Pyridoxine Structure
Pyridoxine is the chemical name for the most common form of Vitamin B6. Available from a wide variety of foods, pyridoxine is essential for proper digestion of proteins and brain function. Its structure allows it to interact with over 100 different proteins in the body.
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Functionalized Ring
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Pyridoxine's chemical structure is based on a six-membered ring of five carbon atoms and one nitrogen atom, linked together with alternating double and single bonds (a structure known as a “pyridine” ring). This ring has several chemical groups attached to it. Most important for its biological activity are its two methanol (CH2OH) groups, which undergo chemical modification to create the active form of the molecule. Pyridoxine's ring also has one hydroxyl (OH) and one methyl (CH3) group connected to it, which help the molecule bind to proteins in the body, but are not modified themselves.
Activation
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Pyridoxine is absorbed into the blood through the small intestine and carried to the liver, brain and other tissues. Once it reaches its target organ, the pyridoxine structure is changed into its active form, pyridoxal-5-phosphate. This transformation has two parts: first, one of the methanol groups is changed to an aldehyde group, in which the methanol is stripped of a hydrogen and the carbon-oxygen bond is reinforced to a double bond. The second step is the attachment of a phosphate group to the remaining methanol group, also known as “phosphorylation.”
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Activities
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The modification to pyridoxal-5-phosphate allows the pyridoxine to bind to, and activate, many different enzymes that participate in over a hundred different reactions throughout the body. In the liver, pyridoxine-dependent enzymes break down proteins from food into components that the body can use. In the brain, phosphorylated pyridoxine participates in the manufacture of neurotransmitters that pass signals between nerve cells. Pyridoxine also has other crucial functions in the muscles, blood and around the body.
Stability
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Pyridoxine is found in a variety of plant- and animal-based foods, especially liver, bean sprouts, soybeans and brown rice. Pyridoxine from plant sources is unphosphorylated and relatively stable to light, heat and food processing. On the other hand, pyridoxine from animal sources is in the phosphorylated form, which is much more reactive and susceptible to deactivation under normal food storage and preparation conditions. Milk stored in glass bottles is especially vulnerable and loses an estimated 50 percent of its Vitamin B6 potential over its shelf life.
Overdose
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Pyridoxine is water-soluble in both its forms, but unlike most water-soluble vitamins it can be toxic in large amounts. An overdose of pyridoxine can cause interfere with nerve function, leading to numbness and difficulty in movement and coordination. The mechanism by which this occurs is not clear, but as noted in “Introduction to Clinical Nutrition,” most molecules with a pyridine ring (like pyridoxine) are neurotoxic at some level.
Deficiency
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True pyridoxine deficiency is rare except in cases of extreme malnutrition. Chemical interference with pyridoxine's activity in the body, as in alcoholism and hydrazine poisoning, can also lead to symptoms of pyridoxine deficiency, as can genetic disease. Pyridoxine deprivation is characterized by headache, seizures and death.
Other Sources of Vitamin B6
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Pyridoxine is not the only source of Vitamin B6. Two other similar molecules can be metabolized into pyridoxal-5-phosphate and are also grouped into the Vitamin B6 classification. One is pyridoxamine, which is exactly like pyridoxine but with a methylamine (CH2NH3) group substituted for one of the methanols. The other is pyridoxal, in which an aldehyde (COH) has been substituted for one of the methanols in the same way as occurs after the first step of pyridoxine phosphorylation.
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References
Resources
- Photo Credit brown rice image by Steve Lovegrove from Fotolia.com
