Vitamin B6 refers to a group of closely related compounds that function as a single vitamin system in the body. The most commonly recognized form is pyridoxine, but biologically active derivatives are equally important in human metabolism.

It is a water-soluble vitamin and is generally heat stable, meaning it can withstand normal cooking temperatures to a reasonable extent. However, it is sensitive to light, ultraviolet (UV) radiation, and alkaline conditions, under which it may degrade.

Chemically, vitamin B6 exists in three interconvertible forms:

• Pyridoxine (alcohol form)
• Pyridoxal (aldehyde form)
• Pyridoxamine (amino form)

Among these, pyridoxal and pyridoxamine can exist as phosphate esters and demonstrate higher biological activity. These phosphorylated derivatives represent the true coenzyme forms of vitamin B6 in the body.

Although all three forms are vitamin-active and interconvertible within tissues, the term “vitamin B6” is often used interchangeably with pyridoxine in general discussion. However, from a biochemical standpoint, the most important active coenzyme is pyridoxal phosphate (PLP).

The principal biologically active coenzyme forms are:

• Pyridoxal phosphate (PLP)
• Pyridoxamine phosphate (PMP)

Of these, PLP is the dominant coenzyme involved in most enzymatic reactions.

Pyridoxine itself is a basic compound, appearing as colorless crystalline substance. In humans, it is metabolized in the liver and ultimately excreted in urine mainly as 4-pyridoxic acid, which is the end product of vitamin B6 metabolism.

   Occurrence of Vitamin B6

Vitamin B6 is widely distributed in both animal and plant foods.

Animal sources:

• Egg yolk
• Meat
• Fish
• Milk

Plant sources:

• Yeast
• Whole grains
• Cabbage
• Legumes

Although widely available, vitamin B6 is susceptible to loss during food processing, especially during:

• Refining of grains
• Prolonged cooking

This is mainly due to its water solubility, which allows it to leach into cooking water.

   Biochemical Role of Vitamin B6

In the body, pyridoxal is converted into its phosphorylated form through enzymatic reactions. The resulting pyridoxal phosphate (PLP) acts as a versatile coenzyme involved in a wide range of metabolic processes, particularly those related to amino acid metabolism.

1. Major reactions involving PLP

(1) Transamination

Transamination reactions involve the transfer of amino groups between amino acids and keto acids. This process is essential for amino acid synthesis and degradation.

(2) Decarboxylation

PLP is also essential in decarboxylation reactions, where carboxyl groups are removed from amino acids.
Important examples include:

• Formation of histamine from histidine
• Formation of dopamine from L-DOPA

A clinically important implication of this role is that vitamin B6 is contraindicated during L-DOPA therapy (used in Parkinsonism). This is because vitamin B6 enhances peripheral decarboxylation of L-DOPA, reducing the amount reaching the brain and thereby decreasing its therapeutic effectiveness.

2. Enzymatic roles of PLP

PLP acts as a coenzyme for a broad group of enzymes, including:

• Transaminases
• Decarboxylases
• Racemases
• Certain dehydratases

This makes vitamin B6 essential in the overall metabolism of amino acids.
Additional enzymatic functions include:

• PLP acts as a coenzyme for diamine oxidase, which catalyzes oxidative deamination of diamines such as:
• Cadaverine
• Putrescine
  This pathway is also involved in the breakdown of histamine.
• It participates in tryptophan metabolism, serving as a coenzyme for kynureninase, which catalyzes conversion of kynurenine.
  In vitamin B6 deficiency:
• Urinary excretion of xanthurenic acid increases
• Conversion of tryptophan to nicotinamide decreases
• It is required for synthesis of δ-aminolevulinic acid (δ-ALA), an intermediate in porphyrin and heme synthesis.
  This explains why anemia may occur in vitamin B6 deficiency.

3. Structural and metabolic roles

Vitamin B6 also plays less commonly emphasized but important roles:

• It is tightly associated with glycogen phosphorylase, where PLP is firmly bound as a structural cofactor.
• It facilitates movement of amino acids and potassium ions (K⁺) into cells against concentration gradients.
• It participates in biosynthesis of arachidonic acid from linoleic acid.
• Glycogen phosphorylase is a classic example of a PLP-dependent enzyme where vitamin B6 acts as a structural coenzyme rather than a transient catalyst.

4. Neurotransmitter synthesis

Vitamin B6 is essential for synthesis of several key neurotransmitters, including:

• Serotonin (from tryptophan)
• Gamma-aminobutyric acid (GABA)
• Dopamine
• Norepinephrine
• Epinephrine

Because of these roles, vitamin B6 is critical for normal brain function, mood regulation, and neurological stability.

5. Homocysteine metabolism

Vitamin B6 acts as a cofactor for cystathionine β-synthase, an enzyme involved in the transsulfuration pathway. This pathway converts:

• Homocysteine → Cystathionine

Adequate vitamin B6 levels help maintain normal homocysteine metabolism and may contribute to reduced cardiovascular risk.

6. Immune and gene regulation roles

Vitamin B6 also has broader physiological roles, including:

• Support of immune function, particularly lymphocyte proliferation
• Participation in gene expression regulation
• Modulation of steroid hormone activity through PLP-dependent enzyme systems

   Effects of Vitamin B6 Deficiency

Vitamin B6 deficiency produces a wide spectrum of clinical effects, depending on severity and age group.

1. Animal studies

In experimental animals such as rats, dogs, and pigs, deficiency may lead to:

• Acrodynia-like symptoms (historically described; now understood more broadly as dermatitis-like lesions in older literature)
• Swelling and necrosis of ears and paws
• Loss of muscle tone
• Convulsions

2. Human deficiency

In humans, deficiency typically develops slowly and is more commonly seen in:

• Very young infants
• Pregnant women

Clinical features include:

• Hypochromic microcytic anemia
• Lymphocytopenia
• Skin lesions, particularly on the face

In infants:

• Digestive disturbances
• Convulsions

The seizures are believed to result from reduced conversion of glutamic acid to GABA, since PLP is required for this reaction. Because GABA is an inhibitory neurotransmitter in the central nervous system, its deficiency leads to neuronal excitability and seizures.

3. Clinical deficiency manifestations

Common signs and symptoms include:

• Seborrheic dermatitis
• Angular cheilitis
• Stomatitis
• Glossitis
• Irritability
• Depression
• Peripheral neuropathy

Additionally, deficiency may result in:

• Elevated homocysteine levels, due to impaired transsulfuration metabolism

   Factors Affecting Requirement

Vitamin B6 requirements increase in several physiological and pharmacological conditions.

1. Nutritional and physiological factors

• High protein intake increases requirement due to increased amino acid metabolism
• Some infants have an unexplained higher requirement for vitamin B6
• Women using oral contraceptives may develop relative deficiency
• Some may require 10–20 mg/day or more to counteract this effect

2. Drug-related factors

• Isoniazid (INH) therapy:
  INH forms a complex with pyridoxal, reducing available vitamin B6 and potentially causing deficiency.
• Only 2–3% of patients on standard doses are affected
• Risk increases with higher doses
• Prophylactic supplementation of 50 mg pyridoxine/day is usually sufficient
• Other drugs increasing requirement or causing functional deficiency:
• Penicillamine
• Cycloserine
• Theophylline

These interfere with PLP-dependent metabolic processes.

   Toxicity

Vitamin B6 toxicity is uncommon from dietary intake but may occur with prolonged high-dose supplementation.
Key toxic effect:

• Sensory neuropathy, characterized by:
• Numbness
• Tingling sensations
• Impaired coordination

Toxicity is generally associated with excessive supplemental intake, rather than normal dietary consumption.