folate and vitamin B12 metabolism

Question 1

Tetrahydrofolate (THF)

Answer 1

• Functional form of folate
• formed from folate by Dihydrofolate reductase
  
  • converts folate to dihydrofolate and converts dihydrofolate to THF

Question 2

Methotrexate (drug)

Answer 2

• Folate analog
• inhibitor of dihydrofolate reductase
  
  • PREVENTS GENERATION OF THF
    
    • prevents purine and pyramidine synthesis
• antiproliferative effects (anticancer drug)

Question 3

describe the absorption of dietary folate

Answer 3

• monoglutamate form is taken up
• becomes reduced and methylated to form N5-methyl THF
• N5-methyl THF is released in blood (most reduced form)

Question 4

Uptake of folate from blood

Answer 4

• Receptor mediated endocytosis
• receptors have HIGH affinity for FOLAT MONOGLUTAMATES
  
  • N5-methyl THF monoglutamate is most abundant form in circulation (retention mechanisms)
• Intracellular folate rapidly metabolized
  
  • polyglutamate added
  • mechanisms of folate retention

Question 5

role of thymidylate synthase

Answer 5

• converts dUMP to dTMP
• essential for DNA synthesis
• most important reaction clinically

**inhibiting this reaction –> inhibits DNA replication**

Question 6

vitamin B12 general

Answer 6

• ONLY synthesized by certain bacteria
• Best dietary sources are from liver, kidney, other meats, dairy products, shellfish
• Plant foods do NOT supply vitamin B12
  
  • unless fortified
  • contamined with bacteria/soil
• ADENOSYLCOBALAMIN and METHYLCOBALAMIN are biologically active forms

Question 7

Absorption of dietary vitamin B12

Answer 7

• dietary B12 is bound to R-protein and carried into duodenum
• in duodenum pancreatic proteases degrad R-protein
• free B12 is bound rapidly to intrinsic factor
• B12 absorbed in ILEUM
• intrinsic factor degraded, receptor recycled

Question 8

Pernicious anemia

Answer 8

• Lack of ability to absorb vitamin B12 form ileum
  
  • autoimmune disease
    
    • gastric atrophy
    • no intrinsic factor produced
• Prior to discovery of vitamin B12
  
  • treated with special diet
    
    • 1/4 to 1/2 lb beef liver per day
• Now intramuscular injection or 1mg/day oral

Question 9

Transport and uptake of B12 from the blood

Answer 9

• Cells of Ileal mucosa make transcobalamin (TC)
  
  • B12 likely secreted into blood as B12/TC complex
• Taken up by cells via receptor-mediated endocytosis
  
  • TC receptor expressed on many cell types

Question 10

describe Haptocorrin role

Answer 10

• Bind B12 to TC required for uptake into cells
• BUT MOST B12 in circulation is bound to haptocorrin
• Haptocorrin/B12 complex is taken up by LIVER
  
  • enterohepatic B12 cycle
    
    • haptocorrin degraded and B12 secreted into bile
    • B12 binds to haptocorrin again
    • Haptocorrin/B12 complexes digested in duodenum
    • B12 binds intrinsic factor
    • Reabsorbed in ileum
  • haptocorrins form a circulating store of B12?

Question 11

Schilling test

Answer 11

• Measures ability of patient to absorb B12
• Part 1
  
  • oral load of radioactive B12
  • injection of non-radioactive B12 to saturate circulating B12-binding proteins
  • 24 hr urine collection started
  • normal individuals excrete at least 7% of radioactivity within 24 hr
• Part 2
  
  • oral load of radioactive B12
  • oral load of PURIFIED INTRINSIC FACOTR
  • Proceed as for part 1

Question 12

Schilling test results

Answer 12

• Abnormal Part 1, NORMAL part 2
  
  • pernicious anemia (instrinsic factor production defect
• ABnormal PART 1 and ABNORMAL part 2
  
  • defect in B12 absorption INDEPENDENT of instrinisc factor production (defect in TCII production, TCII receptor, etc

Question 13

Deficiency of B12

Answer 13

• Conversion of L-methylmalonyl-CoA to succinyl0CoA INHIBITED
• L-methylmalonyl-CoA accumulates
  
  • hydolysis yields methylmalonic acid
    
    • ORGANIC ACIDEMIA

Question 14

describe the methyl trap hypothesis

Answer 14

• practical purposes
  
  • only the methionine synthase reaction can convert N5-methyl THF back to THF
  • lack of B12 prevents synthesis of methylcobalamin
  • lack of methylcobalamin blocks methionine synthase reaction
  • folate becomes “trapped” in N5-methyl THF form
• FUNCTIONAL FOLATE DEFICIENCY ARISES
  
  • may have enough folate in diet but not enough folate in correct oxidation state

Question 15

neurological consequences of vitamin B12 deficiency

Answer 15

• vitamin B12 deficiecny results in demyelination
• Mech
  
  • methionine synthase reaction
    
    • some pts undergoing demyelination due to lack of B12 have show improvement upon methionine administation
  • Related to lack of S-adenosylmethionine
    
    • inability to correctly methylate key substrates?

Question 16

Folate, Vitamin B12 and megaloblastic anemia

Answer 16

• vitamin B12 deficiency leads to functional folate defiiceny
  
  • accumulation of N4-methyl THF
• dietary insufficiency may lead to ACtual folate deficiceny
• EITHER CASE
  
  • thymidine synthesis blocked
    
    • thymidylate synthase requires N5,M10-methylene THF
  • purine syntheiss block
    
    • two steps require N10-formyl THF
• DNA SYNTHESIS IS PREVENTED

Question 17

Megaloblastic anemia

Answer 17

• In cases of FOLATE DEFICIENCY results in Thymidylate synthase incorporates dUTP into DNA instead of dTTP
  
  • Cells GROW but CANNOT DIVIDE
    
    • become megaloblastic
    • eventually leads to DNA gramentation and celld eath
    • Most obviously apparent in rapidly-dividing cells
      
      • MEGALOBLAStIC ANEMIA (red blood cells)

Question 18

what conditions need increased levels of folate

Answer 18

• Pregnancy
• lactation
• periods of growth
• chronic hemolytic anemia

Question 19

conditions resulting in folate deficiency

Answer 19

• alcoholism
• old age
• poverty
• celiac disease/tropical sprue/other conditions of malabsorption
• Strict vegans (no B12)
• pernicious anemia
• gastric acid insufficiency
• fish tapeworm infestation
• competing intestinal flora

Question 20

Acute megaloblastic anemia

Answer 20

• Nitrous oxide destorys methylcobalamin resulting in acute presentation of megaloblastic anemia
• reveiw case of the kid