In this article we will discuss about:- 1. Historical Review of Pyridoxine 2. Chemistry of Pyridoxine 3. Properties and Distribution 4. Functions 5. Deficiency 6. Daily Requirement.
Historical Review of Pyridoxine:
(Vitamin B6) (Rat Antidermatitis Factor)
In 1934 Gyorgyi observed that an extract of yeast cured dermatitis in rats which was not curable with other members of B vitamins. Isolated from natural sources in 1938. Stiller and others characterised its structure in 1939.
Chemistry of Pyridoxine:
It is a pyridine derivative which was synthesised in 1939 by Harris et al independently, and by Kuhn and others.
Two allied compounds, pyridoxal and pyndoxamme, (natural) and synthetic, have the same action. All the three varieties are known as vitamin B6. Pyridoxal phosphate is the active form.
Properties and Distribution of Pyridoxine:
This vitamin is a white, crystal, soluble in water and heat-stable in both acidic and alkaline solutions.
i. Vegetable Sources:
Germs of various grains and seeds, leafy vegetables, etc.
ii. Animal Sources:
Liver, egg-yolk, meat, kidney, yeast, etc. Traces are present in all common articles of food. It has also been produced synthetically.
Functions of Pyridoxine:
Essential for lower mammals. May be important for man but not definitely known.
The functions are as follows:
i. It takes part in normal tryptophan metabolism.
(a) Kynurenine and xanthurenic acid are excreted by rats made deficient in pyridoxine. The above products are not on the main metabolic pathway of tryptophan metabolism and considered as products of side reactions.
(b) The enzyme kynureninase, responsible for the breakdown of kynurenine to anthranilic acid and 3-hydroxykynurenine to 3-hydroxyanthranilic acid requires pyridoxal phosphate as a coenzyme.
ii. Pyridoxal phosphate also acts as a coenzyme for the following enzyme systems:
(a) Transaminases or aminotransferases which are responsible for the transfer of amine groups from amino to keto acids,
(b) Decarboxylases which decarboxylate tyrosine, arginine, glutamic acid, 3, 4-dihydroxyphenylalanine (DOPA), etc.
(c) Deaminases or dehydrases, e.g. serine and threonine dehydrases.
(d) Desulphydrases, responsible for transfer of sulphur group.
iii. Pyridoxal phosphate in some way is related to the formation of long chain polyunsaturated fatty acids.
iv. Helps in the synthesis of fats from proteins and carbohydrates. [3 and 4 possibly explain—rat acrodynia in vitamin B6 deficiency].
v. It plays some part in the interconversion of serine and glycine where there is involvement of tetrahydrofolic acid also.
vi. Helps in transulphurase and thionase reactions converting homocysteine and cystathionine to cysteine
vii. The coenzyme is required in the synthesis of δ-amino levulinic acid—an intermediate in porphyrin synthesis.
viii. Pyridoxal phosphate is an intimate component of phosphorylase-a of muscle.
ix. It is suggested that pyridoxine is involved in the active transport of amino acids and certain metallic ions across cell membranes.
x. There is increased evidence that vitamin B6 is intimately concerned with the metabolism of the central nervous system.
Thus it is indicated that it definitely takes part in protein metabolism and possibly also in fat and carbohydrate metabolism.
Deficiency of Pyridoxine:
Peculiar dermatitis (acrodynia), reduced growth, degeneration of the nerves, reproductive failure and hypochromic microcytic anaemia, weakness of muscles and convulsive seizures occur in absence of this vitamin. In 1951, deficiency of this vitamin was noted in infants due to destruction of this vitamin in a preparation of baby food which produced epileptic type of convulsions.
In subjects suffering from Pellagra and Beriberi, synthetic B6 can easily cure insomnia, irritability, abdominal pain, difficulty in walking and such other symptoms—which fail to be cured by other vitamins. This vitamin seems to be essential also for lower animals, bacteria and mosquito larve. The drug isonicotinic acid hydrazide (INH) used in the treatment of tuberculosis has similar chemical structure as pyridoxal and acts as an antivitamin to pyridoxal kinase reaction.
Daily Requirement of Pyridoxine:
In infants 0.3 mgm and in adults 2 mgm per day. It is sufficiently present in normal diet.