Vitamin B7 - Biotin

Question 1

List dietary sources of biotin

Answer 1

Liver, milk & egg yolks
Whole grains
Legumes
Royal jelly and brewery's yeast
Produced by colon bacteria (not near adequate)

Question 2

Describe the digestion of biotin

Answer 2

Biotin is protein bound
Requires proteolytic enzymes to form free biotin, biocytin or biotinyl peptides
Biocytin (biotin & lysine) needs to be hydrolyzed by biotinidase to free biotin and lysine
Biotinidase found on brush border, in pancreatic juices, in plasma and in most tissues

Question 3

Describe the absorption of biotin

Answer 3

Free biotin is absorbed in the jejunum
Dietary biotin is usually nearly completely absorbed, but alcohol can inhibit its absorption

Two mechanisms:

1. Carrier mediated sodium dependent (SMVT) - at physiological intakes (active transport)
2. Passive diffusion - at pharmacologic doses

Biotin synthesized by colonic bacteria is absorbed in transverse colon

Question 4

Describe the transport of biotin

Answer 4

Found in plasma as mostly free biotin
Lesser amounts are bound to proteins
Carriers are required for transport into cells

Question 5

Describe the function and mechanism of action of biotin

Answer 5

Functions as coenzyme

Step 1. Biotinyl adenosine monophosphate (active biotin) & pyrophosphate are formed in a Mg dependent retraction

Step 2. Biotinyl reacts with any of four apo-carboxylases to form four holo-carboxylases

• pyruvate carboxylase converts pyruvate to oxaloacetate
• acetyl CoA carboxylase converts acetate to Malonyl CoA (FA synthesis)
• propionyl CoA carboxylase converts propionyl CoA to methylmalonyl CoA
• beta-methylcrotonyl CoA carboxylase converts beta-methylcrotonyl CoA to beta-methylglutaconyl – metabolism of leucine

Question 6

Describe the metabolism and excretion of biotin

Answer 6

Biotin holocarboxylases are catabolyzed to biocytin which is then degraded to lysine and biotin
Biotin is further broken down and its metabolites are excreted in the urine
Smoking has been shown to increase biotin catabolism in women
Biotin from colonic bacteria that is not absorbed is excreted in the feces

Question 7

Describe consequences and symptoms of deficiency of biotin

Answer 7

Deficiency is rare, but serious - most common in infants

Symptoms:
Alopecia, brittle nails, scaly red dermatitis, greyish pallor
Anorexia, nausea 
Myalgia, paresthesias
Lethargy, depression, hallucinations

Question 8

What is there an increased risk of biotin deficiency with?

Answer 8

Ingestion of excessive amounts of raw eggs and alcohol
Chronic antibiotic therapy
Cigarette smoking and pregnancy - increase excretion
People with genetic defects involving biotinidase and holocarboxylase synthetase activities - need to be treated with high doses of biotin

Question 9

Describe consequences and symptoms of toxicity of biotin

Answer 9

None reported

Question 10

Describe the assessment of nutrient status of biotin

Answer 10

Urinary biotin or urinary 3-hydroxyisovaleric acid

Question 11

Discuss some of the therapeutic uses (clinical indications) of biotin

Answer 11

Brittle hair or nails

Seborrheic dermatitis of infancy