Folate and B12 Flashcards
What vitamin is folate?
vitamin B9
How was folate initially investigated?
(vitamin B9) was initially investigated as a dietary factor that prevented anemia of pregenancy and as a growth factor present in green leafy vegetables (Foliage)
* folate = “folium” (leaf)
Metabolic functions of folate
Intracellular folate is important for many cellular process:
* Methylation reactions (Lipids, proteins, DNA, RNA)
* Nucleic acid synthesis (synthesis of DNA precursors)
* Amino acid metabolism (Eg. methionine, cysteine, histidine, serine/glycine)
What are the two common forms of folate?
- folate found naturally in foods
- folic acid which is the form found in supplements and fortifie foods
Structure of folate vs. folic acid
The double bond on N5 and N8 is folic acid but with a single bond it is folate. Can also add on more glutamate which give more negative charges
* folate: single bond at N3, N5, N8, 4 Hs
* folic acid double bonds at N3, N5, N8
Where is the active site of folate?
N-5 and N-10
* where substances are added or donated. So methyl groups can be added or donated from here
What are the folate derivatives?
Essentially acts as a substrate for these reactions; so not bound to enzymes like other B vitamins
* most common is THF - tetrahydrofolate?
What role does folate have with histidine?
histidine catabolism to glutamate which is a gluconeogenic precursor
* there is a transfer of the formimino group from formiminoglutamate to THF whereby formimino-THF is converted to 5,10-methenyl-THF
What happens to histidine catabolism with folate deficiency?
buildup of formiminoglutamate in urine
Role of folate in synthesis of DNA precursors
- Pyrimidine synthesis
- purine biosynthesis
Role of folate in pyrimidine synthesis
Thymidylate
* A methyl group on serine is added to uracil to make thymine via THF.
* Converts THF to DHF so need to covert back to THF before it can be used again. This uses NADPH (niacin) to bring back folate. Maintaining active form of THF is important.
What other vitamin is important in the pyrimidine synthesis?
Conversion of DHF back to THF uses niacin (NADPH).
What is Methotrexate?
anti-cancer drug & folate antagonist
* traps DHF in cell, which impairs folate metabolism and reduces cell division when this recycling pattern gets out of hand with cancer
Folate role in purine biosynthesis
Adenine & Guanine
* both require folate
What base pairs require folate?
- thymine
- adenine
- guanine
What is the idea behind folates role in ‘one-carbon metabolism’?
Should be able to draw diagram
Methionine synthase: Folate plays a role in maintaining the essential AA methionine by adding a methyl group to homocysteine via methionine synthase, this cycle is important for many processes.
* methionine is required for cell proliferation, protein synthesis, RNA synthesis and transamination
* methionine is converted to S-adenosyl-Met (AdoMet or SAM) which is important for transmethylation reactions
* AdoMet → AdoHcy → homocysteine
* homocystein can be converted to cystein but ultimately the body wants to maintain as much methionine as possible so homocysteine can be converted back to methionine and the methyl group comes from folate
What are other important vitamins that a play a role in the folate dependant methionine resynthesis cycle?
- Niacin & Riboflavin play a role in making methyl-THF via MTHFR which then donates the methyl group to homocysteine to form methionine
- B12 play a role in methionine synthase
- Pyrodixine plays a role in proteins synthesis (glycine, serine, cystein) within the folate cycle
How might B12 deficiency effect folate metabolism?
B12 is neccessary for methionine synthase in assisting the methyl THF to donate its methyl group to homocysteine. This is the only thing that methyl-THF is used for so there can be a build up of methyl-THF and of homocysteine without B12
* MS (methionine synthase) only way for methyl-THF to be demethylated in tissues to yield free THF
What are some transmethylation reactions which folate is important for?
folate plays a role by ensuring methionine production in order to make SAM which is used in the transmethylation reactions
* epigenetic methylations (DNA/ RNA)
* sterol methylation (catecholamines)
* phospholipid methylation
* creatine synthesis
* protein synthesis
Folate role in epigenetics
- DNA methylation
- histone methylation
Folate in DNA methylation?
Epigenetic pattern during developement whereby some genes are turned off when methylated or activated without the methylation. So without folate can have disproportionate genes turned on or off.
Folate in histone methylation?
Genes wrapped around histones and in order to turn on they have to be unwrapped. Methylating turns off the genes associated so that genes wrapped around cannot be activated but with no methylation they can then unwrap.
folate role in sterol methylation
synthesis of catacholamines include dopamine, epinephrine (adrenaline), and norepinephrine (noradrenaline)
* important for stress responses
* Difference is a methyl group so methylate nor to epi
folate role in PL methylation
In PL metabolism, phosphocholine is the main head group which has 3 methyl groups and all of these use SAM for synthesis
* PC is important for membranes and transport of the lipids;
What would happen in Phospholipid Methylation with folate deficiency?
folate deficiency would inhibit the reaction for synthesis of PC via SAM so would get huge fatty livers because would not be able to move VLDL out of the liver
What would happen with sterol methylation if folate deficient?
Would not be able to form epinephrin
Folate role in creatine synthesis
transmethylation reaction whereby SAM donate methyl group to Guanidinoacetate (starts from arginine) to form creatine
How much of SAM (AdoMet) goes towards creatine synthesis?
This reaction actually consumes most of AdoMet ~30-50%
Folate metabolism overview; draw diagram
How is the RDI of folate determined?
The RDI, is the amount of dietary folate sufficient to maintain red cell folate levels.
How is folate requirements expressed?
We dont use ug, because bioavailability of supplement is different from naturaI sources so it is expressed in Dietary Folate Equivalents (DFE) to take into account the different availabilities of food, supplements, and fortification.
* Synthetic folic acid taken in as supplements and fortified cereal gain foods are 1.7 times more bioavailable as most food folate.
* μg DFE = μg food folate + (1.7 x μg folic acid)
What are the folate requirements?
Folate requirements rise with age.
* 400µg M/F
* greater in pregnancy and lactation
folate upper limit
UL = 1 mg/day folic acid
* high doses folate can mask development of B12 deficiency - Anemia is first sign of deficiency of both folate and B12. But if folic acid is is really high and B12 deficient it does not cause anemia and never know and are then at risk for neuropathy
Dietary sources of folate
found in plant & animal tissues
* Dietary sources include beans, nuts, meat (especially organ meat), dairy products, fruits, grains, and cereals.
principle sources of folate
liver, dark green leafy veg, legumes
How is folate absorbed?
dietary folate as poly- and mono- Glu folates must first be converted to THF which can then be absorbed to enterocyte. folic acid can be transported in enterocyte as is, where it is converted to THF before being absorbed for further circulation. In circulation goes to liver first where THF may become bile (goes to gut), or becomes poly-Glu (metabolically trapped) and used for function or sent to other tissues for use
Important of folate in the folate dependant methionine cycle?
to maintain methylation cycle
How do genetics effect folate requirements?
polymorphisms in methylene tetrahydrofolate reductase (MTHFR) which converts 5,10-methylene-THF to 5-methyl THF (which is what is used on MS)
What other vitamins are required for MTHFR?
Reaction requires Riboflavin (B2) and Niacin (B3)
What are the different MTHFR polymorphisms and prevalence?
- 9% have TT - homozygous affect polymorphism (mutation in both)
- 44% have CT - heterozygous (one mutation)
- 47% have CC - normal
What food requires mandatory fortification of folate and when did this happen?
In 1997, US and Canada instituted mandatory folate fortification of flour and grains.
How did mandatory folate fortification change the rate of NTDs?
Rate of NTDs was reduced but not as good as was projected
What are the cut offs for folate status and what is the prevalence of this in Canada?
- 305 mol/L Deficiency cut-off; so very few in Canada are deficient
- 1360 mol/L High concentration cut-off; so a lot of people have excess
How might folate mask B12 deficiency
pre- vs. post- folic acid fortification has seen an increase in women who show B12 defiency but excess folate
* fortification essentially created this group
How might folate deficiency and excess contribute to cancer?
essential if folate is taken earlier on when condition are still normal it will likely decrease risk but if taken in adenoma (benign tumour) stage it may increase risk
* Folate deficiency limits the synthesis of thymidine and leads to misincorporation of uracil or thymide into DNA leaving breaks the predispose the DNA to mutation and initiation of cancer
* However…Folate plays a central role in DNA methylation since it provides methyl groups for the synthesis of AdoMe so excess may cause hypermethylation of promotor regions with global DNA hypomethylation which is a common observation in tumors.
Structure of Cobalamin
Large molecule with a central cobalt
What is the active site of B12?
The cobalt
B12 role in MS
accepts the methyl from methyl-THF and donates it to homocysteine to form methionine
Role of B12 in Methylmalonyl CoA Mutase
Methylmalonyl CoA Mutase enzyme is dependant on B12 for isomerization of methylmalonyl CoA to succinyl CoA in order for assocaited proteins to enter the CAC
Absorption of B12
Absorption of vitamin B12 requires the action of an Intrinsic Factor (IF) and transcobalamin–II (TC-II)
* B12 in ingested and IF is released from stomach parietal cells. IN the distal iluem IF binds B12 and IF receptor then recognized it and carries it into enterocyte. A lysosome in the enterocyte releases B12 where it then binds with TC-II and leaves the enterocyte TC-II binds to the TC-II receptor of peripheral tissue and is transported into cells
Best dietary sources of B12
- meats (beef liver, beef products, chicken products)
- dairy products
- fish and seafood
- eggs
non-meat sources of B12
- mushrooms
- tempeh
- saurkraut
B12 DRIs
- 2.4 ug/d M/F
- increased need in pregnancy and lactation
- ↑ in elderly due to malabsorption
- no UL set
What is the schilling test for determining vitamin B12 status?
Schilling test = tests for B12 malabsorption
* stage I - 57Co+B12 load: if 57Co excretion is low the problem is absorption
* stage II - 57Co+IF: if 57Co excretion normalized the problem is IF; if not – may be bacterial overgrowth
* stage III - Itreatment with antibiotics & retest
factors relating to B12 deficiency
dietary deficiency is uncommon and we have large B12 stores which are depleted slowly (turnover = months)
* malabsorption more likely
Best indicator for B12 deficiency due to dietary intake
methylmalonic acid is most sensitive
* comparing status of methylmalonic acid od control vs. vegans
