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Know More About Vitamin B6 Or Pyridoxine
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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.
In 1934 a 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. 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.
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. The liver is the site for vitamin B6 metabolism.
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. 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.
Vitamin B6 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 S1P lyase, the enzyme responsible for breaking down sphingosine-1-phosphate, is also PLP-dependent.
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