Biotinussy

Question 1

Active form

Answer 1

D-biotin

Question 2

Forms of biotin

Answer 2

Biocytin - bound to lysine
Carboxybiotin - active form

Question 3

Metabolic role

Answer 3

Acts as the prostethic groups for 4 carboxylases, catalyzed by holocarboxylase synthetase:
- Pyruvate carboxylase (PC)
- Propionyl CoA carboxylase (PCC)
- Acetyl CoA carboxylase (ACC)
- 3-methylcrotonyl CoA carboxylase (MCC)

Question 4

PC and what happens if there is a deficiency?

Answer 4

Pyruvate carboxylase carboxylates pyruvate into oxaloacetate; deficiency causes lactic acidosis and increased ammonia

Question 5

PCC

Answer 5

Propionyl CoA carboxylase carboxylates propionyl CoA to methylmalonyl CoA; deficiency causes propionic acidosis and ketoacidosis

Question 6

ACC

Answer 6

Acetyl CoA Carboxylase carboxylates acetyl CoA to malonyl CoA; the rate limiting step in FA synthesis (in cytosol); acts as regulation of FA uptake for oxidation (in mitochondria); deficiencies have not been documented

Question 7

MCC

Answer 7

3-methyl-crontonyl CoA carboxylase carboxylates 3-methyl-crotonyl CoA to 3-methyl-glutaconyl CoA (for leucine catabolism); deficiency causes severe acidosis

Question 8

Breakdown/absorption of biotin

Answer 8

Protein-biotin complexes are broken down via GI proteases into biotinyl peptides and biocytin; biotinidase breaks biocytin down into biotin; biotin and biocytin can be taken up into the enterocytes via. a multivitamin transporter (carrier-mediated and Na dependent)

Question 9

What is the major site of biotin metabolism?

Answer 9

The liver extracts the majority of biotin and is the major site of use and metabolism.

Question 10

How is biotin homeostasis maintained?

Answer 10

Renal reabsorption is essential for biotin conservation

Question 11

Food sources of biotin

Answer 11

Available as free biotin in vegetables, fruit, milk; or protein-bound in animal products, nuts, cereals
- Most is found bound to proteins

Question 12

Requirements

Answer 12

AI at 30 for m and f; more during lactation; no UL set; TPN patients - 60mcg/day; gut microbial synthesis and in almost all foods, so deficiency is unlikely

Question 13

Determination of status

Answer 13

• Blood/serum not sensitive
• 3-hydroxyisovalerate - measure of MCC activity
• lymphocyte PCC activity
• lymphocyte FA composition

Question 14

Holocarboxylate synthetase deficiency

Answer 14

• Multiple carboxylate deficiency that results in dec. activity of all 4-biotin carboxylases
• Causes dermatitis, alopecia, severe ketoacidosis, seizures

Question 15

Biotinidase deficiency

Answer 15

• Unable to release free biotin from food
• seizures, ataxia, developmental delay, skin rash and hair loss

Question 16

Who’s at risk for biotin deficiency?

Answer 16

• Rare
• Low levels in infants, alcoholics, and pregnancy
• Antibiotics
• Patients on TPN
• Raw egg whites contain avidin which binds biotin; heat labile so cooking eggs destroys it