 catalyze 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 (EC; http://www.chem.qmul.ac.uk/iubmb/enzyme/) has catalogued more than 140 PLP-dependent activities, corresponding to ~4% of all classified activities.

Sources

The United States RDA varies between 1.3 mg and 2.0 mg, depending upon age and gender.

Vitamin B-6 is prevalent in both animal and vegetable food sources. Liver, chicken, fish, green beans, field salad, wheat germ, nutritional yeast and bananas are particularly good food sources.

Deficiencies

Pyroluria is one potential cause of vitamin B6 deficiency. Another cause of vitamin B6 deficiency is the use of the tuberculostatic medication isoniazid, and for this reason, it is recommended to supplement with vitamin B6 when using this drug.

In women, another potential cause for vitamin B6 deficiency is use of oral contraceptives and other medications containing estro-progestational hormones (such as those prescribed as part of Hormone Replacement Therapy). Other contraceptive medications that may cause vitamin B6 deficiency include: the patch (Ortho Evra), vaginal ring (Nuvaring), hormonal IUD (Mirena) and shot (Depo Provera). Specifically, habitual use of estro-progestational hormones inhibit absorption of vitamin B6 (due to a disturbance of tryptophan metabolism), necessitating a larger daily doseage of B6 into the bloodstream. Signs of a Vitamin B6 deficiency include: depression, anxiety, loss of libido, insomnia, water retention, inability to process glucose (weight loss/gain). Physicians are now beginning to recommend routine vitamin B6 administation during hormonal contraception/medication.

A meta-analysis of three databases (MEDLINE, EMBASE, and Cochrane Library), including only double-blind, randomized controlled trials, found that vitamin B6 has a significant effect compared to placebos in treating morning sickness, similar to that of gingerPregnancy Morning Sickness - Ginger as Effective as Vitamin B6 (open) Effectiveness and Safety of Ginger in the Treatment of Pregnancy-Induced Nausea and Vomiting (subscription), Borrelli et al., Obstetrics & Gynecology, 2005;105:849-856.

Diagnostic Testing for B6 Deficiency

A positive diagnosis test for pyroxidine deficiency can be assertained by measuring erythrocyte levels of aspartate aminotransferase in serum.

Overdose

An overdose of pyridoxine can cause a temporary deadening of certain nerves such as the proprioceptory nerves; causing a feeling of disembodiment common with the loss of proprioception.

At least one preliminary study has found that this vitamin may increase dream vividness or the ability to recall dreamsEbben, M., Lequerica, A., & Spielman A. (2002). Effects of pyridoxine on dreaming: a preliminary study. Perceptual & Motor Skills, 94(1), 135–140.. It is thought that this effect may be due to the role this vitamin plays in the conversion of tryptophan to serotonin.

External links

Facts about Vitamin B6 from NIH Office of Dietary Supplements
The B6 database A database of B6-dependent enzymes

References