EXAM #1: FOLATE & VITAMIN B12 METABOLSIM

Question 1

List the major dietary sources of folate.

Answer 1

Spinach
Lettuce
Broccoli

Think FOLATE i.e. FOLIAGE. Also, remember that folate is removed from vegetables by prolonged cooking/ boiling.*

Question 2

What structural feature distinguishes dietary folate from folate found in supplements?

Answer 2

• Dietary= polyglutamate

- Supplement= monoglutamate

Question 3

What enzyme converts folate to tetrahydrofolate (THF)? What drug inhibits this enzyme?

Answer 3

• Diydrofolate reductase
• Methotrexate

Remember that THF and THF derivatives of THF are the active forms of THF in the body.

Question 4

What is the most oxidized form of THF?

Answer 4

N10-formyl THF

Question 5

What is the most reduced form?

Answer 5

N5-methyl THF

Question 6

What is the one-carbon pool?

Answer 6

One carbon groups attached to THF that can be oxidized or reduced while bound to THF

Question 7

Which reaction is the source of most of the carbon in the one-carbon pool?

Answer 7

Serine hydroxymethyltransferase reaction.

Serine + THF = Glycine + N5,N10 methylene-THF + H20

Question 8

How is dietary folate absorbed in the intestine?

Answer 8

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

Question 9

How is folate released into the circulation?

Answer 9

Monoglutamate form of folate is eventually methylated to form “N5-methyl THF” that enters the circulation

Question 10

What is the major form of THF in the circulation?

Answer 10

N5-methyl THF

Question 11

How is folate taken up from the circulation?

Answer 11

Receptor-mediated endocytosis

• Receptors have high affinity for folate monoglutamate
• Receptors/N5-methyl THF are taken up
• Recycling of the receptor

Question 12

What happens to folate inside the cell? Why is this modification important?

Answer 12

1) Conversion from monoglutamate to polyglutamate

Polyglutamate form keeps folate IN the cell i.e. prevents diffusion through the cell membrane

Question 13

Which reaction requiring folate derivatives appears to be of greatest clinical importance?

Answer 13

Thymidylate synthase

• dUMP to dTMP, which is essential for DNA synthesis

Question 14

What is the ultimate source of all vitamin B12?

Answer 14

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

Question 15

What are some important dietary sources?

Answer 15

• Liver
• Kidney
• Meat
• Dairy
• Shellfish

Though it is made by BACTERIA, B12 is stored in these organs

PLANTS DO NOT CONTAIN B12

Question 16

Describe how vitamin B12 is liberated from food?

Answer 16

1) HCl and pepsin liberate B12 from food
2) B12 binds R-proteins
3) B12-R-proteins travel to the intestine

Question 17

How is B12 absorbed in the intestine?

Answer 17

1) Pancreatic proteases remove R-proteins
2) B12 binds intrinsic factor
3) B12/IF undergoes receptor mediated endocytosis in the ILEUM

Question 18

Describe how vitamin B12 is transported in the blood.

Answer 18

• 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

Question 19

How is B12 taken up by tissues?

Answer 19

Receptor mediated endocytosis of B12/transcobalamin

Question 20

What are the metabolically active forms of vitamin B12?

Answer 20

Adenosylcobalamin

Methycobalamin

Question 21

Describe how Part 1 and Part 2 of the Schilling test are performed.

Answer 21

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

Question 22

What does an abnormal Part 1 but normal Part 2 tell you?

Answer 22

Pernicious anemia

• Not getting into the blood b/c of lack of IF
• Normal part 2 b/c of supplemented IF

Question 23

How about abnormal Part 1 and Part 2?

Answer 23

NOT pernicious anemia b/c it was not fixed by IF administration

Question 24

Describe the role of adenosylcobalamin in propionate metabolism. What does a deficiency in adenosylcobalamin result in?

Answer 24

methylmalonyl-CoA mutase in propionyl-CoA metabolism require ADENOSYLCOBALAMIN

Deficiency= methylmaloyl acidemia

Question 25

Describe the role of methylcobalamin in homocysteine metabolism.

Answer 25

Methylcobalamin is required for Methionine synthase

Question 26

What is meant by the ‘methyl trap hypothesis’?

Answer 26

• 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

Question 27

What is megaloblastic anemia?

Answer 27

Anemia characterized by overgrowth of RBCs and RBC destruction

Question 28

Why does deficiency of either folate or vitamin B12 result in a megaloblastic anemia?

Answer 28

• 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*

Question 29

What circumstances might increase folate needs?

Answer 29

1) Pregnancy
2) Lactation
3) Growth
4) Chronic hemolytic anemia

Question 30

Under what circumstances may folate deficiency be seen in a patient?

Answer 30

1) Alcoholism
2) Old age
3) Poverty
4) Malabsorption syndrome

Question 31

What circumstances might increase vitamin B12 needs?

Answer 31

1) Growth

2) Pregnancy

Question 32

Under what circumstances may vitamin B12 deficiency be seen in a patient?

Answer 32

1) Strict vegans
2) Pernicious anemia
3) Celiac disease/sprue
4) Ileal resection
5) Dihydrobothrium lathum i.e. fish tapeworm
6) Competing intestinal flora

Question 33

By what mechanism can nitrous oxide anesthesia provoke an acute megaloblastic anemia?

Answer 33

• 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*

Question 34

How is folate typically found?

Answer 34

Folate polyglutamate

Question 35

What is the functional form of folate in the body?

Answer 35

THF and derivatives of THF

Question 36

Where are 1-carbon groups attached to THF?

Answer 36

N5 and N10

Question 37

What is the mechanism of action of Methotrexate?

Answer 37

• Inhibitor of dihydrofolate reducatase
• Cannot make folate i.e. cannot make DNA
• Antiproliferative drug

Question 38

List the metabolic processes that require folate.

Answer 38

1) Methionine synthesis
2) Thymidilate synthesis
3) Pruine synthesis
4) Histisdine catabolism

Question 39

What are the two reactions in the body that use B12?

Answer 39

• Methylmalonyl-CoA Mutase (Propionyl-CoA metabolsim)

- Methionine Synthase

Question 40

What is pernicious anemia?

Answer 40

• Lack of ability to absorb B12 from the ILEUM due to a lack of IF
• Autoimmune attack of parietal cells that produce IF

Question 41

Where do B12/Haptocorrin complexes traffic?

Answer 41

Liver for storage of B12

Question 42

How does B12 deficiency result in demyelination?

Answer 42

• Failure of methionine synthase reaction
• Lack of SAM

Mechanism is unclear