B12/Folate - Fitz Flashcards
The conversion of folate to dihydrofolate and dihydrofolate to tetrahydrofolate is catalyzed by what enzyme?
DHFR
Interfering with folate results in what?
- Shutting down:
- purine synthesis
- pyrimidine synthesis
What reaction is catalyzed in the liver by DHFR and requires Vitamin B12?
Methylated tetrahydrofolate (CH3THF) → Tetrahydrofolate (THF)
What is the rate limiting step for DNA synthesis? What is it catalyzed by?
dUMP → dTMP
catalyzed by: Thymidylate Synthase
(pyrimidine sythesis)
What enzyme in the Pyrimidine synthesis pathway is associated with resistance to 5-FU?
pyrimidine monophosphate kinase
What enzyme in the Pyrimidine synthesis pathway is associated with greatly increased sensitivity to 5-FU?
dihydropyrimidine dehydrogenase
Stopping what two reactions in Purine Synthesis results in the inhibition of the “de novo pathway” and the “salvage pathway” and eliminate IMP (inhibiting the ability to generate both ATP and GTP)?
- PRPP → 5‑phosphoribosylamine
- via the enzyme glutamyl amidotransferase (GPAT)
- de novo pathway
- PRPP + hypoxanthine/
- via the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT)
- salvage pathway
What is the “pseudofeedback inhibition” reaction?
- 6-MP and 6-TG (and their naturally occurring analogues) inhibit guanylyl kinase
- preventing the conversion of GMP to GDP
- stop PRPS, GPAT, HGPRT and the 2 steps that lead to the formation of XMP and AMP from IMP.
What reaction is catalyzed by ribonucleotide reductase and is inhibited by the antineoplastic drug Hydroxyurea?
CONVERSION OF RIBONUCLEOTIDES TO DEOXYRIBONUCLEOTIDES
What two things are required for DNA synthesis and for maintenance of neurons and RBCs?
Folate and Vitamin B12
What is the critical precursor in the neosynthesis of tetrahydrofolate (TH4)?
Folate
TH4 is a one carbon donor in what three reactions?
- Thymidylate and purine synthesis
- Methionine synthesis
- Amino acid metabolism
Vitamin B12 is an intermediate in
- a reaction that converts methylTH4 to TH4
- homocysteine to methionine converted in the process
- conversion of methylmalonyl to succinyl CoA
How is folate give as a supplement to a patient (2 options)?
- Folic Acid = synthetic form
- more stable
- oral, or IV/IM/SQ if GI malabsorption
- Leucovorin
- is a naturally ocurring compound that is used to replace folate in rescue therapy
- doesn’t require dihydrofolate reductase for conversion to tetrahydrofolate (i.e., its use can’t be blocked by antifolates, bypasses DHFR)
Increased levels of what are diagnostic for Vitamin B12 deficency?
Homocysteine
How is Vitamin B12 given as a supplement to a patient (2 forms)?
- HYDROXOCOBALAMIN:
- used for treatment of cyanide poisoning
- more plasma protein bound, therefore remains in circulation longer
- CYANOCOBALAMIN
About how long do the normal folate stores in the body last?
1-6 month supply
About how long do the normal Vitamin B12 stores in the body last?
~5 year supply is stored in the liver (so deficiency is rare, except in the elderly)
What are the two major transporters that get folate across the cell?
- Reduced Folate Carrier
- high capacity
- low affinity
- Folate Receptor
- high affinity
- low capacity
Why is Methotrexate a better antineoplastic than 5-FU?
- Methotrexate works on both purine & pyrimidine synthesis
- Can use Leucovorin for folate rescue
- 5-FU has the narrowest therapeutic dose
What four biochemical pathways utilize folate?
- Amino acid metabolism
- Methionine synthesis
- Purine synthesis
- Pyrimidine synthesis
(indirectly involvedc with purine salvage pathway)
What protein is overexpressed in some epithelial cancers, making them more sensitive to methotrexate?
Folate receptor
Why does folate stay in cells?
Polyglutamination keeps the folate inside the cell.
(also increases the affinity of TH4 for target enzymes)
*cancer cells are better polyglutaminators
Why is folate supplementation good for pregnant women but potentially bad for the elderly?
If you don’t check for B12 deficiency before you supplement you don’t know about possible/potential neural defects.
How does the Reduced Folate Carrier’s (RFC) affinity to Folate, Methotrexate, and Leucovorin compare?
Leucovorin > Folate > Methotrexate
Primary resistance to what drug is due to reduced expression or mutation of the RFC?
Methotrexate
How do you treat cancer cells that have decreased function of the RFC due to primary resistance of Methotrexate?
- Must give extremely high doses of methotrexate to kill the resistant cancer/bacterial cells
- LEUCOVORIN can be used to rescue the normal cells
How does the affinity of the Folate Receptor to Folate and Methotrexate compare?
Folate > Methotrexate
What deficiency does not result in neurologic damage in adults?
Folate deficiency
(can cause neural tube defects in fetuses)
What has Folate fortification in flour been good and bad?
- Decreased occurence of neural tube defects in infants
- Exacerbated neurological damage that occurs in adults/elderly because it masks Vitamin B12 deficiency
What are the four therapeutic uses of Methotrexate?
- Antineoplastic - treat cancer
- Immunosuppresent - RA, psoriasis, IBD/Crohn’s
- Antibiotic
- Arbortifacient - will cause abortion in 1st trimester (typically combined with Misoprostol
What drug is a selective, competitive inhibitor of BACTERIAL dihydrofolate reductase (DHFR)?
Trimethoprim
(folic acid inhibitor)
