Vitamin B6, also known as Pyridoxine is water soluble, heat stable, a basic substance with colorless crystals. It is sensitive to light, ultraviolet rays and alkalies. This vitamin exists in three forms; pyridoxine, pyridoxal, and pyridoxamine. It is convertible to its aldehyde form which is pyridoxal and amino form which is pyridoxamine. These three forms are interconvertible and pyridoxal and pyridoxamine occur as phosphates. Pyridoxine is excreted in urine as pyridoxic acid.
- Animal Kingdom: Egg yolk, meat, fish, poultry and milk in the animal kingdom are good sources.
- Plant Kingdom: Yeast, whole grains, legumes, cabbage, cereals and many other vegetables in the plant kingdom are its good sources.
This vitamin B6 plays an in important function in the metabolism of amino acids, fats, and carbohydrates. Pyridoxal is converted into phosphate. The pyridoxal phosphate so formed acts as a coenzyme for various important reactions taking place in our body especially those involving amino acids. These reactions include:
- Transamination: Enzymes catalyze transamination reaction which is AST and ALT. The finding of low ALT activity of red blood cells is of help in diagnosing vitamin B6 deficiency; this enzyme activity is increased when B6 is administered.
- Decarboxylation: This reaction results in the formation of serotonin and gamma amino butyric acid and these are very important neurotransmitters for our brain. Other reactions of decarboxylation are involved in the formation of histamine and dopamine from histindine and DOPA respectively. Vitamin B6 is contraindicated in the treatment of Parkinson’s disease with L-DOPA since it antagonizes the effect of this drug.
- It is a coenzyme for diamine oxidases which catalyze the other reaction which is oxidative deamination of diamines like cadaverine and putrescine; these enzymes are also involved in the catabolism of histamine.
- Vitamin B6 takes part in the metabolism of tryptophan because it is a coenzyme for kynureninase which catalyzes the conversion of kynurenine to anthranilic acid. Administration of excess tryptophan to pyridoxine deficient people increases the urinary excretion of xanthurenic acid.
- Vitamin B6 is involved in the formation of niacin from tryptophan. In B6 deficiency tryptophan instead of forming niacin forms xanthurenic acid.
- This vitamin takes part in the metabolism of Sulphur-containing amino acids. The enzymes catalyzing the formation of cystathionine from homocysteine and serine contain B6 as a coenzyme.
- It is also a coenzyme for threonine aldolase as well.
- Pyridoxine is involved in the synthesis of delta-aminolevulinic acid which is an intermediate in the formation of porphyrins which give rise to heme. This explains anemia occurring in case of vitamin B6 deficiency.
- Pyridoxine is a part of the molecule of glycogen phosphorylase and makes this enzyme stable.
- Vitamin B6 helps in the entry of amino acids and potassium ion into the cells against a concentration gradient.
- It is also involved in the manufacturing of arachidonic acid from linolenic acid.
In the human being, deficiency symptoms appear only after a prolonged deficiency and occur mainly in young infants and pregnant women. Erythropoiesis, skin and nervous system are the ones worst affected. There occurs anemia of hypochromic, microcytic type along with lymphocytopenia; skin lesions occur on the face and nasolabial seborrhea is present. The tongue shows glossitis. In young infants, their digestive system shows troubles and convulsions. These convulsions are caused by an inhibition of decarboxylation of glutamic acid to GABA. Its deficiency also produces a deficiency of neurotransmitters. Peripheral neuropathies with symmetrical motor and sensory derangements are also seen.
Its excess leads to neurotoxicity. The symptoms include those of peripheral neuropathies, including ataxia, peri-oral numbness, and clumsiness of the hands and feet.