Metabolic functions of folate Domina Petric, MD

Coenzymes in single-carbon metabolism November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Introduction The folates function as enzyme co-substrates in many reactions of the metabolism of amino acids and nucleotides, as well as the formation of the primary donor for biological methylations, S-adenosylmethionine (SAM). In each of these functions, the fully reduced (tetrahydro-) form of each serves as an acceptor or donor of a single-carbon unit. These reactions are referred to as single-carbon metabolism. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Methionine and S-adenosylmethionine (SAM) synthesis As 5-methyl-FH4, which is freely produced from 5,10-methenyl-FH4, folate provides labile methyl groups for methionine synthesis from homocysteine. Methionine is essential for the synthesis of proteins and polyamines, and is the precursor of SAM, which serves as a donor of labile methyl groups for more than 100 enzymatic reactions that have critical roles in metabolism. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Methionine and S-adenosylmethionine (SAM) synthesis S-adenosylmethionine also serves as a key regulator of the transsulfuration and remethylation pathways. This metabolism is catalyzed by two enzymes: methylenetetrahydrofolate reductase (MTHFR) and methionine synthase. Methylenetetrahydrofolate reductase is necessary for the production of 5-methyl-FH4, the obligate single-carbon donor for methionine synthesis. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Histidine catabolism Cytosolic formiminotransferase catalyzes the final reaction in the catabolism of histidine by transferring the formimino group from formiminoglutamate (FIGLU) to FH4. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Nucleotide metabolism Folates are required for the synthesis of thymidylate and for purine synthesis. Both roles are necessary for de novo synthesis of DNA and thus for DNA replication and cell division. Disruption of these functions impairs cell division and results in the macrocytic anemia of folate deficiency. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Thymidylate synthesis Folates transport formaldehyde (as 5,10-methylene-FH4) for thymidylate synthesis. The enzyme thymidylate synthase catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), involving the transfer of formaldehyde from 5,10-methylene-FH4 to dUMP. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Thymidylate synthesis This step is rate limiting to DNA replication, thus enabling the normal progression of the cell cycle. Thymidylate synthase is expressed only in replicating tissues. The highest expression occurs during the S phase of the cell cycle. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Purine synthesis Folates transport formate (as 10-formyl-FH4, produced from 5,10-methylene-FH4) in two steps in the synthesis of purines: synthesis of adenine and guanine. In this way, formyl groups are used to provide the C-2 and C-8 positions of the purine ring, and FH4 is regenerated. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Purine synthesis These reactions are catalyzed by aminoimidazolecarboxamide ribonucleotide tranformylase and glycinamide ribonucleotide tranformylase. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Regulation of single-carbon metabolism The regulation of single-carbon metabolism is effected by the interconversion of oxidation states of the folate intermediates. β-carbon of serine is the major source of single-carbon units for these aspects of metabolism. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Regulation of single-carbon metabolism That carbon fragment is accepted by FH4 to form 5,10-methylene-FH4 (by serine hydroxymethyltransferase), which has a central role in single-carbon metabolism. The latter derivative can be used directly for the synthesis of thymidylate (by thymidylate synthetase). It can be oxidized to 5,10-methenyl-FH4 (by 5,10-methylene-FH4 dehydrogenase) for use in the de novo synthesis of purines, or it can be reduced to 5-methyl-FH4 (by MTHFR) for use in the biosynthesis of methionine. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Regulation of single-carbon metabolism The result is the channeling of single-carbon units in several directions: to methionine, to thymidylate (for DNA synthesis) or to purine biosynthesis. Folyl polyglutamates have been found to inhibit a number of the enzymes of single-carbon metabolism. Variation in their polyglutamate chain lengths may play a role in the regulation of single-carbon metabolism. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

The methyl-folate trap The major cycle of single-carbon flux appears to be the serine hydroxymethyltransferase/5,10-methylene-FH4 reductase/methionine synthetase cycle, in which the methionine synthetase reaction is rate limiting. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

The methyl-folate trap The committed step (5,10-methylene-FH4 reductase) is feedback inhibited by S-adenosylmethionine and product inhibited by 5-methyl-FH4. Methionine synthetase depends on the transfer of labile methyl groups from 5-methyl-FH4 to vitamin B12. Methyl-B12 serves as the immediate methyl donor for converting homocysteine to methionine. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

The methyl-folate trap Without adequate levels of vitamin B12 to accept methyl groups from 5-methyl-FH4, homocysteine accumulates at the expense of the other metabolically active folate pools. This is known as the methyl-folate trap. The loss of FH4, that results from this blockage in folate recycling, blocks transfer of the histidine-formino group to folate (as 5-formimino-FH4) during the catabolism of that amino acid. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

The methyl-folate trap This results in the accumulation of the intermediate formiminoglutamate (FIGLU). Elevated urinary FIGLU levels after an oral histidine load are diagnostic of vitamin B12 deficiency. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Health effects II. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Homocysteinemia High circulating levels of homocysteine have been associated with increased risks to cardiovascular disease, recurrent early pregnancy loss and hip fracture. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Homocysteinemia Accumulation of homocysteine can occur through: elevated production of homocysteine from methionine impaired disposal of homocysteine through transsulfuration to cystathionine This results in homocysteinemia and homocysteinuria. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Homocysteinemia Homocysteine can be converted to homocysteine thiolactone by methionyl tRNA synthetase, in an error-editing reaction that prevents its incorporation into the primary structure of proteins. At high levels the thiolactone can react with protein lysyl residues. Protection against the damage to high-density lipoproteins that results from homocysteinylation is effected by a Ca2+-dependent homocysteine thiolactonase associated with those particles. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Homocysteinemia Homocysteinemia can have congenital causes and can also be related to nutritional status with respect to vitamin B6, vitamin B12 and folate. It causes displacement of protein-bound cysteine, which results in changes in redox thiol status, probably via thiol-disulfide exchange and redox reactions. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Homocysteinemia Homocysteinemia can respond to supplementation of folic acid. The relationship is linear up to daily folate intakes of about 0.4 mg. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Homocysteinemia The MTHFR genotype can also affect plasma homocysteine levels. Individuals with the C677T T/T genotype have slightly lower levels of folate and slightly higher levels of homocysteine in comparison to other (C/C and C/T) genotypes. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Cardiovascular disease Folate supplementation has been shown to reverse endothelial dysfunction independent of its effect in lowering plasma homocysteine level, to reduce arterial pressure and increase coronary dilation. These effects likely involve the stimulation of nitric oxide production by 5-methylfolic acid, and perhaps also inhibition of lipoprotein oxidation by folic acid. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Neural tube defects (NTD) Folate has an important role in normal embryogenesis, apparently involving its support of normal cell division. Studies in the United States demonstrated the efficacy of folate supplements (400-4000 µg) in preventing second NTDs in women with prior NTD pregnancies. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Other serious pregnancy complications Low-folate status has been associated with other congenital defects, including cleft lip and palate and defective development of limbs and the heart. Homocysteinemia has been associated with increased risks of hypertension, preeclampsia and placental abruption. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Erythropoiesis Folate is required for the production of new erythrocytes through its functions in: synthesis of purines and thymidylate required for DNA synthesis as well as through its function regenerating methionine, the methyl donor for DNA methylation November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Erythropoiesis Folate deficiency causes: arrest of erythropoiesis prior to the late stages of differentiation apoptotic reduction of cells surviving to postmitotic, terminal stages The condition is called megaloblastic anemia. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Erythropoiesis Addition of folate to iron supplements can improve the treatment of anemia in pregnancy. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

DNA methylation Folate appears to have a role in the methylation of DNA by virtue of its function in single-carbon metabolism, as the methylation of cytosine is important in regulating gene transcription. Folate deprivation has been found to produce chromosomal breaks in megaloblastic bone marrow, reflecting DNA strand breaks and hypomethylation. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Cancer Folate deprivation, by enhancing genomic instability and dysregulated gene transcription, appears to enhance predisposition to neoplastic transformation. Low-folate status has been associated with increased risk of cancers of the colon, cervix, lung, oral and pharyngeal, head and neck, stomach and gastrointestinal tract, and brain. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Immune system Folate supplementation can stimulate natural killer (NK) cell cytotoxicity among subjects of low-folate status. Among subjects consuming a high-folate diet the NK cytotoxicity was inversely related to the plasma concentration of the non-metabolized form of the vitamin, folic acid. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Cognitive function Folate is required to maintain homocysteine at low levels in the central nervous system. Hyperhomocysteinemia has been associated with increased risks to psychiactic and neurodegenerative disorders including depression, schizophrenia, Alzheimer’s disease and Parkinson’s disease. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

MTHFR polymorphisms November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Vascular disease The T/T C677T MTHFR genotype has been identified as a risk factor for carotid intima-media thickening, a risk factor to vascular disease. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Neural tube defects The protective effect of folate supplementation against NTD risk may be limited to a subset of subjects with defective folate metabolism caused by MTHFR mutations. The T/T C677T genotype is associated with NTD risk, but the effect appears to be dependent on folate status. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Down syndrome T/T C677T genotype elevates risk in individuals also carrying a mutation in methionine synthase reductase. Such individuals have also been found to have elevated risks of Down syndrome. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Cognitive disorders Individuals of the T/T C677T genotype have increased risks of depression and schizophrenia. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Cancer Homozygosity for T/T C677T MTHFR has consistently been found to be associated with reduced risk for colorectal cancer. The T/T genotype does not appear to affect risk for colorectal adenoma unless folate status is low, in which case it is related to increased risk. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Cancer These risks appear to be even greater in individuals with the combined C677T/A1298C heterozygous genotype. The T/T genotype has also been associated with increased risk for lymphocytic leukemia. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Methotrexate therapy Individuals with the T/T C677T genotype have a higher risk of discontinuing methotrexate treatment for rheumatoid arthritis owing to adverse effects. Individuals with the C/C A1298C genotype typically show improved efficacy of methotrexate therapy without increased side effects. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Bone mineral density One study found that women of the T/T C677T genotype showed reduced bone mineral density if they had relatively low intakes of folate, vitamin B12 and riboflavin. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Masking vitamin B12 deficiency November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Masking vitamin B12 deficiency High-level intakes of folate can interfere with the clinical diagnosis of vitamin B12 deficiency by preventing the common presenting sign, megaloblastic anemia. Folate supplementation does not mask the irreversible progression of neurological dysfunction and cognitive decline of vitamin B12 deficiency. Neurologic signs develop over a longer timeline than does the anemia produced by the deficiency. November 11, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Literature Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008. November 11, 2018