pyridoxine Flashcards
structure of pyridoxine
Vitamin B6 consists of a mixture of three different closely
related pyridine derivatives namely:
1. Pyridoxine
2. Pyridoxal
3. Pyridoxamine.
All the three have equal vitamin activity, as they can
be interconverted in the body.
Active Form of Vitamin B6
Pyridoxal phosphate (PLP) is the active form of vitamin B6. PLP is formed from phosphorylation of all
three forms of vitamin B6.
Sources of vitamin B6
- Pyridoxine occurs mainly in plants, whereas pyridoxal
and pyridoxamine are present mainly in animal products. - Major dietary sources of vitamin B6 are yeast,
unrefined cereals, pulses, meat, poultry fish, potatoes
and vegetables. - Dairy products and grains contribute lesser amounts.
Functions of vitamin B6
Active form of vitamin B6, pyridoxal phosphate
(PLP) acts as coenzyme in large number of reactions
of amino acid metabolism. For example:
– Transamination
– Decarboxylation
– Nonoxidative deamination
– Trans-sulfuration
– Condensation reactions of amino acids.
vitamin B6 involved in transamination reactions
Transamination reactions: Transamination reactions are
catalyzed by transaminases and PLP acts as coenzyme
converting amino acid to keto acid, e.g. aspartate
transaminase (AST) and alanine transaminase (ALT).
Vitamin B6 involved in decarboxylation reactions
PLP acts as coenzyme in
decarboxylation of some amino acids. The amino acids
are decarboxylated to corresponding amines. The
important biogenic amines synthesized by PLP
decarboxylation
* γ-Amino butyric acid (GABA): It is an inhibitory
neurotransmitter derived from glutamate on
decarboxylation hence in vitamin B6 deficiency
underproduction of GABA leads to convulsions
(epileptic seizures) in infants and children.
* Serotonin and melatonin: These are produced from
tryptophan. Serotonin is a neurotransmitter and
stimulates the cerebral activity. Melatonin is a sleep
inducing substance and is involved in regulation of
circadian rhythm of body.
* Histamine: Histamine produced by decarboxylation
of histidine. It is a vasodilator and lowers blood
pressure. It is involved in allergic reactions.
* Catecholamines (dopamine, norepinephrine and
epinephrine) Synthesis of catecholamines from
tyrosine requires PLP-dependent DOPA decarboxylase. Catecholamines are neurotransmitters and
involved in metabolic and nervous regulation.
Vitamin B6 involved in non oxidative deamination
Hydroxyl group containing amino acids (serine, threonine) are non-oxidatively
deaminated to α-keto acids and ammonia, which requires PLP.
Vitamin B6 involved in trans-sulfuration reactions
PLP is a coenzyme for
cystathionine synthase involved in synthesis of cysteine from methionine. In these reactions transfer of sulfur from methionine to serine occurs to produce cysteine.
Vitamin B6 involved in condensation reactions
Pyridoxal phosphate is required for the condensation reaction of L-glycine and
succinyl CoA to form δ-aminolevulinic acid, a precursor of heme.
Other PLP-dependent reactions
- Pyridoxal phosphate is required for niacin coenzyme (NAD+/NADP+) synthesis from tryptophan.
- PLP is required for synthesis of serine from glycine.
- The enzyme glycogen phosphorylase contains
covalently bound PLP. - PLP is required for the synthesis of sphingosine, a
component of sphingomyelin.
Nutritional Requirement
The RDA for vitamin B6 is 1.6 to 2.0 mg. Requirements
increase during pregnancy and lactation.
Deficiency Manifestations
As pyridoxine occurs in most foods, the dietary
deficiency of vitamin B6 is rare. The main clinical
symptoms of deficiency are given below:
* Vitamin B6 deficiency causes neurological disorders
such as depression, nervousness and irritability.
These symptoms are due to decreased production of
neurotransmitters, catecholamines, GABA and
serotonine.
* Severe deficiency of pyridoxine causes epileptic
seizures (convulsions) in infants due to reduced
production of GABA.
* Demyelination of nerves causes peripheral neuropathy. Since vitamin B6 is required for synthesis of
sphingolipids needed for myelin formation.
* Vitamin B6 deficiency causes hypochromic microcytic anemia due to decreased heme synthesis. Since
PLP is required for the synthesis of heme.
* The commonest cause of pyridoxine deficiency is:
– Drug antagonism, e.g. isoniazide (INH), used in
the treatment of tuberculosis and penicillamide
used in the treatment of Wilson’s disease and
rheumatoid arthritis can combine with pyridoxal
phosphate forming an inactive derivative with
pyridoxal phosphate.
– Alcoholism: Alcoholics may be deficient owing
to metabolism of ethanol to acetaldehyde, which
stimulates hydrolysis of the phosphate of the
pyridoxal phosphate.
Vitamin B6 Assay
Activities of blood transaminases have been used frequently as indirect measurements of vitamin B6 status. Erythrocyte levels of aspartate and alanine aminotransferase provide a better information of vitamin B6 status.
Therapeutic Uses of vitamin B6
Pyridoxine is used for the treatment of:
* Seizures.
* Down’s syndrome, a state of mental subnormality
(incomplete development of mind) due to chromosome defect.
* Autism, psychiatric disorder of childhood.
* Premenstrual tension syndrome (PMS).
Toxicity of vitamin B6
Pyridoxine seems to be safe at levels of 100–150 mg but taking 500–5000 mg per day, has shown peripheral neuropathy within 1–3 years.
