Vitamin B6 (Pyridoxine)

It is water soluble, heat stable but is sensitive to light, ultra-violet rays and alkalies. Vitamin B6 is also called Pyridoxine. Its aldehyde form pyridoxal and the amino form prydoxamine which occurs as phosphates have even higher biological activity and represent the coenzyme form of this vitamin.

Pyridoxine, pyridoxal and pyridoxamine are inter-convertible in the body; all three have the vitamin activity but the term B6 is reserved for pyridoxine.

Pyridoxine is a basic substance with colorless crystals. It is excreted in urine as pyridoxic acid which is produced in the liver.

   Occurrence of Vitamin B6

Egg yolk, meat, fish and milk in the animal kingdom and yeast, whole grains, cabbages and legumes in plant kingdom are good sources of this vitamin.

   Biochemical Role of Vitamin B6

Pyridoxal (the aldehyde form of the vitamin B6) is converted to its phosphate by the reaction.

The pyridoxal phosphate so formed acts as coenzyme for many important reactions in the body especially those involving amino acids. These are given below:

  1. Transamination: This reaction has been described earlier.
  2. Decarboxylation


In addition to these other decarboxylation reactions are involved in the formation of histamine and dopamine from histidine and DOPA respectively. Vitamin B6 is contraindicated in treatment with L-DOPA (for Parkinsonism) because it antagonizes the effects of this drug.

  1. It is a coenzyme for diamine oxidase which catalyze the oxidative deamination of diamines like cadaverine and putrescine; these enzyme are also involved in the catabolism of histamine.
  2. It takes part in metabolism of trypophan becaue it is a coenzyme for kynureninase which catalyses the conversion of Vitamin B6 deficiency, the urinary excretion of xanthurenic acid another product derived from kynurenine, is increased; a decreased production of nicotinamide from tryptophan also results.
  3. It is also involved in the synthesis of delta-aminolevulinic acid which is an intermediate in the synthesis of porphyrins which give rise to home. This explains anemia occurring in vitamin B6 deficiency.
  4. Other functions: it appears to be a part of the molecule of glycogen phosphorylase. Vitamin B6 helps in the entry of amino acids and K into the cells against a concentration gradient. It is also involved in the biosynthesis of arachidonic acid from linolenicacid.

   Effects of Vitamin B6 Deficiency

In rat, dog and pig there is acrodynia (swelling and necrosis of ears, paws), loss muscle tone and convulsions.

In man deficiency symptoms appear only after a prolonged deficiency and occur mainly in very young infants and pregnant women. There is anemia of the hypochromic, microcytic type, lymphocytopenia and skin lesions on face. The young infant shows digestive troubles and convulsions. The convulsions are thought to be caused by an inhibition of decarboxylation of glutamic acid to GABA for which vitamin B6 acts as a coenzyme. GABA normally acts as an inhibitor of the spinal cord synaptic activity.

   Factors affecting requirement

The requirement of pyridoxine is increased in the following conditions:

  1. Greater intake of proteins
  2. During therapy with isonicotinic acid hydrazide or INH: INH forms a complex with pyridoxal which tends to produce a deficiency of the vitamin which thus has to given in extra amounts. However, this increased requirement is shown only by 2 to 3% of patients on the usual therapeutic dose of INH. The number of patients of proteins thus affected increases if the dose of INH is increased. 50 mg pyridoxine per day is sufficient as a prophylactic for such patients.
  3. Some infants, for some unknown reason, also show much greater requirement of vitamin B6.
  4. Women on oral contraceptives show a relative B6 deficiency. Some women need 10 to 20 mg or even more of vitamin B6 daily to counteract the effects of oral contraceptives.