Regulation of Folate-mediated One-carbon Metabolism by 10-Formyltetrahydrofolate Dehydrogenase

Abstract

10-Formyltetrahydrofolate dehydrogenase (FDH) catalyzes the NADP+-dependent conversion of 10-formyltetrahydrofolate to CO2 and tetrahydrofolate (THF) and is an abundant high affinity folate-binding protein. Although several activities have been ascribed to FDH, its metabolic role in folate-mediated one-carbon metabolism is not well understood. FDH has been proposed to: 1) inhibit purine biosynthesis by depleting 10-formyl-THF pools, 2) maintain cellular folate concentrations by sequestering THF, 3) deplete the supply of folate-activated one-carbon units, and 4) stimulate the generation of THF-activated one-carbon unit synthesis by channeling folate cofactors to other folate-dependent enzymes. The metabolic functions of FDH were investigated in neuroblastoma, which do not contain detectable levels of FDH. Both low and high FDH expression reduced total cellular folate concentrations by 60%, elevated rates of folate catabolism, and depleted cellular 5-methyl-THF and S-adenosylmethionine levels. Low FDH expression increased the formyl-THF/THF ratio nearly 10-fold, whereas THF accounted for nearly 50% of total folate in neuroblastoma with high FDH expression. FDH expression did not affect the enrichment of exogenous formate into methionine, serine, or purines and did not suppress de novo purine nucleotide biosynthesis. We conclude that low FDH expression facilitates the incorporation of one-carbon units into the one-carbon pool, whereas high levels of FDH expression deplete the folate-activated one-carbon pool by catalyzing the conversion of 10-formyl-THF to THF. Furthermore, FDH does not increase cellular folate concentrations by sequestering THF in neuroblastoma nor does it inhibit or regulate de novo purine biosynthesis. FDH expression does deplete cellular 5-methyl-THF and S-adenosylmethionine levels indicating that FDH impairs the folate-dependent homocysteine remethylation cycle. 10-Formyltetrahydrofolate dehydrogenase (FDH) catalyzes the NADP+-dependent conversion of 10-formyltetrahydrofolate to CO2 and tetrahydrofolate (THF) and is an abundant high affinity folate-binding protein. Although several activities have been ascribed to FDH, its metabolic role in folate-mediated one-carbon metabolism is not well understood. FDH has been proposed to: 1) inhibit purine biosynthesis by depleting 10-formyl-THF pools, 2) maintain cellular folate concentrations by sequestering THF, 3) deplete the supply of folate-activated one-carbon units, and 4) stimulate the generation of THF-activated one-carbon unit synthesis by channeling folate cofactors to other folate-dependent enzymes. The metabolic functions of FDH were investigated in neuroblastoma, which do not contain detectable levels of FDH. Both low and high FDH expression reduced total cellular folate concentrations by 60%, elevated rates of folate catabolism, and depleted cellular 5-methyl-THF and S-adenosylmethionine levels. Low FDH expression increased the formyl-THF/THF ratio nearly 10-fold, whereas THF accounted for nearly 50% of total folate in neuroblastoma with high FDH expression. FDH expression did not affect the enrichment of exogenous formate into methionine, serine, or purines and did not suppress de novo purine nucleotide biosynthesis. We conclude that low FDH expression facilitates the incorporation of one-carbon units into the one-carbon pool, whereas high levels of FDH expression deplete the folate-activated one-carbon pool by catalyzing the conversion of 10-formyl-THF to THF. Furthermore, FDH does not increase cellular folate concentrations by sequestering THF in neuroblastoma nor does it inhibit or regulate de novo purine biosynthesis. FDH expression does deplete cellular 5-methyl-THF and S-adenosylmethionine levels indicating that FDH impairs the folate-dependent homocysteine remethylation cycle. Tetrahydrofolate (THF) 2The abbreviations used are: THF, tetrahydrofolate; FDH, 10-formyltetrahydrofolate dehydrogenase; MTHFD1, C1-tetrahydrofolate synthase; MTHFS, methenyltetrahydrofolate synthetase; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; pABG, para-aminobenzoylglutamate; FR, folate receptor; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; tet, tetracycline; SH-SY5YFDH, a neuroblastoma cell line that stably integrated the human FDH cDNA under the control of a tet-inducible promoter; SH-SY5YFDH+tet, a neuroblastoma cell line that stably integrated the human FDH cDNA under the control of a tet-inducible promoter that is cultured in the presence of 1 μg/ml tet; αMEM, α minimal essential medium; HPLC, high-performance liquid chromatography; MTT, 3-(4,5)-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. 2The abbreviations used are: THF, tetrahydrofolate; FDH, 10-formyltetrahydrofolate dehydrogenase; MTHFD1, C1-tetrahydrofolate synthase; MTHFS, methenyltetrahydrofolate synthetase; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; pABG, para-aminobenzoylglutamate; FR, folate receptor; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; tet, tetracycline; SH-SY5YFDH, a neuroblastoma cell line that stably integrated the human FDH cDNA under the control of a tet-inducible promoter; SH-SY5YFDH+tet, a neuroblastoma cell line that stably integrated the human FDH cDNA under the control of a tet-inducible promoter that is cultured in the presence of 1 μg/ml tet; αMEM, α minimal essential medium; HPLC, high-performance liquid chromatography; MTT, 3-(4,5)-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. polyglutamates are cofactors that function as one-carbon donors and acceptors in a set of reactions known as folate-mediated one-carbon metabolism, which occurs both in the cytoplasm and in mitochondria (see Fig. 1) (1Barlowe C.K. Appling D.R. BioFactors. 1988; 1: 171-176PubMed Google Scholar). THF cofactors carry one-carbon units at three oxidation states ranging from formate to methanol (2Wagner C. Folate in Health and Disease. 1995; (Bailey, L. B., ed) pp. , Marcel Dekker, Inc., New York: 23-42Google Scholar). The biologically active THF derivatives contain a reduced pteridine and a polyglutamate peptide consisting of five to eight glutamate residues linked by γ-peptide bonds (3Shane B. Folate in Health and Disease. 1995; (Bailey, L. B., ed) pp. , Marcel Dekker, Inc., New York: 1-22Google Scholar). Cytoplasmic folate-mediated one-carbon metabolism is required for the de novo synthesis of purines (supplies the carbon-2 and carbon-8 of the purine ring) and thymidylate (methylation of dUMP to dTMP), and also for remethylation of homocysteine to methionine (2Wagner C. Folate in Health and Disease. 1995; (Bailey, L. B., ed) pp. , Marcel Dekker, Inc., New York: 23-42Google Scholar, 3Shane B. Folate in Health and Disease. 1995; (Bailey, L. B., ed) pp. , Marcel Dekker, Inc., New York: 1-22Google Scholar). Methionine can be converted to S-adenosylmethionine (SAM) and serve as a methyl donor for numerous methylation reactions, including the methylation of DNA, RNA, and proteins. Serine is a primary source of folate-activated one-carbon units (4Davis S. Stacpoole P. Williamson J. Kick L. Quinlivan E. Coats B. Shane B. Bailey L. Gregory III J.F. Am. J. Physiol. 2004; 286: E272-E279Crossref PubMed Scopus (126) Google Scholar). Cytoplasmic serine hydroxymethyltransferase (cSHMT) catalyzes the THF-dependent aldol cleavage of serine to methylene-THF and glycine. Alternatively, serine can be converted to glycine and formate in the mitochondria through mitochondrial folate-mediated one-carbon metabolism (5Suh J.R. Herbig A.K. Stover P.J. Annu. Rev. Nutr. 2001; 21: 255-282Crossref PubMed Scopus (222) Google Scholar) (Fig. 1). The formate produced in the mitochondria is a major source of one-carbon units for cytoplasmic one-carbon metabolism through its conversion to 10-formyl-THF by the enzyme C1-THF synthase (MTHFD1) (6Appling D.R. FASEB J. 1991; 5: 2645-2651Crossref PubMed Scopus (303) Google Scholar). Folate deficiency impairs dTMP, methionine, and SAM synthesis, resulting in increased rates of uracil misincorporation into DNA and DNA hypomethylation (5Suh J.R. Herbig A.K. Stover P.J. Annu. Rev. Nutr. 2001; 21: 255-282Crossref PubMed Scopus (222) Google Scholar). Therefore, maintenance of cellular folate concentrations is critical for both DNA synthesis and genome stability and expression. Intracellular folate concentrations are influenced by folate availability, uptake, polyglutamylation, turnover, and export (5Suh J.R. Herbig A.K. Stover P.J. Annu. Rev. Nutr. 2001; 21: 255-282Crossref PubMed Scopus (222) Google Scholar). Folate monoglutamates are transported into the cell either by folate receptors (FRs) or carrier-mediated proteins (7Sirotnak F.M. Tolner B. Annu. Rev. Nutr. 1999; 19: 91-122Crossref PubMed Scopus (260) Google Scholar, 8Antony A.C. Annu. Rev. Nutr. 1996; 16: 501-521Crossref PubMed Scopus (502) Google Scholar) and then converted to folate polyglutamates by the enzyme folylpolyglutamate synthetase. The addition of the polyglutamate chain is required for cellular retention of folate cofactors (9Chen L. Qi H. Korenberg J. Garrow T.A. Choi Y.J. Shane B. J. Biol. Chem. 1996; 271Google Scholar, 10Lin B.-F. Huang R.-F.S. Shane B. J. Biol. Chem. 1993; 268: 21674-21679Abstract Full Text PDF PubMed Google Scholar, 11Lin B.-F. Shane B. J. Biol. Chem. 1994; 269: 9705-9713Abstract Full Text PDF PubMed Google Scholar, 12Titus S.A. Moran R.G. J. Biol. Chem. 2000; 275: 36811-36817Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar). Reduced folate polyglutamates are sequestered by folate-binding proteins inside the cell that exhibit Kd values in the low micromolar to nanomolar range (13Strong W.B. Schirch V. Biochemistry. 1989; 28: 9430-9439Crossref PubMed Scopus (45) Google Scholar). Studies have indicated that the concentration of cellular folate-binding proteins exceeds the concentration of total intracellular folate. In liver, for example, the concentration of folate-binding proteins exceeds the concentration of folate by 5- to 10-fold (5Suh J.R. Herbig A.K. Stover P.J. Annu. Rev. Nutr. 2001; 21: 255-282Crossref PubMed Scopus (222) Google Scholar, 13Strong W.B. Schirch V. Biochemistry. 1989; 28: 9430-9439Crossref PubMed Scopus (45) Google Scholar); it has been suggested that the concentration of cellular folate-binding proteins may set the upper limit for intracellular folate accumulation (5Suh J.R. Herbig A.K. Stover P.J. Annu. Rev. Nutr. 2001; 21: 255-282Crossref PubMed Scopus (222) Google Scholar). 10-Formyltetrahydrofolate dehydrogenase (FDH; EC 1.5.1.6) is one of the most abundant folate-binding proteins present in liver (14Min H. Shane B. Stokstad E.L.R. Biochim. Biophys. Acta. 1988; 967: 348-353Crossref PubMed Scopus (49) Google Scholar). FDH catalyzes the NADP+-dependent conversion of 10-formyl-THF to THF and CO2 (1Barlowe C.K. Appling D.R. BioFactors. 1988; 1: 171-176PubMed Google Scholar, 15Cook R.J. Lloyd R.S. Wagner C. J. Biol. Chem. 1991; 266: 4965-4973Abstract Full Text PDF PubMed Google Scholar). Mice lacking FDH activity are viable and fertile, indicating that one or more FDH functions are not essential in the laboratory environment (16Champion K.M. Cook R.J. Tollaksen S.L. Giometti C.S. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 11338-11342Crossref PubMed Scopus (56) Google Scholar); FDH may function to regulate the folate-mediated one-carbon metabolic network in specific cell-types. FDH expression is restricted normally to the liver and brain, and its range of tissue-specific expression parallels that of cSHMT (17Girgis S. Nasrallah I.M. Suh J.R. Oppenheim E. Zanetti K.A. Mastri M.G. Stover P.J. Gene. 1998; 210: 315-324Crossref PubMed Scopus Google Scholar, Biol. 1999; Google whereas folate-dependent that reactions are to be more and functions have been ascribed to FDH. B. J. PubMed Scopus Google Scholar) suggested that FDH to one-carbon units from folate metabolism by depleting of folate-activated one-carbon In of with a FDH deficiency exhibit THF concentrations and 10-formyl-THF concentrations with the of control (16Champion K.M. Cook R.J. Tollaksen S.L. Giometti C.S. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 11338-11342Crossref PubMed Scopus (56) Google Scholar). FDH has been suggested to and cellular folate in the of THF (2Wagner C. Folate in Health and Disease. 1995; (Bailey, L. B., ed) pp. , Marcel Dekker, Inc., New York: 23-42Google Scholar, B. Schirch V. Biophys. 1999; PubMed Scopus Google Scholar). FDH from liver THF polyglutamates and its by THF Huang Schirch Schirch V. Biochemistry. 1996; PubMed Scopus Google Scholar). In liver, FDH and of total R.J. Lloyd R.S. Wagner C. J. Biol. Chem. 1991; 266: 4965-4973Abstract Full Text PDF PubMed Google Scholar). In liver, the concentration of FDH has been to be intracellular folate concentrations are to be in liver Shane B. Biochemistry. PubMed Scopus Google Scholar). intracellular FDH concentrations THF cytoplasmic THF is to be by FDH in Huang Schirch Schirch V. Biochemistry. 1996; PubMed Scopus Google Scholar). Mice lacking FDH exhibit a in total folate concentrations folate indicating that FDH may to maintain cellular folate concentrations by sequestering cellular THF (16Champion K.M. Cook R.J. Tollaksen S.L. Giometti C.S. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 11338-11342Crossref PubMed Scopus (56) Google Scholar). FDH has been proposed to inhibit purine biosynthesis by depleting cellular 10-formyl-THF S.A. Google Scholar). FDH has been suggested to the generation of folate-activated by channeling THF to the cSHMT and Huang Schirch Schirch V. Biochemistry. 1996; PubMed Scopus Google Scholar). are the primary for one-carbon units into one-carbon metabolism (Fig. 1). FDH by THF and a that is from the active B. Schirch V. Biophys. 1999; PubMed Scopus Google Scholar). Both cSHMT and suppress of FDH and the of FDH by Huang Schirch Schirch V. Biochemistry. 1996; PubMed Scopus Google Scholar). the and functions of FDH, the of FDH expression the folate-mediated one-carbon metabolic network and cellular folate concentrations were in neuroblastoma, a cell line that does not detectable levels of the from Inc., and from from the of essential and lacking and as were from by of at for with then to folate. and human neuroblastoma cell line has been S. Suh J.R. J. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). were cultured in with for FDH expression in cell by the addition of 1 μg/ml for a of to of from a liver used as the to the human FDH in The of the human FDH cDNA from the the and the The were for for and for for a total of The of the human FDH cDNA the and the The were for for and for for a total of and were into the the The nucleotide of and were from three and were also to the of the human FDH the from the with and and into the from the with and the and with is present in the of both and The with and then with and the human FDH cDNA in the The FDH cDNA from the with and and into the cell the human FDH cDNA from the were were with of DNA at and then cultured with for The with and to for to were and and and human FDH were by of in were into of the with of were at by and at Folate were in by as Suh J.R. H. Nasrallah I.M. J. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). were with pABG, and to the of the at at and 5-methyl-THF at were into and the a liquid of FDH, GAPDH, and were cultured to in were a 1 The were and concentration of the were an FDH or a and then to a a of FDH, the at with primary consisting of in a peptide of human FDH residues of and FR, the at with that the human or The in then for with for the of FDH and for the of and were the The were from of human FDH cDNA expression cell a In the is reduced to an by mitochondrial in viable in were and in a were in at 1 in either serine, and or with and of and an of FDH expression with 1 μg/ml and of to were at in The for at then in of and a values the and of with values were to the which a of of Folate SAM, SAH, and were cultured to in that cellular the of folate and folate polyglutamate chain were by as P.J. J. Chem. 2000; PubMed Scopus Google Scholar). SAM and were cultured to in to were cultured in lacking methionine to SAM levels cellular homocysteine remethylation and not exogenous methionine SAM and were from the and by with to as J. Shane B. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the enrichment of into and methionine, were at 50% in and cultured in which of lacking and with methionine, 1 and the enrichment of into and methionine, were at 50% in and cultured in which of lacking and with methionine, 1 were cultured for a total of and the were by with then and by and DNA were as J. Shane B. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). were in in and at for The were by converted to and then an enrichment in by a and at a ratio for serine, for for for methionine, and for glycine. DNA were under and in and at for in at under the were in of a of and and at for The derivatives were an enrichment in by a and at a ratio for for and for to the of de novo purine nucleotide biosynthesis by the incorporation of into purine to purine nucleotide biosynthesis through the by the incorporation of into purine were in minimal essential α with The of that serine, methionine, and folate with methionine, and were and in at CO2 in and The cell were at DNA a DNA from and levels were a in of were into in to and DNA to to by HPLC, S. Choi J. Chem. PubMed Scopus Google Scholar). to were from and levels of the FDH cDNA and of a cell line that the human FDH cDNA from a promoter to as FDH not in cultured in the of low detectable levels of FDH (Fig. cultured with FDH levels present in cell were used to the ascribed metabolic of FDH in folate-mediated one-carbon the of both low and high FDH expression 1) intracellular folate 2) the 3) SAM and 4) the folate-mediated one-carbon metabolic network were of FDH Folate cultured in the and presence of a and in total cellular folate concentrations with the cell 1). do not a role for FDH in cellular folate by sequestering or cellular THF. in cellular folate can from rates of cellular increased rates of (5Suh J.R. Herbig A.K. Stover P.J. Annu. Rev. Nutr. 2001; 21: 255-282Crossref PubMed Scopus (222) Google Scholar). Therefore, the of FDH expression both expression and folate were facilitates of into the cell 1999; Google and its expression with cellular folate deficiency in cell Wagner C. J. 2001; PubMed Scopus Google Scholar). cultured in the presence or of increased levels to as by (Fig. with FDH expression a folate deficiency in also that the in cellular folate concentrations is not the of folate of FDH expression total cellular folate levels and polyglutamate chain in polyglutamate of total folate values are three cell of total folate values are three cell of total folate values are three cell of of total folate values are three cell of in a The of FDH expression rates of folate catabolism, as by the generation of the folate were in to as Suh J.R. H. Nasrallah I.M. J. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). cultured in the and presence of and more with (Fig. that FDH expression folate in which may to the folate the increase in generation the and cultured not that increased rates of folate may not for the in folate concentrations that is the primary of folate (Fig. Furthermore, FDH expression the of intracellular folate polyglutamate 1). rates of folate have been to increase the folate polyglutamate chain folate which have a affinity for folate-binding are more to Suh J.R. H. Nasrallah I.M. J. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). of FDH the of Folate and has been proposed to regulate the supply of folate-activated one units by the are nearly in the mitochondria and and are not in B.-F. Huang R.-F.S. Shane B. J. Biol. Chem. 1993; 268: 21674-21679Abstract Full Text PDF PubMed Google Scholar). is the primary folate in the mitochondria of S. Suh J.R. J. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) and not be to FDH in the the low of FDH expression in increased the cellular formyl-THF/THF ratio by 10-fold with accounted for of total cellular and the levels of both THF and 5-methyl-THF were reduced by In high FDH expression the THF accounted for of total folate in of FDH expression the of folate derivatives in THF and of of of of in a to metabolic been ascribed to 1) the of one-carbon units from the folate-activated one-carbon pool by depleting 10-formyl-THF B. J. PubMed Scopus Google and 2) the generation of folate-activated by channeling THF to cSHMT and MTHFD1, that the synthesis of folate-activated one-carbon units (Fig. 1). The from that both of metabolic may be in the expression of FDH. Low levels of FDH expression increased the with a role for FDH in the generation of folate-activated one-carbon the other high levels of FDH expression reduced the supply of folate one-carbon units that THF the primary cellular folate are with the known of FDH. FDH a affinity for THF polyglutamates 10-formyl-THF polyglutamates FDH, at low is to cellular THF. total FDH activity is to be with its activity to and levels of FDH expression for to the of 10-formyl-THF through the conversion of 10-formyl-THF to THF specific activity of FDH exceeds the of 10-formyl-THF In FDH can either or folate-activated one-carbon units its of expression. Both high and low FDH expression reduced the concentrations of 5-methyl-THF from in to in other have that in 5-methyl-THF concentrations cellular folate The human of the a enzyme with reduced to to 5-methyl-THF P. P. Am. J. 1995; Google Scholar). The impairs the accumulation of 5-methyl-THF in in of S. Choi S. J. P.J. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). of the also exhibit folate in S. Choi S. J. P.J. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). Therefore, the to maintain cellular 5-methyl-THF concentrations may be an of total cellular folate The of of the 5-methyl-THF pool the homocysteine remethylation investigated by cellular SAM and concentrations 5-methyl-THF is for folate-dependent homocysteine in 5-methyl-THF concentrations may SAM synthesis (Fig. 1). FDH expression at both high and low levels reduced SAM concentrations and the ratio by indicating that the in 5-methyl-THF total cellular folate concentrations the homocysteine remethylation and reduced the methylation of the of FDH homocysteine remethylation is a that not of FDH expression SAM and levels in in a of FDH and one-carbon unit of 10-formyl-THF the and for de novo purine biosynthesis by the and (2Wagner C. Folate in Health and Disease. 1995; (Bailey, L. B., ed) pp. , Marcel Dekker, Inc., New York: 23-42Google Scholar). FDH catalyzes the conversion of 10-formyl-THF to THF, it has been proposed that increased FDH expression may cell by depleting the supply of one-carbon units the of for purine biosynthesis. The of increased FDH expression cell rates in the human FDH cDNA were cultured with that serine, and glycine (Fig. FDH rates to indicating that increased levels of FDH cell in (Fig. The purine serine, and The presence of purine in the did not rates in cell FDH rates with (Fig. Therefore, expression of FDH in cell line cell the did not from the of de novo purine biosynthesis. The of FDH expression folate-dependent de novo purine biosynthesis a which the of the purine to suppress the incorporation of formate into purines through the de novo In are in the presence and is into purine through de novo purine nucleotide which is whereas is into purine through the Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). FDH expression the ratio in DNA or purine FDH can deplete 10-formyl-THF pools, activity does not the incorporation of formate into purines novo purine to biosynthesis in neuroblastoma that the enzyme methenyltetrahydrofolate is a high affinity and that 10-formyl-THF rates of purine biosynthesis to synthesis in Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). may and 10-formyl-THF to the de novo purine biosynthesis from of 10-formyl-THF ratio for in and in a of FDH the of the and Methionine or other folate-dependent reactions were for the in 10-formyl-THF the incorporation of into methionine and serine in cellular and purines in in as a function of FDH expression (Fig. and The one-carbon used for synthesis of serine, and methionine can be either from exogenous formate or from serine (Fig. FDH expression into methionine or serine Furthermore, FDH expression the enrichment of the 10-formyl-THF pool used for purine synthesis that FDH expression does not the enrichment of into the folate-activated one-carbon pool (Fig. and of FDH expression formate metabolism in of serine and methionine in to enrichment in enrichment of 10-formyl-THF pool used for purine from the ratio of the and of and of that of the and of the purine were from to enrichment in from the ratio of the and of and of that of the and of the purine were from in a the of one-carbon units into the and methionine in the presence and of FDH metabolic were also with that is by cSHMT and into methionine or is to the that are present the of serine (Fig. Alternatively, the the be from the as and formate contain a have that to of one-carbon units used for cytoplasmic folate metabolism in are from formate by mitochondrial serine metabolism J. Shane B. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the of serine for most of the serine is converted to with of the at the J. Shane B. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the of the at the of that of the cellular serine pool that serine for synthesis both the by Serine that one in the the of serine that from glycine the pool (Fig. the methionine and one-carbon units were from the of be of the The in that the are that most of the one-carbon units used for cytoplasmic methionine and synthesis were from formate that from mitochondrial serine have that in cSHMT methylene-THF to synthesis as by the of in with methionine J. Shane B. Stover P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The in that cSHMT also one-carbon units for synthesis in as by the incorporation of into Low levels of FDH expression in cultured did not affect the incorporation of into either methionine or dTMP, whereas with high levels of FDH expression a increase in in methionine and with the cell that FDH expression increased the of cSHMT into methionine and dTMP, the enrichment of cSHMT into methionine and did not Furthermore, the that FDH rates of formate incorporation into methionine and dTMP, the cSHMT enzyme can by to the one-carbon network by one-carbon units for methionine and of FDH expression metabolism in in a several metabolic functions of FDH in a cell from its in FDH to affect the 10-formyl-THF in in 10-formyl-THF levels did not de novo purine biosynthesis or a purine to as has been We also that increased FDH expression did not in increased cellular folate accumulation in neuroblastoma, and its folate do not enzyme to serve as a for cellular folate. The of have a and metabolic role for both 5-methyl-THF and SAM levels were to FDH expression The of FDH cellular methylation reactions the metabolic role of FDH the folate-dependent one-carbon nor its total cellular folate is from its or Furthermore, its function and one-carbon metabolism and cellular folate may in cell and be the expression of other tissue-specific folate-dependent and proteins.