Vitamin B6 occurs in three forms–pyridoxine, pyridoxal, and pyridoxamine. B6 is absorbed in the large intestine. The muscle stores approximately 75-80% of the vitamin while the live stores approximately 5-10%.
B6 is required for transaminations, decarboxylations, dehydratases, side-chain cleavage reactions, and ammonia release. Its aldehyde group acts as a Schiff base to react with the amino groups of amino acids. It basically acts to shuttle nitrogen between compounds.
Functions- The three forms of B6 can all be converted to the coenzyme PLP that aids in transamination and protein metabolism. PLP is essential for glycogen degradation; it also helps with the formation of the neurotransmitter, serotonin, the nonprotein portion of hemoglobin (heme), nucleic acids, and lecithin. Vitamin B6 is essential for the metabolism of tryptophan to niacin.
· Transaminations These reactions are needed to recycle and reuse nitrogen in the body. They are the first step in amino acid catabolism and the last step in the synthesis of nonessential amino acids. Example: a-ketoglutarate + asparate glutamate + oxaloacetate
· Serine/ Threonine Deamination (Dehydratases) This reaction provides the production of the a-keto acids of serine and threonine through oxidative removal of N as ammonia Example: Serine a-keto-serine + NH4+
· Decarboxylation These reactions often occur on the neuroactive amines of seratonin, tyramine, histamine, and GABA. They also are important in porphyrin synthesis, intermediates in the synthesis of sphingomyelin, lecithin and taurine, and also for the breakdown and desulfuration of cysteine.
· Glycogen Phosphorylase 50% of all B6 in the body is bound to glycogen phosphorylase but the importance is unknown. The reaction is important for recycling of folate and is as follows:
Serine glycine + 5,10-methylene-THF
Deficiency Deficiencies of vitamin B6 are relatively rare. People with a diet deficient in vitamin B6 will first show signs of weakness, irritability, insomnia, nervousness, and moist skin lesions. More advanced symptoms include growth failure, impaired motor function, hypochromic microcytic anemia, smooth tongue, and convulsions.
The impaired motor function and other mental problems can be attributed to a decrease in amino acid-based neurotransmitters (serotonin, GABA, tyramine, epinephrine). You see an increase in urea excretion because of a decrease synthesis of nonessential amino acids. The nitrogen from these amino acids is not recycled by transamination and instead is excreted and lost. You see an increase in urinary excretion particularly in the following amino acids: metabolites of glycine, methionine, and tryptophan. The hypochromic microcytic anemia is due to the fact that there is a decreased synthesis of the B6 dependent porphyrin ring of hemoglobin.
People who are at most risk include the elderly with poor vitamin intake, alcoholics, hemodialysis patients, and those who are on drug therapy. Overall, individuals with a high metabolic stress.
Sources Foods richest in vitamin B6 include:
o whole grains o legumes o nuts o navy beans o walnuts o meat (sirloin and chicken) o fish (salmon and shellfish) o bananas and apples o broccoli and spinach
Therapeutics Vitamin B6 may provide therapeutic benefits for the following syndromes: carpal tunnel syndrome, glucose intolerance, sideroblastic anemia, neurologic disorders, hyperoxaluria, convulsive seizures, monosodium glutarate (MSG) intolerance, premenstral syndrome (PMS) and immune function. Furthermore, vitamin B6 may help to decrease homocysteine levels and thus the risk of developing cardiovascular disease.