The intestinal absorption of folates

Do Mammalian Cells Really Need to Export and Import Heme?

Heme is a cofactor that is essential to almost all forms of life. The production of heme is a balancing act between the generation of the requisite levels of the end-product and protection of the cell and/or organism against any toxic substrates, intermediates and, in this case, end-product. In this review, we provide an overview of our understanding of the formation and regulation of this metallocofactor and discuss new research on the cell biology of heme homeostasis, with a focus on putative transmembrane transporters now proposed to be important regulators of heme distribution. The main text is complemented by a discussion dedicated to the intricate chemistry and biochemistry of heme, which is often overlooked when new pathways of heme transport are conceived.

Impact of posttranslational modifications of engineered cysteines on the substituted cysteine accessibility method: evidence for glutathionylation

The substituted cysteine accessibility method (SCAM) is widely used to study the structure and function of channels, receptors and transporters. In its usual application, a cysteine residue is introduced into a protein which lacks native cysteines following which the accessibility of the residue to the aqueous compartment is assessed. Implicit, and generally assumed, is that if the cysteine-substituted residue is not available to react with sulfhydryl reagents it is not exposed to the extracellular compartment or within the aqueous translocation pathway. We demonstrate here, in a Hela-derived cell line, that some cysteine-substituted residues of the proton-coupled folate transporter (PCFT, SLC46A1) that are inaccessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate are glutathionylated by biotinylated glutathione ethyl ester in the absence of an oxidizing agent. Intramolecular disulfide formation involving cysteine-substituted residues was also identified in some instances. These posttranslational modifications limit the accessibility of the cysteine residues to sulfhydryl-reactive reagents and can have a profound impact on the interpretation of SCAM but may not alter function. When a posttranslationally modified residue is used as a reference extracellular control, the high level of exposure required for detection on Western blot results in erroneous detection of otherwise inaccessible intracellular cysteine-substituted residues. The data indicate that in the application of SCAM, when a cysteine-substituted residue does not appear to be accessible to sulfhydryl-reactive reagents, the possibility of a posttranslational modification should be excluded. The data explain the discrepancies in the assessment, and confirm the localization, of the first intracellular loop of PCFT.

Development of stable isotope dilution assays for the quantitation of intra- and extracellular folate patterns of Bifidobacterium adolescentis

Folate-producing bifidobacteria have been studied extensively but appropriate methods for detailed quantitation of intra- and extracellular pteroylmono- and pteroylpolyglutamate patterns are lacking. Therefore, B. adolescentis DSM 20083^T was cultivated in folate-free medium (FFM) for 24h to develop and validate stable isotope dilution assays (SIDAs) coupled with LC-MS/MS for the determination of 5-formyltetrahydrofolic acid (5-HCO-H₄folate), 10-formylfolic acid (10-HCO-PteGlu), tetrahydrofolic acid (H₄folate), folic acid (PteGlu) and 5-methyltetrahydrofolic acid (5-CH₃-H₄folate) including its di-, tri-, and tetraglutamic vitamers (5-CH₃-H₄PteGlu_2-4). The respective monoglutamylated isotopologues labelled with deuterium were used as internal standards for quantitation. Limits of detection and quantitation (LOD/LOQ) were sufficiently low to quantify 48.2nmol L^-1 5-CH₃-H₄folate (5.7/17nmolL^-1) and 71.0nmolL^-1 5-HCO-H₄folate (10/30nmolL^-1) as major folate vitamers extracellularly and 124nmolL^-1 5-CH₃-H₄folate (3.4/10nmolL^-1), 213nmolL^-1 5-HCO-H₄folate (4.8/14nmolL^-1), and 61.4nmolL^-1 H₄folate (2.3/7.0nmolL^-1) intracellularly after deconjugation. The major portion of native 5-CH₃-H₄folate vitamer was ascribed to its tetraglutamate ( > 95%). Concentrations of mono-, di-, tri-, and pentaglutamylated folates were below LOD or LOQ. Intra-assay precision coefficients of variation (CVs) ranged from 7% (at a concentration of 53.9nmolL^-1 for 5-CH₃-H₄PteGlu₄), 15% (25.5nmolL^-1 5-CH₃-H₄folate) to 18% (78.5nmolL^-1 5-HCO-H₄folate), extracellularly, and from 6% (60.7nmolL^-1 5-CH₃-H₄PteGlu₄), 7% (202nmolL^-1 5-HCO-H₄folate), 10% (67.1nmolL^-1 H₄folate) to 11% (127nmolL^-1 5-CH₃-H₄folate), intracellularly. Inter-assay precision CVs ranged from 2% (54.7nmolL^-1 5-CH₃-H₄PteGlu₄), 3% (71nmolL^-1 5-HCO-H₄folate) to 11% (48.2nmolL^-1 5-CH₃-H₄folate), extracellularly, and from 1% (61.4nmolL^-1 H₄folate), 5% (213nmolL^-1 5-HCO-H₄folate), 6% (63.5nmolL^-1 5-CH₃-H₄PteGlu₄) to 10% (124nmolL^-1 5-CH₃-H₄folate), intracellularly, thus showing excellent reproducibility. Recoveries for all analytes under study ranged between 81 and 113%. These newly developed methods enable reproducible, precise and sensitive quantitation of eight bacterially synthesized folate vitamers in two totally different matrices, including both monoglutamates and polyglutamates. Furthermore, we here present the first assay using solely monoglutamylated [²H₄]-5-CH₃-H₄folate to quantify native polyglutamate patterns of this vitamer in bacteria which might replace time-consuming determination of monoglutamates in the future.

The proton-coupled folate transporter (PCFT-SLC46A1) and the syndrome of systemic and cerebral folate deficiency of infancy: Hereditary folate malabsorption

The proton-coupled folate transporter (PCFT-SLC46A1) is the mechanism by which folates are absorbed across the brush-border membrane of the small intestine. The transporter is also expressed in the choroid plexus and is required for transport of folates into the cerebrospinal fluid. Loss of PCFT function, as occurs in the autosomal recessive disorder "hereditary folate malabsorption" (HFM), results in a syndrome characterized by severe systemic and cerebral folate deficiency. Folate-receptor alpha (FRα) is expressed in the choroid plexus, and loss of function of this protein, as also occurs in an autosomal recessive disorder, results solely in "cerebral folate deficiency" (CFD), the designation for this disorder. This paper reviews the current understanding of the functional and structural properties and regulation of PCFT, an electrogenic proton symporter, and contrasts PCFT properties with those of the reduced folate carrier (RFC), an organic anion antiporter, that is the major route of folate transport to systemic tissues. The clinical characteristics of HFM and its treatment, based upon the thirty-seven known cases with the clinical syndrome, of which thirty have been verified by genotype, are presented. The ways in which PCFT and FRα might interact at the level of the choroid plexus such that each is required for folate transport from blood to cerebrospinal fluid are considered along with the different clinical presentations of HFM and CFD.

Lansoprazole Exacerbates Pemetrexed-Mediated Hematologic Toxicity by Competitive Inhibition of Renal Basolateral Human Organic Anion Transporter 3

Pemetrexed, a multitargeted antifolate, is eliminated by tubular secretion via human organic anion transporter 3 (hOAT3). Although proton pump inhibitors (PPIs) are frequently used in cancer patients, the drug interaction between PPIs and pemetrexed remains to be clarified. In this study, we examined the drug interaction between pemetrexed and PPIs in hOAT3-expressing cultured cells, and retrospectively analyzed the impact of PPIs on the development of hematologic toxicity in 108 patients who received pemetrexed and carboplatin treatment of nonsquamous non-small cell lung cancer for the first time between January 2011 and June 2015. We established that pemetrexed was transported via hOAT3 (Km = 68.3 ± 11.1 µM). Lansoprazole, rabeprazole, pantoprazole, esomeprazole, omeprazole, and vonoprazan inhibited hOAT3-mediated uptake of pemetrexed in a concentration-dependent manner. The inhibitory effect of lansoprazole was much greater than those of other PPIs and the apparent IC50 value of lansoprazole against pemetrexed transport via hOAT3 was 0.57 ± 0.17 µM. The inhibitory type of lansoprazole was competitive. In a retrospective study, multivariate analysis revealed that coadministration of lansoprazole, but not other PPIs, with pemetrexed and carboplatin was an independent risk factor significantly contributing to the development of hematologic toxicity (odds ratio: 10.004, P = 0.005). These findings demonstrated that coadministration of lansoprazole could exacerbate the hematologic toxicity associated with pemetrexed, at least in part, by competitive inhibition of hOAT3. Our results would aid clinicians to make decisions of coadministration drugs to avoid drug interaction-induced side effects for achievement of safe and appropriate chemotherapy with pemetrexed.

When barriers ignore the "rule-of-five"

Why are a few drugs with properties beyond the rule of 5 (bRo5) absorbed across the intestinal mucosa while most other bRo5 compounds are not? Are such exceptional bRo5 compounds exclusively taken up by carrier-mediated transport or are they able to permeate the lipid bilayer (passive lipoidal diffusion)? Our experimental data with liposomes indicate that tetracycline, which violates one rule of the Ro5, and rifampicin, violating three of the rules, significantly permeate a phospholipid bilayer with kinetics similar to labetalol and metoprolol, respectively. Published data from experimental work and molecular dynamics simulations suggest that the formation of intramolecular H-bonds and the possibility to adopt an elongated shape besides the presence of a significant fraction of net neutral species facilitate lipid bilayer permeation. As an alternative to lipid bilayer permeation, carrier proteins can be targeted to improve absorption, with the potential drawbacks of drug-drug interactions and non-linear pharmacokinetics.

No major effects of vitamin D3 (1,25 dihydroxyvitamin D3) on absorption and pharmacokinetics of folic acid and fexofenadine in healthy volunteers

PURPOSE

In Caco-2 cells, folate uptake via the proton-coupled folate transporter (PCFT) increases significantly by a 3-day treatment with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). Additionally, mRNA content and protein expression of the transporter OATP1A2 were increased up to ninefold with 1,25(OH)2D3. We investigated whether these in vitro findings can be confirmed in humans in vivo.

METHODS

Ten healthy volunteers (six women) received 5 mg folic acid orally once before and once together with the last intake of a 10-day course of 0.5 μg 1,25(OH)2D3 orally. One hundred twenty milligrams fexofenadine, an OATP1A2 substrate, was taken in 1 day before the first folic acid intake, and again on the ninth day of 1,25(OH)2D3 intake. Duodenal biopsies were taken for transporter mRNA assessments once before and once on the ninth or tenth day of the vitamin D3 course. Serum folic acid and fexofenadine concentrations were quantified with a chemiluminescence immunoassay and LC-MS/MS, respectively. Pharmacokinetics were compared between periods with standard bioequivalence approaches.

RESULTS

While geometric mean folic acid AUC0-2h, which mainly reflects absorption, was 0.403 and 0.414 mg/L·h before and after the vitamin D3 course (geometric mean ratio (GMR), 1.027; 90 % confidence interval (90 % CI), 0.788-1.340), the geometric mean fexofenadine AUC0-2h was 1.932 and 2.761 mg/L·h, respectively (GMR, 1.429; 90 % CI, 0.890-2.294). PCFT- and OATP1A2-mRNA expressions in duodenal biopsies were essentially unchanged.

CONCLUSIONS

No significant changes in folic acid and fexofenadine absorption were observed after a 10-day course of 1,25(OH)2D3 in humans in vivo. This study underlines the importance of confirming in vitro findings in vivo in humans.

Folates in Fruits and Vegetables: Contents, Processing, and Stability

Folates play a key role in human one-carbon metabolism and are provided by food. It is well established that folates are beneficial in the prevention of neural tube defects and cardiovascular and neurodegenerative diseases. Fruits and vegetables, and especially green vegetables, are the main sources of folates. In parallel, fruits and vegetables, with high contents of folates, are mostly consumed after processing, such as, canning, freezing, or home-cooking, which involve folate losses during their preparation. Hence, it is important to know the percentage of folate losses during processing and, moreover, the mechanisms underlying those losses. The current knowledge on folate losses from fruit and vegetables are presented in this review. They depend on the nature of the respective fruit or vegetable and the respective treatment. For example, steaming involves almost no folate losses in contrast to boiling. Two main mechanisms are involved in folate losses: (i) leaching into the surrounding liquid and (ii) oxidation during heat treatment, the latter of which depending on the nature of the vitamer considered. In this respect, a vitamer stability decreases in the order starting from folic acid followed by 5-HCO-H₄ folate, 5-CH₃ -H₄ folate, and, finally, H₄ folate. Further studies are required, especially on the diffusion of the vitamers in real foods and on the determination of folate degradation products.

Identification of an Extracellular Gate for the Proton-coupled Folate Transporter (PCFT-SLC46A1) by Cysteine Cross-linking

The proton-coupled folate transporter (PCFT, SLC46A1) is required for intestinal folate absorption and folate homeostasis in humans. A homology model of PCFT, based upon theEscherichia coliglycerol 3-phosphate transporter structure, predicted that PCFT transmembrane domains (TMDs) 1, 2, 7, and 11 form an extracellular gate in the inward-open conformation. To assess this model, five residues (Gln(45)-TMD1, Asn(90)-TMD2, Leu(290)-TMD7, Ser(407)-TMD11 and Asn(411)-TMD11) in the predicted gate were substituted with Cys to generate single and nine double mutants. Transport function of the mutants was assayed in transient transfectants by measurement of [(3)H]substrate influx as was accessibility of the Cys residues to biotinylation. Pairs of Cys residues were assessed for spontaneous formation of a disulfide bond, induction of a disulfide bond by oxidization with dichloro(1,10-phenanthroline)copper (II) (CuPh), or the formation of a Cd(2+)complex. The data were consistent with the formation of a spontaneous disulfide bond between the N90C/S407C pair and a CuPh- and Cd(2+)-induced disulfide bond and complex, respectively, for the Q45C/L290C and L290C/N411C pairs. The decrease in activity induced by cross-linkage of the Cys residue pairs was due to a decrease in the influxVmaxconsistent with restriction in the mobility of the transporter. The presence of folate substrate decreased the CuPh-induced inhibition of transport. Hence, the data support the glycerol 3-phosphate transporter-based homology model of PCFT and the presence of an extracellular gate formed by TMDs 1, 2, 7, and 11.

Beyond Ussing's chambers: contemporary thoughts on integration of transepithelial transport

In the mid-20th century, Hans Ussing developed a chamber that allowed for the simultaneous measurement of current and labeled probe flux across epithelia. Using frog skin as a model, Ussing used his results to propose mechanisms of transcellular Na(+) and K(+) transport across apical (exterior/luminal) and basolateral (interior) membranes that is essentially unchanged today. Others took advantage of Ussing's chambers to study mucosal tissues, including bladder and intestines. It quickly became clear that, in some tissues, passive paracellular flux, i.e., across the tight junction, was an important component of overall transepithelial transport. Subsequent work demonstrated that activation of the apical Na(+)-glucose cotransporter SGLT1 regulated paracellular permeability such that intestinal paracellular transport could coordinate with and amplify transcellular transport. Intermediates in this process include activation of p38 MAPK, the apical Na(+)/H(+) exchanger NHE3, and myosin light chain kinase (MLCK). Investigators then focused on these processes in disease. They found that TNF induces barrier dysfunction via MLCK activation and downstream caveolin-1-dependent endocytosis of the tight junction protein occludin. TNF also inhibited NHE3, and both barrier loss and PKCα-dependent NHE3 inhibition were required for TNF-induced acute diarrhea, emphasizing the interplay between transcellular and paracellular transport. Finally, studies using immune-mediated inflammatory bowel disease models showed that mice lacking epithelial MLCK were initially protected, but became ill as epithelial damage progressed and provided a tight junction-independent means of barrier loss. None of these advances would have been possible without the insights provided by Ussing and others using Ussing's ingenious, and still useful, chambers.

Absorption and blood/cellular transport of folate and cobalamin: Pharmacokinetic and physiological considerations

The systems involving folate and cobalamin have several features in common: 1) their dietary forms require luminal digestion for absorption; 2) intestinal bacteria in the upper intestine synthesize and utilize both vitamins, creating possible competition for the nutrients; 3) there is one major intestinal brush border protein essential for absorption; 4) both are subject to extensive entero-hepatic circulation. Finally, human mutations have confirmed the role of specific transporters and receptors in these processes. There are other features, however, that distinguish the metabolism of these vitamins: 1) upper intestinal bacteria tend to produce folate, while cobalamin (cbl) utilization is more common; 2) cbl absorption requires a luminal binding protein, but folate does not; 3) folate absorption can occur throughout the small bowel, but the cbl receptor, cubilin, is restricted to the distal half of the small bowel; 4) movement into cells uses transporters, exchangers, and symporters, whereas cbl is transferred by receptor-mediated endocytosis; 5) folate is carried in the blood mostly in red blood cells, whereas cbl is carried on specific binding-proteins; 6) folate can enter cells via multiple systems, but cbl uptake into all tissues use the transcobalamin receptor (TC-R), with the asialoglycoprotein receptor (ASGP-R) present in hepatocytes for uptake of haptocorrin-cbl (HC-cbl) complexes. In summary, the systems for absorption and distribution of folate and cobalamin are complex. These complexities help to explain the variable clinical responses after oral administration of the vitamins, especially when provided as supplements.

Folic acid supplementation during high-fat diet feeding restores AMPK activation via an AMP-LKB1-dependent mechanism

AMPK is an endogenous energy sensor that regulates lipid and carbohydrate metabolism. Nonalcoholic fatty liver disease (NAFLD) is regarded as a hepatic manifestation of metabolic syndrome with impaired lipid and glucose metabolism and increased oxidative stress. Our recent study showed that folic acid supplementation attenuated hepatic oxidative stress and lipid accumulation in high-fat diet-fed mice. The aim of the present study was to investigate the effect of folic acid on hepatic AMPK during high-fat diet feeding and the mechanisms involved. Male C57BL/6J mice were fed a control diet (10% kcal fat), a high-fat diet (60% kcal fat), or a high-fat diet supplemented with folic acid (26 mg/kg diet) for 5 wk. Mice fed a high-fat diet exhibited hyperglycemia, hepatic cholesterol accumulation, and reduced hepatic AMPK phosphorylation. Folic acid supplementation restored AMPK phosphorylation (activation) and reduced blood glucose and hepatic cholesterol levels. Activation of AMPK by folic acid was mediated through an elevation of its allosteric activator AMP and activation of its upstream kinase, namely, liver kinase B1 (LKB1) in the liver. Consistent with in vivo findings, 5-methyltetrahydrofolate (bioactive form of folate) restored phosphorylation (activation) of both AMPK and LKB1 in palmitic acid-treated HepG2 cells. Activation of AMPK by folic acid might be responsible for AMPK-dependent phosphorylation of HMG-CoA reductase, leading to reduced hepatic cholesterol synthesis during high-fat diet feeding. These results suggest that folic acid supplementation may improve cholesterol and glucose metabolism by restoration of AMPK activation in the liver.

Therapeutic potential of folic acid supplementation for cardiovascular disease prevention through homocysteine lowering and blockade in rheumatoid arthritis patients

Rheumatoid arthritis (RA) is a chronic inflammatory disease that preferentially affects joints, and characterized by an approximately two-fold increased risk of cardiovascular diseases compared with the general population. Beyond classical cardiovascular risk factors, systemic inflammatory markers are primarily involved. Hence, anti-inflammatory strategies such as homocysteine-lowering interventions are warranted. Indeed, hyperhomocysteinemia is commonly found in RA patients as a result of both genetic and non-genetic factors including older age, male gender, disease-specific features and disease-modifying antirheumatic drugs. Most importantly in the pathophysiology of hyperhomocysteinemia and its related cardiovascular diseases in RA, there is a bi-directional link between immuno-inflammatory activation and hyperhomocysteinemia. As such, chronic immune activation causes B vitamins (including folic acid) depletion and subsequent hyperhomocysteinemia. In turn, hyperhomocysteinemia may perpetrate immuno-inflammatory stimulation via nuclear factor ƙappa B enhancement. This chronic immune activation is a key determinant of hyperhomocysteinemia-related cardiovascular diseases in RA patients. Folate, a homocysteine-lowering therapy could prove valuable for cardiovascular disease prevention in RA patients in the near future with respect to homocysteine reduction along with blockade of subsequent oxidative stress, lipid peroxidation, and endothelial dysfunction. Thus, large scale and long term homocysteine-lowering clinical trials would be helpful to clarify the association between hyperhomocysteinemia and cardiovascular diseases in RA patients and to definitely state conditions surrounding folic acid supplementation. This article reviews direct and indirect evidence for cardiovascular disease prevention with folic acid supplementation in RA patients.

Can folic acid have a role in mitochondrial disorders?

Cellular folate metabolism is highly compartmentalized, with mitochondria folate transport and metabolism being distinct from the well-known cytosolic folate metabolism. There is evidence supporting the association between low folate status and mitochondrial DNA (mtDNA) instability, and cerebral folate deficiency is relatively frequent in mitochondrial disorders. Furthermore, folinic acid supplementation has been reported to be beneficial not only in some patients with mitochondrial disease, but also in patients with relatively common diseases where folate deficiency might be an important pathophysiological factor. In this review, we focus on the evidence that supports the potential involvement of impaired folate metabolism in the pathophysiology of mitochondrial disorders.

Determinants of the activities of antifolates delivered into cells by folate-receptor-mediated endocytosis

PURPOSE

Elements in the endocytic process that are determinants of the activities of antifolates delivered by folate-receptor alpha (FRα) were explored.

METHODS

Antifolate growth inhibition was assessed with a 1- or 5-day exposure in reduced folate carrier-null HeLa cell lines that express a high level of FRα in the presence or absence of the proton-coupled folate transporter (PCFT). pH-dependent rates of dissociation from FRα were also determined.

RESULTS

With a 1-day drug exposure which is relevant to the pulse clinical administration of these drugs, FRα expression enhanced raltitrexed activity and modestly enhanced ZD9331 activity, but did not significantly augment the activity of pemetrexed or lomotrexol. With a 5-day drug exposure, FRα-mediated growth inhibition was increased for raltitrexed and ZD9331 and emerged for lomotrexol. While the FRα-augmented activity of lomotrexol and raltitrexed did not require PCFT, augmentation of ZD9331 activity required the co-expression of PCFT with both 1- and 5-day exposures. In contrast, there was no augmentation of pemetrexed activity by FRα under any condition. The activities of these agents correlated with their rate of dissociation from the receptor at acidic pH: raltitrexed > ZD9331 > lomotrexol > pemetrexed consistent with insufficient pemetrexed release from FRα for export from the endosomes.

CONCLUSIONS

FRα is unlikely to contribute to the pharmacological activity of antifolates, such as pemetrexed, that bind tightly to, and dissociate slowly from, the receptor particularly when the exposure time is brief. While PCFT was required for FRα-mediated ZD9931 activity, the activities of the other antifolates was independent of PCFT.

Novel insights in folate receptors and transporters: implications for disease and treatment of immune diseases and cancer

Folate receptors and transporters as well as folate enzymes play an essential role in human disease and form important targets for the treatment of immune diseases and cancer. To discuss new developments in this area, every 2 years a multidisciplinary meeting is held, which aims to be an informal forum for fundamental scientists and clinicians. During this meeting, the regulation of folate transporters and folate enzymes is discussed at the level of expression, transcription, translation, post-translational modification, and splicing and enzyme regulation. Importantly, this knowledge is applied and translated into exciting clinical applications by clinicians with various backgrounds, such as surgeons, nephrologists, rheumatologists and oncologists. Moreover, the meeting provides an excellent forum for a scientific interaction between academia and industry.

Identification of Tyr residues that enhance folate substrate binding and constrain oscillation of the proton-coupled folate transporter (PCFT-SLC46A1)

The proton-coupled folate transporter (PCFT) mediates intestinal folate absorption and transport of folates across the choroid plexus. This study focuses on the role of Tyr residues in PCFT function. The substituted Cys-accessibility method identified four Tyr residues (Y291, Y362, Y315, and Y414) that are accessible to the extracellular compartment; three of these (Y291, Y362, and Y315) are located within or near the folate binding pocket. When the Tyr residues were replaced with Cys or Ala, these mutants showed similar (up to 6-fold) increases in influx Vmax and Kt/Ki for [(3)H]methotrexate and [(3)H]pemetrexed. When the Tyr residues were replaced with Phe, these changes were moderated or absent. When Y315A PCFT was used as representative of the mutants and [(3)H]pemetrexed as the transport substrate, this substitution did not increase the efflux rate constant. Furthermore, neither influx nor efflux mediated by Y315A PCFT was transstimulated by the presence of substrate in the opposite compartment; however, substantial bidirectional transstimulation of transport was mediated by wild-type PCFT. This resulted in a threefold greater efflux rate constant for cells that express wild-type PCFT than for cells that express Y315 PCFT under exchange conditions. These data suggest that these Tyr residues, possibly through their rigid side chains, secure the carrier in a high-affinity state for its folate substrates. However, this may be achieved at the expense of constraining the carrier's mobility, thereby decreasing the rate at which the protein oscillates between its conformational states. The Vmax generated by these Tyr mutants may be so rapid that further augmentation during transstimulation may not be possible.

Folate is absorbed across the human colon: evidence by using enteric-coated caplets containing 13C-labeled [6S]-5-formyltetrahydrofolate

BACKGROUND

Folate intakes that do not meet or greatly exceed requirements may be associated with negative health outcomes. A better understanding of contributors that influence the input side will help establish dietary guidance that ensures health benefits without associated risks. Colonic microbiota produce large quantities of folate, and [(13)C5]5-formyltetrahydrofolate infused during colonoscopy is absorbed. However, it is unclear if significant quantities of folate are absorbed in an intact microbiome.

OBJECTIVE

We determined whether and how much of a physiologic dose of [(13)C5]5-formyltetrahydrofolate delivered in a pH-sensitive enteric caplet to an intact colonic microbiome is absorbed.

DESIGN

Healthy adults ingested a specially designed pH-sensitive acrylic copolymer-coated barium sulfate caplet that contained 855 nmol (400 μg) [(13)C5]5-formyltetrahydrofolate. After a washout period ≥ 4 wk, subjects received an intravenous injection of the same compound (214 nmol). Serially collected blood samples before and after each test dose were analyzed by using a microbiological assay and liquid chromatography-tandem mass spectrometry.

RESULTS

Caplet disintegration in the colon was observed by fluoroscopic imaging for 6 subjects with a mean (± SD) complete disintegration time of 284 ± 155 min. The mean (± SEM) rate of appearance of [(13)C5]5-methyltetrahydrofolate in plasma was 0.33 ± 0.09 (caplet) and 5.8 ± 1.2 (intravenous) nmol/h. Likely because of the significant time in the colon, the mean apparent absorption across the colon was 46%.

CONCLUSIONS

Folate is absorbed across the colon in humans with an undisturbed microbiome. This finding and previous observations of the size of the colonic depot of folate and its potential for manipulation by diet (eg, dietary fiber, oligosaccharides, and probiotics) suggest that an individual's dietary folate requirement may differ depending on the consumption of dietary constituents that affect the size and composition of their gastrointestinal microbiota. In addition, a systematic investigation of the role of colonic folate on gastrointestinal development and the prevention of colorectal cancer is warranted. This trial was registered at clinicaltrials.gov as NCT00941174.

Mammalian mitochondrial and cytosolic folylpolyglutamate synthetase maintain the subcellular compartmentalization of folates

Folylpoly-γ-glutamate synthetase (FPGS) catalyze the addition of multiple glutamates to tetrahydrofolate derivatives. Two mRNAs for the fpgs gene direct isoforms of FPGS to the cytosol and to mitochondria in mouse and human tissues. We sought to clarify the functions of these two compartmentalized isoforms. Stable cell lines were created that express cDNAs for the mitochondrial and cytosolic isoforms of human FPGS under control of a doxycycline-inducible promoter in the AUXB1 cell line. AUXB1 are devoid of endogenous FPGS activity due to a premature translational stop at codon 432 in the fpgs gene. Loss of folates was not measurable from these doxycycline-induced cells or from parental CHO cells over the course of three CHO cell generations. Likewise, there was no detectable transfer of folate polyglutamates either from the cytosol to mitochondria, or from mitochondria to the cytosol. The cell line expressing cytosolic FPGS required exogenous glycine but not thymidine or purine, whereas cells expressing the mitochondrial isoform required exogenous thymidine and purine but not glycine for optimal growth and survival. We concluded that mitochondrial FPGS is required because folate polyglutamates are not substrates for transport across the mitochondrial membrane in either direction and that polyglutamation not only traps folates in the cytosol, but also in the mitochondrial matrix.