4 - Folate and Vit B12 metabolism Flashcards Preview

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Flashcards in 4 - Folate and Vit B12 metabolism Deck (35):


(folic acid, or pteroylglutamate)
- comes from Green leafy vegetables
--- spinach, lettuce, broccoli
- Water-soluble and readily removed by prolonged cooking



- Functional form is tetrahydrofolate (THF)
- formed from folate by dihydrofolate reductase
--- coverts folate to dihydrofolate and converts dihydrofolate to THF



- folate analog
- inhibitor of dihydrofolate reductase
- prevents generation of THF
- antiproliferative effects


THF and the one-carbon pool

- THF carries one-carbon groups
--- oxidized/reduced whilst bound to THF
-----> can modify enzymes
--- very important for purine/pyrimidine synthesis


Major source of one carbon groups

serine hydroxymethyltransferase reaction
Serine + THF <=> Glycine + N5, N10-methylene-THF + H2O


Absorption of dietary folate

- Folic acid converted from polyglutamate form to monoglutamate form by brush border enzymes
- monoglutamate form taken up into intestinal epithelial cell
- monoglutamate folic acid reduced and methylated into N5-methyl THF
--- most reduced form of folate
- N5-methyl THF released into mesenteric circulation


Uptake of folate from blood

- Receptor-mediated endocytosis
- Receptors have high affinity for folate monoglutamates
--- N5-methyl THF monoglutamate is most abundant form in circulation
- Intracellular folate rapidly metabolized
--- polyglutamate added
-----> to keep the folate in the cell


Thymidylate synthase

- converts dUMP to dTMP
- essential for DNA synthesis
- most important reaction clinically
--- hurt first if there's a deficiency of folate. Results in symptoms


Vitamin B12

- multiple forms
--- including cyanocobalamin which is included in vitamin supplements
- Complex structure
- Only synthesized by certain bacteria
- Best dietary sources: liver, kidney, other meats, dairy products, shellfish
*- Plant foods do not supply vitamin B12
--- unless fortified
--- contaminated with bacteria/soil


Four cobalamins in human metabolism


- Modifications occur at position labelled R


biologically active forms of Vitamin B12

Adenosylcobalamin and methylcobalamin


Absorption of dietary vitamin B12

- gastric acid and pepsin release dietary B12
- B12 bound to Haptocorrins in stomach
--- Haptocorrins present in many secretions in body
- Haptocorrin/B12 complex moves into duodenum and is broken apart by pancreatic proteases
- Free B12 bound by Intrinsic factor
--- intrinsic factor made in stomach but acts in intestine
- Intrinsic factor/B12 complex absorbed in ileum
- Intrinsic factor degraded, receptor recycled


Pernicious anemia

- Lack of ability to absorb vitamin B12 from ileum
--- autoimmune disease
-----> gastric atrophy
-----> no intrinsic factor produced
- takes a long time to develop because B12 stored in liver


Pernicious anemia tx

- intramuscular injection
- or 1 mg/day oral

- previously treated by eating beef liver


Transport and uptake of B12 from the blood

- 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



- Binding of B12 to TC required for uptake into cells
- But most B12 in circulation is bound to haptocorrin
- Haptocorrin/ B12 complexes taken up by liver
- Haptocorrins may be forming a circulating store of B12

- aside from action of binding B12 in stomach


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


The Schilling test

- No longer available in the USA!
--- inability to procure labeled vitamin B12
- Measures ability of patient to absorb B12


The Schilling test: 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


The Schilling test: Part 2

- oral load of radioactive B12
- oral load of purified intrinsic factor
- proceed as for part 1


Abnormal Schilling test: Abnormal part 1, normal part 2

pernicious anemia (intrinsic factor production defect)


Abnormal Schilling test: Abnormal part 1, abnormal part 2

defect in B12 absorption independent of intrinsic factor production e.g. defect in TCII production, TCII receptor, etc.


Metabolic processes requiring vitamin B12

- Propionyl-CoA metabolism
- Homocysteine metabolism


Where is Vit B12 needed in Propionyl-CoA metabolism

--- methylmalonyl-CoA mutase requires adenosylcobalamin
-----> Conversion of L-methylmalonyl-CoA to succinyl-CoA


B12 Deficiency effects on Propionyl-CoA metabolism

- L-methylmalonyl-CoA accumulates
--- hydrolysis yields methylmalonic acid
-----> organic acidemia


Where is Vit B12 needed in homocysteine metabolism?

- Methionine synthase converts homocysteine to methionine
--- concomitantly, THF regenerated from N5-methyl THF


The methyl trap hypothesis

- For 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 the correct oxidation state inside cells


Neurological consequences of vitamin B12 deficiency

progressive demyelination
- failure of methionine synthase reaction
--- Some patients undergoing demyelination due to lack of B12 have shown improvement upon methionine administration
- Mechanism unclear
--- possibly related to lack of S-adenosylmethionine
----> inability to correctly methylate key substrates?


folate deficiency

- Vitamin B12 deficiency leads to functional folate deficiency
--- accumulation of N5-methyl THF
- Dietary insufficiency may lead to actual folate deficiency


effects of folate deficiency

- thymidine synthesis blocked
--- thymidylate synthase requires N5,N10-methylene THF
-----> clinically most important
-----> dUTP incorporated into DNA instead of dTTP
- purine synthesis blocked
--- two steps require N10-formyl THF

DNA synthesis is prevented
--- Cells grow but cannot divide
-----> become megaloblastic
-----> eventually leads to DNA fragmentation and cell death
--- Most obviously apparent in rapidly-dividing cells
-----> megaloblastic anemia


Folate needs increased by

chronic hemolytic anemia


folate Deficiency often seen in

old age
celiac disease/tropical sprue/other conditions of malabsorption


Vitamin B12 needs increased by

periods of growth

(folate too)


Vitamin B12 Deficiency often seen in

strict vegans (who shun supplements/fortified foods)
pernicious anemia
celiac disease/tropical sprue/impaired absorption
gastric acid insufficiency
ileitis/resection of the ileum
fish tapeworm infestation
competing intestinal flora


Acute megaloblastic anemia: nitrous oxide anesthesia

- indicators:
--- megaloblastic abnormalities
--- methylmalonic aciduria

- acute course b/c nitrous oxide destroys methylcobalamin
--- nitrous oxide anesthesia
--- transiently lose methylcobalamin but if already B12 deficient then serious