Folate, Vitamin B12 and Inhibitors

Question 1

What are the two ways we synthesize tetrahydrofolate in our bodies?

Answer 1

1. folate is converted to dihydrofolate and then to tetrahydrofolate by DHFR
2. methyltetrathydrofolate from liver stores in converted to tetrahydrofolate (requires vitamin B12)

Question 2

What reactions use tetrahydrofolate as a carbon donor?

Answer 2

1. thymidilate and purine synthesis
2. methionine synthesis
3. amino acid metabolism

Question 3

What is the rate limiting step in DNA synthesis? What enzyme?

Answer 3

conversion of UMP to TMP with thymidylate synthase (in pyrimidine synthesis)

Question 4

What reactions in pyrimidine synthesis is important for the development of resistance of 5-FU?

Answer 4

UMP to UDP by pyrimidine monophosphate kinase

Question 5

An inherited deficiency in what enzyme responsible for thymidine nucleotide degradation causes greately increased sensitivity to 5;FU?

Answer 5

dihydropyrimidine dehydrogenase

Question 6

What two kinds of cells particularly need folate and vitamin B12 for maintenance/

Answer 6

RBCs and neurons

Question 7

B12 is necessary for a reaction that converts methylTH4 to TH4, converting homocysteine to what?

Answer 7

methionine (that’s why homocysteine levels are diagnostic of B12 deficiency)

Question 8

B12 is also necessary for the conversion of methylmalonyl CoA to what?

Answer 8

succinyl CoA

Question 9

What’s the synthetic form of folate typically used in supplements?

Answer 9

folic acid

Question 10

What’s the naturally occurring form of folate that’s used to replace folate in rescue therapy?

Answer 10

leucovorin

Question 11

What about leucovorin makes it better for folate rescue than just folic acid?

Answer 11

It doesn’t require dihydrofolate reductase for its conversion to tetrahydrofolate

Question 12

What are the two naturally occurring forms of B12?

Answer 12

adenosylcobalamin, methylcobalamin

Question 13

What are the two medicinal forms of B12?

Answer 13

hydroxocobalamin, cyanocobalamin

Question 14

Where in the body is folate absorbed?

Answer 14

small intestine

Question 15

Why is there a high daily requirement for folate?

Answer 15

a relatively small quantity is stored in the liver - only 1-6 month supply

Question 16

What is required for absorption of B12?

Answer 16

acidic pH and intrinsic factor made in the stomach

Question 17

Where in the body does absopriton of B12 occur?

Answer 17

small intestine

Question 18

Why is B12 deiciency rare except in the elderly?

Answer 18

there’s about a 5 year supply stored in the liver

Question 19

Reduced expression or mutation of what causes a primary resistance to low doses of folate inhibitors like methotrexate?

Answer 19

reduced folate carrier

Question 20

What do we give to overcome resistance from mutated reduced folate carrier?

Answer 20

HIGH doses of methotrexate (forces uptake via folate receptor), followed by leucovorin to rescue healthy cells (since leukocovorin won’t be taken up by the mutant form - haha sucker)

Question 21

Comparing the reduced folate carrier and the folate receptor…Which has higher affinity? which has higher capacity?

Answer 21

reduced folate carrier has low affinity but high capacity

folate receptor has high affinity but low capacity and is only in specific tissues

Question 22

Overexpression of the folate receptor (as seen in some leukemias, gynecological cancers and epithelial cancers) will conver a higher or lower sensitivity to folate inhibitors?

Answer 22

high sensitivity

Question 23

What does polyglutamation do for the folates inside the cell?

Answer 23

It occurs through the enzyme folylpolyglutamate synthase

It is required for biological activity and facilitates retention and icnreases affinity for folate-dependent enzymes like TS

Question 24

How does polyglutamation confer selective toxicity for the folate inhibitors?

Answer 24

Cancer cells are more capable of polyglutamation, so they’re more likely to retain the folate inhibitors once they’re in

Question 25

What’s usually the first clinical sign of a folate or B12 deficiency?

Answer 25

megaloblastic anemia

Question 26

What can cause a folate deficiency?

Answer 26

insufficient dieary intake (rare), high demand (pregnancy), alcoholism, intestinal disease like celiacs, inhibitors

Question 27

Why do we fortify with folate now?

Answer 27

to avoid neural tube defects

Question 28

What’s the most common cuase of B12 deficiency?

Answer 28

intrinsic factor deficiency (so you can’t treat it with oral supplementation)

Question 29

Why does B12 deficiency cause a flate deficiency as well?

Answer 29

Without B12 you can’t access the stored TH4

Question 30

IF the B12 deficiency is caused by lack of IF, what type of anemia occurs?

Answer 30

pernicious anemia

Question 31

Which can cause neurological deficits in adults: folate or B12 deficiency

Answer 31

B12

Question 32

Why is it critical that B12 stats be checked before beginning folate supplementaiton?

Answer 32

Because once B12 deficiency reaches neurological stages, it really can’t be reversed

Question 33

What’s the dilemma with folate fortification?

Answer 33

It decreases incidence of neural tube defects, but it masks B12 deficiencies, making it more difficult in millions of elderly people - common cause of senile dementia

Question 34

What does methotrexate inhibit?

Answer 34

DHFR

Question 35

What are the four therapeutic uses of MTX?

Answer 35

1. cancer
2. immunosuppressant (RA, IBS, etc)
3. antibiotic
4. abortifacient

Question 36

Trimethoprim is usually combined with what other antibiotic to work synergistically against bacteria?

Answer 36

sulfamethoxazole - blocks dihydropoteroate synthetase

together they block sucessive steps in the folate synthesis pathway

Question 37

What does trimethoprim inhibit?

Answer 37

competitive inhibitor of DHFR (like methotrexate)