Folate (7 questions) Flashcards
What are the CoE. forms of FOLATE?
DHF
THF
5-Formyl THF
10-Formyl THF
5,10-Methenyl THF
5,10-Methylene THF
5-Methyl THF
(7 total)
How are RDAs for folate expressed?
DFEs
DFE
Dietary Folate Equivalents
What is the DFE for FOLATE?
1 mcg DFE = 1 mcg from food FOLATE
What is the DFE for FOLIC ACID?
1 mcg DFE = 0.6 mcg FOLIC ACID
What is the DFE for FOLIC ACID from dietary supplements taken on an empty stomach?
1 mcg DFE = 0.5 mcg FOLIC ACID (very high bioavailability)
What is the difference between FOLATE and FOLIC ACID?
FOLATE = the form from natural food sources (green leafy vegetables); reduced form
FOLIC ACID = form from fortified foods (flours, breakfast cereals) or supplements; oxidized form
Why is FOLATE more difficult to digest?
2 reasons why folate absorption is impeded:
- FOLATE has very long tails of GLUTAMIC ACIDS
- Reduced form
What is the bioavailability of FOLATE?
50% naturally present form
What is the bioavailability of FOLIC ACID?
~85% when taken with food
What are the best food sources of folate?
- Beef liver
- Spinach, boiled
- Black-eyed peas
- Breakfast cereals
- Green vegetables (asparagus, Brussels, lettuce, etc)
Also, fortified grains
(T/F) There is a federal fortification of folate.
TRUE, since 1998.
140 micrograms folic acid per 100g product
FOLIC ACID MONOGLUTAMATE is the fortified form (it’s oxidized, so very easily absorbed)
What is the formula for calculating DFEs?
mcg DFE = mcg naturally occurring folate + (1.7 x mcg folic acid)
Example 1: Calculating DFEs
A serving (85g) of frozen spinach contains 120 mcg of folate. What is the DFE?
Folate (mcg DFE) = 120 mcg x 1
120 mcg DFE
Example 2: Calculating DFEs
A serving (40g) of ready-to-eat breakfast cereals contains 200 mcg folic acid. What is the DFE?
Folate (mcg DFE) = 200 mcg x 1.7
340 mcg DFE
Example 3: Calculating DFEs
A serving (30g) of enriched wheat flour contains a combination of naturally occurring folate (5 mcg) and folic acid (50 mcg). What is the DFE?
Folate (mcg DFE) = (5 x 1) + (1.7 x 50)
90 mcg DFE
(T/F) Folic acid from supplements and fortification do not need to undergo digestion and are readily absorped.
TRUE
FILL IN THE BLANK
Folate naturally found in food is in XXXXXXXXXX form and must be digested to XXXXXXXXXX form.
POLYGLUTAMATE FORM
MONOGLUTAMATE FORM
What 2 enzymes are needed for folate absorption?
CARBOXYPEPTIDASES are needed to hydrolyze (“remove”) the polyglutamate tail from folate to allow for absorption:
- Soluble carboxypeptidase
- Enzyme bound to brush border membrane
Soluble carboxypeptidase
From pancreatic juices and bile secreted into the jejunum
Needed to hydrolyze polyglutamate tail from folate for absorption
Enzyme bound to brush-border membrane
Contributes to digestion/absorption more than soluble form
—It is zinc-dependent and requires a HIGHER pH (more basic) in the duodenum (thanks to bicarbonate from the pancreas)
—It is inhibited by ETOH ingestion and certain foods (legumes, lentils, cabbage, and oranges)
(T/F) Dietary supplements of folate are inhibited by the ingestion of ETOH due to the enzyme bound to brush-border membrane.
FALSE
Only natural forms of folate require the two carboxypeptidases enzymes to hydrolyze the polyglutamate tail from folate to allow for absorption
The enzyme bound to brush-border membrane is inhibited by ETOH and certain foods (lentils, legumes, oranges, and cabbage)
Proton-coupled folate transporter
A high affinity folate carrier expressed in proximal jejunum and duodenum
Facilitates the transport of folic acid and folate into the enterocytes
What happens once folate and folic acid are in the enterocyte?
Once folate enters the CYTOPLASM of the cell, it is going to be reduced by dihydrofolate reductase to DHF, then THF with NADPH and reductase enzyme
How does folate leave the enterocyte?
THF will bind with a folate binding protein and be transported outside of the enterocyte, VIA MRP3
MRP3
An active-carrier dependent Multi-drug Resistant Protein 3, that binds THF to transport folate outside the enterocyte
What CoE. form will folate be in portal circulation?
5-methyl THF, OR
DHF, OR
the formulated forms as well.
It will be monoglutamated
RFC
Reduced Folate Carriers; ubiquitously expressed
Facilitates carrier-mediated uptake into the liver (majority) and other tissues
FOLR1
Folate Receptor Alpha; expressed in brain
Note defects in gene are associated with neurological disorders
What are the forms of folate in hepatocytes and other cells? What is the breakdown?
~33% THF
~33% 5-methylTHF
~33% 5- or 10- formyl THF
How is folate trapped in cells?
Once in the cell, it will be converted to the polyglutamate form, trapping it into the cell
Polyglutamate synthetase
Enzyme that catalyzes the addition of 4-8 glutamate / glutamic acids
Requires ATP
Traps folate within the cell
Gamma-glutamyl hydrolase
Enzyme that removes glutamate residues prior to folate exiting the cell
FILL IN THE BLANK
In systemic circulation, folate is in XXXXXXXXXX form.
Monoglutamate form
—~1/3 free
—~2/3 bound to protein (albumin, alpha 2 macroglobulin, and high-affinity folate binding protein)
In THF, 5-methylTNF or 10-formylTHF
How much folate is stored in the body? Where?
7 to 30 mg stored
50% in the liver, in THF and 5-methyl THF polyglutamate forms
(T/F) When folate is trapped inside the cell, it’s typically also bound to some type of folate-binding protein. This will keep it anchored into the cell, as well as the polyglutamate tail.
TRUE
Folate CoE. forms function in which 2 types of reactions?
- Nucleotide biosynthesis
- Methylation of AA
How does THF form THF derivatives?
THF accepts one-carbon groups
THF derivatives then serve as donors of one carbon units in a variety of biosynthetic reactions
Folate is critical for…..
pyrimidine synthesis of DNA
What are the 3 pyrimidines?
1) Cytosine
2) Uracil
3) Thymine
Pyrimidine
The base
Will be converted to CYTOSINE (which is used for RNA and DNA biosynthesis)
(T/F) The base uracil can be converted to thymine.
TRUE, when the body needs to make DNA
Uracil
ONLY used in RNA
Thymine
ONLY used in DNA
dUMP
DeoxyURIDINE monophosphate; used in RNA synthesis
Can be converted to dTMP
dTMP
DeoxyTHYMINE monophosphate; used in DNA synthesis
Thymidylate synthetase
An enzyme that requires folate (as N5, N10-methylene THF)***
So in this reaction, a methyl group is going to be added, when uracil —–> thymine
(T/F) The dUMP —> dTMP reaction can take place without folate.
FALSE
If there is a deficiency of folate, this reaction cannot take place.
What is the consequence of utilizing uracil for DNA synthesis?
When there is a deficiency of the CoE form (5-, 10-methyleneTHF), the body will use URACIL into the DNA instead of using thymine, which causes all kinds of problems.
This is considered an AP site (pre-mutagenic apyrimidinic site), which can lead to:
—DNA strand breaks
—DNA fragmentation
—or, the cell can recognize this as an anomaly, can the cell will go through APOPTOSIS (cell suicide)
This accumulation of DNA damage could result in pre-cancerous changes.
What CoE. form is required for the biosynthesis of purines? ***
10-formyl THF form
What are the 2 purines?
1) Adenine
2) Guanine
Both used in DNA and RNA
Explain the process for dUMP —–> dTMP when there is not a folate deficiency present.
5-,10-methylene THF is going to be used for the dUMP —-> dTMP reaction. With thymidylate synthetase being required.
The 5,10-methylene THF is going to donate the methyl groups, and then be converted back to DHF
What is the base, that will be converted to adenine and guanine? **
INOSINE MONOPHOSPHATE (IMP) **
The biosynthesis of IMP is extremely important because it REQUIRES FOLATE; specifically, the 10-formyl THF form
(T/F) The body must biosynthesis purines every time it needs to make RNA and DNA.
FALSE. There are salvage pathways, so when the body breaks down old DNA and RNA, it can reuse these purines.
De Novo Synthesis
Meaning “FROM THE NEW”
In reference to the biosynthesis of purines
An example would be a child who is rapidly growing, or during a growth spurt, they are going to need de novo purine biosynthesis, and it is VERY critical.
It is not a concern for older adults
What CoE. form is required to convert glycine <—–> serine?
5, 10-methylene THF (also used in dUMP to dTMP)
Serine hydroxymethyltransferase
PLP-dependent enzyme (B6 COENZYME**) that converts 5, 10-methylene THF <—->THF in the reaction: Glycine <——-> Serine
Note: this is a reversible reaction
Explain folate’s role in homocysteine metabolism.
5-methyl THF is going to be picked up by the enzyme METHIONINE SYNTHASE, and is going to take that methyl group, and add it to COBALAMIN, so now we have METHYLCOBALAMIN. This converts 5-methyl THF —> THF, allowing THF to go through the glycine <—-> serine reaction, via SERINE HYDROXYMETHYLTRANSFERASE.
Explain how SAM plays a role in the methylation of DNA.
SAM donates a methyl group to CYTOSINE, creating 5-methylcytosine.
DNA methyl-transferase
Enzyme in the reaction of converting cytosine —> 5-methylcytosine
Where is the methyl group on 5-methylcytosine, originally come from? ***
Initially this methyl group came from 5-methyl THF, then given to B12, creating methylcobalamin, then given to methionine, and then converted to SAM, and now it’s going to be provided here to make DNA.
**AKA: IT STARTED OUT AS PART OF FOLATE
How does methylation play an important role in gene expression?
The importance of the methyl groups getting added to cytosine is seen here.
If the DNA is wrapped around histones (DNA spools), and the brown spots are methylated cytosines, it keeps the DNA tight and wrapped up. So the DNA cannot be accessed and transcription of DNA cannot occur.
When methylation is removed, the spooling becomes much looser and DNA can now be accessed by other enzymes that CAN go through gene transcription. So gene expression is going to be increased
It is all involved with 5-methyl THF bc that’s where the methyl group originated from.
Folate Deficiency
Megaloblastic macrocytic anemia (very common)
—fewer than normal RBCs, that are large and immature, oval-shaped
What types of cells are likely to be affected by folate deficiency?
Rapidly-dividing cells, such as 1) enterocytes, and 2) blood cells
How does folate deficiency present, clinically?
MCV increases and hypersegmentation of WBCs occur, due to:
Impaired DNA synthesis, negatively effecting GI cells and snowballing into impaired nutrient absorption of other vitamins and minerals.
-Glossitis and bright red tongue, swelling (in severe deficiencies)
-Spina bifida and neural tube defects
What is the upper tolerable intake for folate, in adults? Why?
1 mg for folic acid supplementation of fortified foods, because folic acid supplements can mask potential B12 deficiency associated with megaloblastic anemia but not neurological damage
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Intakes of folate higher than XXXXXX times recommendations may be associated with increased cancer risk and cancer mortality (especially in those with pre-cancerous tumors). Why?
2.5x higher
Because of folate’s ability to be utilized in the synthesis of DNA. So, in those with cancers that are rapidly growing tissues, we do not want that to happen, and so increased intake of folate can actually drive cancer growth in that way.
How is folate assessed?
Plasma folate concentrations (3 to 20 mcg/L). Below 3 = deficiency. This indicates the short-term status of folate
RBCs folate <140 mcg/L considered deficient. This indicates the long-term status of folate because RBCs live for 120 days.
