16. Pantothenic acid, Pyridoxine, and Biotin (Vitamin B Group 2)

What is the actual name of Vitamin B5?

Pantothenic acid

Describe the structure of B5.

Contains D-Pantoic acid and β-Alanine in amide linkage

What does “panto” mean and how does it relate to pantothenic acid?

• Means “everywhere”

• Pantothenic acid widely distributed in nature

What are the roles of B5?

1. Component of Coenzyme A (CoA-SH)

2. Important part of fatty acid synthase complex

What are the components of Coenzyme A (CoA-SH)?

1. Adenosine phosphate

2. Pantothenic acid

3. β-Mercaptoethanolamine

What is the role of CoA-SH in metabolism?

Required as coenzyme for several enzymes in body (oxidation of carbohydrates, FAs, and TCA cycle)

What are the food sources of B5?

• Beef

• Poultry

• Seafood

• Organ meats

• Eggs

• Milk

• Vegetables (mushrooms, avocados, sweet potatoes, and broccoli)

• Whole grains (whole wheat, brown rice, and oats)

What is the RDA for pantothenic acid?

5mg/day for both genders

Is deficiency of pantothenic acid common?

No - it is rare

What are the symptoms of severe deficiency of B5?

• Numbness and burning of hands and feet

• Headache

• Fatigue

• Irritability

• Sleeping problems

• Stomach pain

• Heartburn

• Diarrhea

• Nausea

• Vomiting

• Loss of appetite

What is the term Vitamin B6 applied to?

A family of 3 pyridine derivatives

What are the 3 pyridine derivatives?

1. Pyridoxine

2. Pyridoxamine

3. Pyridoxal

What is the coenzyme form of B6?

Pyridoxal phosphate (PLP)

List the roles of B6 in metabolism.

Required for amino acid metabolism:

1. Transamination and synthesis of non-essential amino acids

2. Deamination of serine and threonine requires PLP

3. Decarboxylation of amino acids requires PLP

4. Metabolism of sulfur-containing amino acids including homocysteine

5. Synthesis of niacin from tryptophan requires PLP

6. Synthesis of hemoglobin

7. Breakdown of glycogen in muscles

8. Synthesis of sphingomyelin and phosphatidylcholine

9. PLP required for methionine and cysteine metabolism

Describe the decarboxylation of amino acids as a role of B6.

• Requires PLP

• Formation of neurotransmitters

  • Glutamate to GABA

  • Tryptophan to serotonin

  • Tyrosine to norepinephrine

  • Histidine to histamine

Describe synthesis of hemoglobin as a role of B6.

• ALA synthase is a PLP-dependent enzyme

• B6 deficiency results in hypochromic microcytic anemia

Describe breakdown of glycogen in muscles as a role of B6.

• 70% of total B6 associated with glycogen phosphorylase enzyme

• Acts as coenzyme

Describe PLP in the metabolism of methionine and cysteine as a role of B6.

Deficiency of PLP may lead to hyperhomocystenemia (risk factor for cardiovascular disease)

What is the RDA of B6?

1.0-2.0mg/day (for a 65g protein diet)

Describe how requirement of B6 varies.

• Requirement proportional to protein intake

• Increases in pregnancy, lactation, and old age

What are the food sources for B6?

• Bananas

• Whole grain cereals

• Legumes

• Meats

• Green leafy vegetables

• Egg yolks

• Milk

• Yeast

What are the deficiency symptoms of B6?

1. Neurological symptoms

2. Peripheral neuritis

3. Anemia

4. Pellagra-like symptoms

Describe neurological symptoms of B6 deficiency.

• Hyperirritability, convulsions, depression, nervousness (due to underproduction of neurotransmitters)

• Convulsions due to impaired synthesis of gamma amino butyric acid (GABA), an inhibitory neurotransmitter in brain

Why does peripheral neuritis occur with deficiency of B6?

Due to demyelination of nerves

Describe anemia as a symptom of B6 deficiency.

• Associated with hypochromic microcytic anemia

• Due to decreased heme synthesis

• Hence, decrease in Hb levels

Describe pellagra-like symptoms as a symptom of B6 deficiency.

Due to block in niacin coenzymes (NAD+ and NADP+) synthesis from tryptophan

How can the symptoms of B6 deficiency be reversed?

With high doses of B6

What drugs cause drug-induced B6 deficiency?

• Isoniazid (anti-TB drug) and penicillamine (treatment for rheumatoid arthritis)

• Cycloserine

• Ethanol (alcohol)

• Oral contraceptives

How do isoniazid and penicillamine cause drug-induced B6 deficiency?

Can lead to symptoms of B6 deficiency

How does cycloserine cause drug-induced B6 deficiency?

• Inhibits pyridoxal kinase, reduces formation of PLP

• Acts as B6 antagonist

How does ethanol (alcohol) cause drug-induced B6 deficiency?

• Inactivates PLP

• Neuritis common in alcohols

How do oral contraceptives cause drug-induced B6 deficiency?

Cause mild B6 deficiency

Describe toxicity of B6.

• Doses over 100mg may lead to sensory neuropathy

• Further excess manifested by imbalance, numbness, muscle weakness, and nerve damage