EXAM #1: FOLATE & VITAMIN B12 METABOLSIM Flashcards
List the major dietary sources of folate.
Spinach
Lettuce
Broccoli
Think FOLATE i.e. FOLIAGE. Also, remember that folate is removed from vegetables by prolonged cooking/ boiling.*
What structural feature distinguishes dietary folate from folate found in supplements?
- Dietary= polyglutamate
- Supplement= monoglutamate
What enzyme converts folate to tetrahydrofolate (THF)? What drug inhibits this enzyme?
- Diydrofolate reductase
- Methotrexate
Remember that THF and THF derivatives of THF are the active forms of THF in the body.
What is the most oxidized form of THF?
N10-formyl THF
What is the most reduced form?
N5-methyl THF
What is the one-carbon pool?
One carbon groups attached to THF that can be oxidized or reduced while bound to THF
Which reaction is the source of most of the carbon in the one-carbon pool?
Serine hydroxymethyltransferase reaction.
Serine + THF = Glycine + N5,N10 methylene-THF + H20
How is dietary folate absorbed in the intestine?
1) Hydrolysis of polyglutamate to monoglutamate
2) Monoglutamate form is absorbed
3) Reduction
4) Methylation
Product is N5-methyl THF, the most abundant form of folate in the circulation
How is folate released into the circulation?
Monoglutamate form of folate is eventually methylated to form “N5-methyl THF” that enters the circulation
What is the major form of THF in the circulation?
N5-methyl THF
How is folate taken up from the circulation?
Receptor-mediated endocytosis
- Receptors have high affinity for folate monoglutamate
- Receptors/N5-methyl THF are taken up
- Recycling of the receptor
What happens to folate inside the cell? Why is this modification important?
1) Conversion from monoglutamate to polyglutamate
Polyglutamate form keeps folate IN the cell i.e. prevents diffusion through the cell membrane
Which reaction requiring folate derivatives appears to be of greatest clinical importance?
Thymidylate synthase
- dUMP to dTMP, which is essential for DNA synthesis
What is the ultimate source of all vitamin B12?
Only synthesized by certain BACTERIA
Note that B12 is also referred to as cyanocobalamin b/c of a cobalt group in the center–this is the form that is contained in supplements for vegans
What are some important dietary sources?
- Liver
- Kidney
- Meat
- Dairy
- Shellfish
Though it is made by BACTERIA, B12 is stored in these organs
PLANTS DO NOT CONTAIN B12
Describe how vitamin B12 is liberated from food?
1) HCl and pepsin liberate B12 from food
2) B12 binds R-proteins
3) B12-R-proteins travel to the intestine
How is B12 absorbed in the intestine?
1) Pancreatic proteases remove R-proteins
2) B12 binds intrinsic factor
3) B12/IF undergoes receptor mediated endocytosis in the ILEUM
Describe how vitamin B12 is transported in the blood.
- Cells of the ileal mucosa make TRANSCOBALAMIN
- B12 is secreted into the blood as B12/transcobalamin
Note that most of the B12 in the blood is bound to HAPTACORRIN
How is B12 taken up by tissues?
Receptor mediated endocytosis of B12/transcobalamin
What are the metabolically active forms of vitamin B12?
Adenosylcobalamin
Methycobalamin
Describe how Part 1 and Part 2 of the Schilling test are performed.
Schilling test is a way of evaluating a patient’s ability to absorb B12.
Part1=
- oral load of radioactive B12
- injection of non-radioactive B12
- 24 hour urine collection
- Radioactive excreted in urine, normal = 7%
Part2=
- oral radioactive B12
- oral purified IF
- same processing
What does an abnormal Part 1 but normal Part 2 tell you?
Pernicious anemia
- Not getting into the blood b/c of lack of IF
- Normal part 2 b/c of supplemented IF
How about abnormal Part 1 and Part 2?
NOT pernicious anemia b/c it was not fixed by IF administration
Describe the role of adenosylcobalamin in propionate metabolism. What does a deficiency in adenosylcobalamin result in?
methylmalonyl-CoA mutase in propionyl-CoA metabolism require ADENOSYLCOBALAMIN
Deficiency= methylmaloyl acidemia
Describe the role of methylcobalamin in homocysteine metabolism.
Methylcobalamin is required for Methionine synthase
What is meant by the ‘methyl trap hypothesis’?
- Methionine synthase converts N5, N10-mehtyl THF back to THF (requires methylcobalamin)
- Without B12, Folate becomes trapped in N5-methyl THF form
A FUNCTIONAL folate deficiency exists b/c this form cannot participate in the necessary biochemical reactions
What is megaloblastic anemia?
Anemia characterized by overgrowth of RBCs and RBC destruction
Why does deficiency of either folate or vitamin B12 result in a megaloblastic anemia?
- Vitamin B12 deficiency leads to functional folate deficiency (trapped as N5-methyl THF)
- Dietary insufficency may lead to actual folate deficiency
1) Thymidylate synthase is impaired dUMP–>dTTP
2) dUTP is incorporated into DNA instead
3) Cells GROW but CANNOT DIVIDE
Eventually leads to DNA fragmentation and cell death and appears in rapidly dividing cells i.e. RBCs*
What circumstances might increase folate needs?
1) Pregnancy
2) Lactation
3) Growth
4) Chronic hemolytic anemia
Under what circumstances may folate deficiency be seen in a patient?
1) Alcoholism
2) Old age
3) Poverty
4) Malabsorption syndrome
What circumstances might increase vitamin B12 needs?
1) Growth
2) Pregnancy
Under what circumstances may vitamin B12 deficiency be seen in a patient?
1) Strict vegans
2) Pernicious anemia
3) Celiac disease/sprue
4) Ileal resection
5) Dihydrobothrium lathum i.e. fish tapeworm
6) Competing intestinal flora
By what mechanism can nitrous oxide anesthesia provoke an acute megaloblastic anemia?
- NO destroys methylcobalamin
- In a normal person this will cause transient B12 deficiency and megaloblastic anemia
In patients with already borderline B12 levels, this can cause full-blown megaloblastic anemia*
How is folate typically found?
Folate polyglutamate
What is the functional form of folate in the body?
THF and derivatives of THF
Where are 1-carbon groups attached to THF?
N5 and N10
What is the mechanism of action of Methotrexate?
- Inhibitor of dihydrofolate reducatase
- Cannot make folate i.e. cannot make DNA
- Antiproliferative drug
List the metabolic processes that require folate.
1) Methionine synthesis
2) Thymidilate synthesis
3) Pruine synthesis
4) Histisdine catabolism
What are the two reactions in the body that use B12?
- Methylmalonyl-CoA Mutase (Propionyl-CoA metabolsim)
- Methionine Synthase
What is pernicious anemia?
- Lack of ability to absorb B12 from the ILEUM due to a lack of IF
- Autoimmune attack of parietal cells that produce IF
Where do B12/Haptocorrin complexes traffic?
Liver for storage of B12
How does B12 deficiency result in demyelination?
- Failure of methionine synthase reaction
- Lack of SAM
Mechanism is unclear
