Cobalamin (B12)

Question 1

What 2 reactions in the body is B12 involved in?

Answer 1

• Methionine synthase to generate SAM
• Methylmalonyl CoA mutase (for gluconeogenesis)

Question 2

What is methylmalonyl CoA converted to in B12 deficiency?

Answer 2

Methylmalonic acid

Question 3

Methyl Trap hypothesis

Answer 3

• Lack of MS activity
• Decrease Syn of methionine and THF
• Decrease AdoMet
• Increase conversion of methylene-THF to CH3-THF
  Trapped: Rxn irreversible, lack of coenzyme

Question 4

Absorption of B12

Answer 4

• IF released from stomach and binds to B12
• Uptaken by IF receptor in the ileum
• Released from IF by lysosome
• Binds to TC II (transcobalamin II) to get released into circulation

Question 5

Most sensitive marker of vitamin B12 status

Answer 5

Methylmalonic acid

Question 6

Requirements

Answer 6

RDA - M and F: 2.4mcg, increase with pregnancy: 2.6, lactation: 2.8
- More needed with age due to decreased IF

Question 7

B12 UL

Answer 7

None

Question 8

Schilling test

Answer 8

Tests for B12 malabsorption
- Stage 1 - 57Co + B12 load; low 57Co indicates low absorption
- Stage 2 - 57Co + IF; normal 57Co indicates problem with IF (or else, bacterial overgrowth)
- Stage 3 - treatment with antibiotics and retest

Question 9

Name for B12 anemia vs folate

Answer 9

B12: megaloblastic
- Addisonian (genetic; IF is absent)
- Non-Addisonian (gastrectomy and age; IF synthesis decreases; IF is inadequate)
Folate: pernicious

Both types are identical

Question 10

Signs of B12 deficiency

Answer 10

• Problems with tissues with high turnover rates (RBC, gut)
• Magaloblastic anemia
• Neuropathy (impaired myelin formation from depletion of AdoMet)

Question 11

Treatment for B12 deficiency

Answer 11

Oral supplements or intramuscular injections (B12 can be stored)