Folate-Binding Protein in Milk: A Review of Biochemistry, Physiology, and Analytical Methods

Untargeted Screening Based on UHPLC-HRMS of Total Folates Produced by Lactic Acid Bacteria in Fermented Milk and During Yogurt Shelf Life

Folate deficiency is a widespread nutritional issue, and biofortifying dairy products through lactic acid bacteria (LAB) is a promising strategy to enhance natural folate levels. This study aimed to develop a reliable method for selecting Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus strains with enhanced folate production for use as functional starter cultures. Initially, a traditional microbiological assay (MA) was used to measure folate production in 36 LAB strains isolated from fermented milks. Due to MA's limitations, an untargeted and semi-quantitative method combining ultra-high-performance liquid chromatography (UHPLC) with high-resolution mass spectrometry (HRMS) was developed for a more comprehensive folate screening. The MA showed higher folate production in S. thermophilus strains (309-639 µg/L) compared to L. delbrueckii subsp. bulgaricus (up to 48 µg/L). Subsequently, nine selected LAB strains were further analyzed using the UHPLC-HRMS approach, which enabled the identification and semi-quantification of six folate metabolites, namely dihydrofolate, tetrahydrofolate (THF), 10-formyl-THF, 5,10-methenyl-THF, 5,10-methylene-THF, and 5-methyl-THF. Lab-scale yogurt production using the top-performing strains, as identified through the HRMS method, demonstrated an increase in folate content over a 14-day shelf life. These findings revealed the potential of UHPLC-HRMS as a high-throughput alternative method for folates detection, offering a promising tool for screening folate-enhanced LAB strains for biofortification.

Biological, dietetic and pharmacological properties of vitamin B9

Humans must obtain vitamin B9 (folate) from plant-based diet. The sources as well as the effect of food processing are discussed in detail. Industrial production, fortification and biofortification, kinetics, and physiological role in humans are described. As folate deficiency leads to several pathological states, current opinions toward prevention through fortification are discussed. Claimed risks of increased folate intake are mentioned as well as analytical ways for measurement of folate.

Assessing a Fermented Whey Beverage Biofortified with Folate as a Potential Folate Source for Humans

Folate, a vital water-soluble vitamin (B9), requires specific attention as its recommended daily intake frequently is not reached in countries without mandatory fortification. In this regard, biofortification with microorganisms like Bifidobacterium and Streptococcus offers a compelling approach for enhancing food with natural folates. A randomized, nonblinded, and monocentric human pilot study is conducted to assess the bioavailability of a folate-biofortified fermented whey beverage, comprising 3 intervention days and a controlled replenishment phase before and during the assay. Folate plasma concentration (5-CH3-H4folate) is determined using a stable isotope dilution assay and LC-MS/MS detection. Biokinetic parameters (cmax and tmax) are determined, and areas under the curve (AUC) normalized to the basal folate plasma concentration are calculated. An average bioavailability of 17.1% in relation to the 5-CH3-H4folate supplement, ranging from 0% to 39.8%, is obtained. These results reiterate the significance of additional research into folate bioavailability in general and dairy products. Further investigations are warranted into folate-binding proteins (FBP) and other potential limiting factors within the food and individual factors. In summary, biofortification via fermentation emerges as a promising avenue for enhancing the natural folate content in dairy and other food products.

How food matrices modulate folate bioaccessibility: A comprehensive overview of recent advances and challenges

The incomplete absorption of dietary folate makes it crucial to understand how food matrices affect folate bioaccessibility. Bioavailability encompasses bioaccessibility, which depicts the proportion that is liberated from the food matrix during digestion and becomes available for absorption. Bioavailability studies are expensive and difficult to control, whereas bioaccessibility studies utilize in vitro digestion models to parameterize the complex digestion, allowing the evaluation of the effect of food matrices on bioaccessibility. This review covers the folate contents in various food matrices, the methods used to determine and the factors affecting folate bioaccessibility, and the advances and challenges in understanding how food matrices affect folate bioaccessibility. The methods for determining bioaccessibility have been improved in the last decade. Current research shows that food matrices modulate folate bioaccessibility by affecting the liberation and stability of folate during digestion but do not provide enough information about folate and food component interactions at the molecular level. In addition, information on folate interconversion and degradation during digestion is scant, hindering our understanding of the impact of food matrices on folate stability. Moreover, the role of conjugase inhibitors should not be neglected when evaluating the nutritional value of food folates. Due to the complexity of food digestion, holistic methods should be applied to investigate bioaccessibility. By synthesizing the current state of knowledge on this topic, this review highlights the lack of in-depth understanding of the mechanisms of how food matrices modulate folate bioaccessibility and provides insights into potential strategies for accurate evaluation of the nutritional value of dietary folate.

Recent Advances in Folates and Autoantibodies against Folate Receptors in Early Pregnancy and Miscarriage

Though firstly identified in cerebral folate deficiency, autoantibodies against folate receptors (FRAbs) have been implicated in pregnancy complications such as miscarriage; however, the underlying mechanism needs to be further elaborated. FRAbs can be produced via sensitization mediated by folate-binding protein as well as gene mutation, aberrant modulation, or degradation of folate receptors (FRs). FRAbs may interfere with folate internalization and metabolism through blocking or binding with FRs. Interestingly, different types of FRs are expressed on trophoblast cells, decidual epithelium or stroma, and macrophages at the maternal-fetal interface, implying FRAbs may be involved in the critical events necessary for a successful pregnancy. Thus, we propose that FRAbs may disturb pregnancy establishment and maintenance by modulating trophoblastic biofunctions, placental development, decidualization, and decidua homeostasis as well as the functions of FOLR2+ macrophages. In light of these findings, FRAbs may be a critical factor in pathological pregnancy, and deserve careful consideration in therapies involving folic acid supplementation for pregnancy complications.

Characterizing the extent human milk folate is buffered against maternal malnutrition and infection in drought‐stricken northern Kenya

Objectives

Folate is an essential nutrient fundamental to human growth and development. Human milk maintains high folate content across the maternal folate status range, suggesting buffering of milk folate with prioritized delivery to milk at the expense of maternal depletion. We investigated whether and how the extent of this buffering may diminish under prolonged nutritional and/or disease stress, while taking into consideration infants' varying vulnerability to malnutrition‐related morbidity/mortality.

Methods

A cross‐sectional study analyzed milk specimens from northern Kenyan mothers (n  = 203), surveyed during a historic drought and ensuing food shortage. Multiple regression models for folate receptor‐α (FOLR1) in milk were constructed. Predictors included maternal underweight (BMI < 18.5), iron‐deficiency anemia (hemoglobin <12 g/dl and dried‐blood‐spot transferrin receptor >5 mg/L), folate deficiency (hyperhomocysteinemia, homocysteine >12 or 14 μmol/L), inflammation (serum C‐reactive protein >5 mg/L), infant age and sex, and mother‐infant interactions.

Results

In adjusted models, milk FOLR1 was unassociated with maternal underweight, iron‐deficiency anemia and inflammation. FOLR1 was positively associated with maternal folate deficiency, and inversely associated with infant age. There was interaction between infant age and maternal underweight, and between infant sex and maternal folate deficiency, predicting complex changes in FOLR1.

Conclusions

Our results suggest that mothers buffer milk folate against their own nutritional stress even during a prolonged drought; however, the extent of this buffering may vary with infant age, and, among folate‐deficient mothers, with infant sex. Future research is needed to better understand this variability in maternal buffering of milk folate and how it relates to folate status in nursing infants.

Human mothers might have the capacity to buffer milk nutrient content against undue fluctuations under nutritional or disease stress. In the case of the micronutrient folate, this buffering is most apparent for younger infants.

Conversion of calcium-l-methylfolate and (6S)-5-methyltetrahydrofolic acid glucosamine salt into dietary folate equivalents

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the conversion of calcium-l-methylfolate and (6S)-5-methyltetrahydrofolic acid glucosamine salt (collectively called 5-MTHF hereafter) into dietary folate equivalents (DFE). Following a systematic review, the conclusions of the opinion are based on one intervention study in adults for intakes < 400 μg/day and three intervention studies in adults for intakes ≥ 400 μg/day. At intakes below 400 μg/day, folic acid (FA) is assumed to be linearly related to responses of biomarkers of intake and status and is an appropriate comparator for deriving a DFE conversion factor for 5-MTHF. It is proposed to use the same factor as for folic acid for conversion of 5-MTHF into DFE for intakes < 400 μg/day. As such intake levels are unlikely to be exceeded through fortified food consumption, the conversion factor of 1.7 relative to natural food folate (NF) could be applied to 5-MTHF added to foods and to food supplements providing < 400 μg/day. At 400 μg/day, 5-MTHF was found to be more bioavailable than folic acid and a conversion factor of 2 is proposed for this intake level and for higher intakes. The derived DFE equations are DFE = NF + 1.7 × FA + 1.7 × 5-MTHF for fortified foods and food supplements providing intakes < 400 μg/day; and DFE = NF + 1.7 × FA + 2.0 × 5-MTHF for food supplements providing intakes ≥ 400 μg/day. Although this assessment applies to calcium-L-methylfolate and 5-MTHF glucosamine salt, it is considered that the influence of the cation on bioavailability is likely to be within the margin of error of the proposed DFE equations. Therefore, the proposed equations can also be applied to 5-MTHF associated with other cations.

Profile of Folate in Breast Milk from Chinese Women over 1-400 Days Postpartum

Folate is an essential nutrient for growth in early life. This study aimed to determine the levels and compositions of folate in Chinese breast milk samples. This study was part of the Maternal Nutrition and Infant Investigation (MUAI) study. A total of 205 healthy mothers were randomly recruited in Chengdu over 1−400 days postpartum. Five different species of folate, including tetrahydrofolate (THF), 5-methyl-THF, 5,10-methenyl-THF,5-formyl-THF and unmetabolized folic acid (UMFA), were measured for liquid chromatography−tandem mass spectrometry (LC-MS). The median levels of total folate ranged from 12.86 to 56.77 ng/mL in the breast milk of mothers at 1−400 days postpartum, gradually increasing throughout the lactating periods. The median levels of 5-methyl-THF, minor reduced folate (the sum of THF, 5,10-methenyl-THF and 5-formyl-THF) and UMFA were in the ranges of 8.52−40.65 ng/mL, 3.48−16.15 ng/mL and 0.00−1.24 ng/mL during 1−400 days postpartum, respectively. 5-Methyl-THF accounted for more than 65% of the total folate in all breast milk samples. The levels of UMFA in mature breast milk samples were higher in supplement users than nonusers, but not for colostrum and transitional milk samples (p < 0.05). In conclusion, the level of total folate in the breast milk changed along with the prolonged lactating periods, but 5-methyl-THF remains the dominant species of folate in the breast milk of Chinese populations across all entire lactating periods.

Heterologous Expression of SiFBP, a Folate-Binding Protein from Foxtail Millet, Confers Increased Folate Content and Altered Amino Acid Profiles with Nutritional Potential to Arabidopsis

The mechanism underlying folate degradation in foxtail millet grains remains unclear. Here, we identified SiFBP (Setaria italica folate-binding protein) from foxtail millet. A phylogenetic tree revealed that FBPs have close genetic relationships among cereal crop species. Docking analysis and heterologous expression of SiFBP in yeast showed that it could bind folic acid (FA). The SiFBP localized to the plasma membrane in tobacco mesophyll cells by transient expression. In Arabidopsis, it was expressed specifically in the roots and germinating seeds. Overexpressing SiFBP in yeast and Arabidopsis significantly increased folate contents. Untargeted metabolome analysis revealed differentially accumulated metabolites between the transgenic lines (TLs) and wild type (WT); these metabolites were mainly enriched in the amino acid metabolism pathway. The relative contents of lysine and leucine, threonine, and l-methionine were significantly higher in the TLs than in WT. Genes related to the folate and lysine synthesis pathways were upregulated in the TLs. Thus, SiFBP can be used for biofortification of folate and important amino acids in crops via genetic engineering.

First Insight into the Variation of the Milk Serum Proteome within and between Individual Cows

Milk contains all nutrients needed for development of calves. One important group of components responsible for this are the milk proteins. Variation due to feed or animal health, has been studied for the most abundant milk proteins. The aim of this study was to determine the variation between and within cows for their milk serum proteome. Sample Set 1 was collected from Holstein Friesian (HF) cows between November 2011 and March 2012 and prepared using filter aided sample preparation (FASP) followed by LC-MS/MS for protein identification and quantification. The results showed that the milk serum proteome was very constant in mid lactation (four cows at five time points, p > 0.05) between 3 and 6 months in lactation. Sample Set 2 was collected from HF cows in Dec 2012 and analyzed using FASP and dimethyl labeling followed by LC-MS/MS. Significant variation in the milk serum proteome (p < 0.05) between 17 individual cows was found in Sample Set 2. The most variable proteins were immune-related proteins, which may reflect the health status of the individual cow. On the other hand, proteins related to nutrient synthesis and transport were relatively constant, indicating the importance of milk in providing a stable supply of nutrients to the neonate. In conclusion, the milk serum proteome was stable over mid lactation, but differed significantly between individuals, especially in immune-related proteins.

Folate in Milk Fermented by Lactic Acid Bacteria from Different Food Sources

Folates are essential micronutrients, and folate deficiency still occurs in many countries. Lactic acid bacteria (LAB) are known to be able to synthesize folates during fermentation, but the folate production is strain-dependent and influenced by the fermentation medium, presence of a folate precursor, and fermentation time. This study aimed to screen extracellular folate-producing LAB from local food sources and evaluate the factors influencing their folate biosynthesis during milk fermentation. The selection of folate-producing LAB was based on their ability to grow in folate-free medium (FACM), with folate concentrations quantified by microbiological assay. Growth of the 18 LAB in FACM varied between isolates, with only 8 isolates growing well and able to synthesize extracellular folate at relatively high concentrations (up to 24.27 ng/mL). The isolates with highest extracellular folate levels, Lactobacillus fermentum JK13 from kefir granules, Lactobacillus plantarum 4C261 from salted mustard, and Lactobacillus rhamnosus R23 from breast milk, were applied to milk fermentation. The last two isolates were probiotic candidates. The three isolates consumed folate when it was present in the milk, and its consumption was in line with their growth. The availability of folate precursors affected the amount of folate consumed, but did not lead to increased folate concentrations in the medium after 72 h fermentation. The results of this study indicate that these isolates cannot be utilized for producing folate in folate-containing milk, as it shows feedback inhibition on folate biosynthesis.

Bioactive Compounds in Infant Formula and Their Effects on Infant Nutrition and Health: A Systematic Literature Review

Infant formulas are an alternative to replace or supplement human milk when breastfeeding is not possible. The knowledge of human milk's bioactive compounds and their beneficial effects has attracted the interest of researchers in the field of infant nutrition, as well as researchers of technology and food sciences that seek to improve the nutritional characteristics of infant formulas. Several scientific studies evaluate the optimization of infant formula composition. The bioactive compound inclusion has been used to upgrade the quality and nutrition of infant formulas. In this context, the purpose of this systematic literature review is to assess the scientific evidence of bioactive compounds present in infant formulas (α-lactalbumin, lactoferrin, taurine, milk fat globule membrane, folates, polyamines, long-chain polyunsaturated fatty acids, prebiotics, and probiotics) and their effects on infant nutrition and health. Through previously determined criteria, studies published in the last fifteen years from five different databases were included to identify the advances in the optimization of infant formula composition. Over the last few years, there has been optimization of the infant formula composition, not only to increase the similarities in their content of macro and micronutrients but also to include novel bioactive ingredients with potential health benefits for infants. Although the infant food industry has advanced in the last years, there is no consensus on whether novel bioactive ingredients added to infant formulas have the same functional effects as the compounds found in human milk. Thus, further studies about the impact of bioactive compounds in infant nutrition are fundamental to infant health.

Is natural (6S)-5-methyltetrahydrofolic acid as effective as synthetic folic acid in increasing serum and red blood cell folate concentrations during pregnancy? A proof-of-concept pilot study

Background

North American health authorities recommend 0.4 mg/day folic acid before conception and throughout pregnancy to reduce the risk of neural tube defects. Folic acid is a synthetic form of folate that must be reduced by dihydrofolate reductase and then further metabolized. Recent evidence suggests that the maximal capacity for this process is limited and unmetabolized folic acid has been detected in the circulation. The biological effects of unmetabolized folic acid are unknown. A natural form of folate, (6S)-5-methyltetrahydrofolic acid (Metafolin®), may be a superior alternative because it does not need to be reduced in the small intestine. Metafolin® is currently used in some prenatal multivitamins; however, it has yet to be evaluated during pregnancy.

Methods/design

This double-blind, randomized trial will recruit 60 pregnant women aged 19–42 years. The women will receive either 0.6 mg/day folic acid or an equimolar dose (0.625 mg/day) of (6S)-5-methyltetrahydrofolic acid for 16 weeks. The trial will be initiated at 8–21 weeks’ gestation (after neural tube closure) to reduce the risk of harm should (6S)-5-methyltetrahydrofolic acid prove less effective. All women will also receive a prenatal multivitamin (not containing folate) to ensure adequacy of other nutrients. Baseline and endline blood samples will be collected to assess primary outcome measures, including serum folate, red blood cell folate and unmetabolized folic acid. The extent to which the change in primary outcomes from baseline to endline differs between treatment groups, controlling for baseline level, will be estimated using linear regression. Participants will have the option to continue supplementing until 1 week postpartum to provide a breastmilk and blood sample. Exploratory analyses will be completed to evaluate breastmilk and postpartum blood folate concentrations.

Discussion

This proof-of-concept trial is needed to obtain estimates of the effect of (6S)-5-methyltetrahydrofolic acid compared to folic acid on circulating biomarkers of folate status during pregnancy. These estimates will inform the design of a definitive trial which will be powered to assess whether (6S)-5-methyltetrahydrofolic acid is as effective as folic acid in raising blood folate concentrations during pregnancy. Ultimately, these findings will inform folate supplementation policies for pregnant women.

Trial registration

ClinicalTrials.gov, ID: NCT04022135. Registered on 14 July 2019.

The MTHFR 677C>T polymorphism is associated with unmetabolized folic acid in breast milk in a cohort of Canadian women

BACKGROUND

Maternal nutrition and genetics are determinants of breast-milk nutrient composition and, as such, are determinants of the nutritional exposure of breastfed infants.

OBJECTIVES

The aim of this study was to determine whether common maternal single nucleotide polymorphisms (SNPs) in folate-dependent enzymes are associated with breast-milk folate content in a cohort of mothers enrolled in the Maternal-Infant Research on Environmental Chemicals (MIREC) study.

METHODS

The MIREC study is a Canadian prospective pregnancy cohort study that recruited 2001 participants between 2008 and 2011. Five folate-related SNPs-MTHFR 677C>T (rs1801133), MTHFR 1298A>C (rs1801131), MTHFR 1793G>A (rs2274976), MTR 2756A>G (rs1805087), and MTRR 66A>G (rs1801394)-were genotyped. Breast milk was sampled ∼1 mo postpartum, and tetrahydrofolate (THF), 5-methyl-THF, 5-formyl-THF, 5,10-methenyl-THF, and unmetabolized folic acid (UMFA) were measured using liquid chromatography-tandem mass spectrometry in a subset of participants (n = 551). Associations were assessed using Wald's test. Associations were considered significant if P ≤ 0.01 (Bonferroni correction for multiple testing).

RESULTS

None of the SNPs were associated with total breast-milk folate. However, the MTHFR 677C>T SNP was associated with breast-milk UMFA (R2 = 0.01; unadjusted P = 0.004), explaining a small portion of total variance; this association remained significant when adjusted for other covariates, including supplemental folic acid consumption. The MTHFR 1793G>A and MTRR 66A>G SNPs tended to be associated with 5-methyl-THF (R2 = 0.008, P = 0.04) and reduced folates (THF + 5-methyl-THF + 5-formyl-THF + 5,10-methenyl-THF; R2 = 0.01, P = 0.02), respectively.

CONCLUSIONS

We found that total breast-milk folate content was not associated with any of the folate-related SNPs examined. The association between the MTHFR 677C>T SNP and breast-milk UMFA, albeit modest, highlights the need to better understand the determinants of breast-milk folate and the impact they might have on milk folate bioavailability.

A Rapid Chemiluminescence Assay for Measurement of Folate in Small Volumes of Breast Milk

Early life exposure to folate has long lasting effects on development and health. Newborns obtain part of their folate from maternal milk. Studies on health effects of milk folate require rapid, affordable and reliable measurements in large numbers of samples from cohort studies. Recently, a competitive chemiluminescence assay for quantification of folate has become available for automated diagnostic measurement of folate in human serum or plasma. We tested if this method (“FOLA” from Siemens Healthcare) could also be used for human milk. To minimize interference and matrix effects, samples had to be skimmed, diluted seven times with demineralized water, and heated for 5 min at 90 °C. Folate could thus be measured in a linear range between 8.4 and 111.7 nM, with recoveries for the most relevant form, 5-methyltetrahydrofolate (5-MeTHF), of 96%–107%. Results were comparable to those with a recently validated Liquid Chromatography/Mass Spectrometry method (Y = 0.998X − 0.2; R² = 0.807). The FOLA method was subsequently used for samples from the LIFE Child cohort in Germany, providing first data of breast milk folate in this country (range: 6.2–100.7 nM). This technique could indeed prove useful for large cohorts with multiple samplings.

Total folate and unmetabolized folic acid in the breast milk of a cross-section of Canadian women

Background: Folate requirements increase during pregnancy and lactation. It is recommended that women who could become pregnant, are pregnant, or are lactating consume a folic acid (FA)-containing supplement.Objectives: We sought to determine breast-milk total folate and unmetabolized folic acid (UMFA) contents and their relation with FA-supplement use and doses in a cohort of Canadian mothers who were enrolled in the MIREC (Maternal-Infant Research on Environmental Chemicals) study.Design: Breast-milk tetrahydrofolate (THF), 5-methyl-THF, 5-formyl-THF, 5,10-methenyl-THF, and UMFA were measured with the use of liquid chromatography-tandem mass spectrometry (n = 561). Total daily supplemental FA intake was based on self-reported FA-supplement use.Results: UMFA was detectable in the milk of 96.1% of the women. Total daily FA intake from supplements was associated with breast folate concentration and species. Breast-milk total folate was 18% higher (P < 0.001) in supplement users (n = 401) than in nonusers (n = 160), a difference driven by women consuming >400 μg FA/d (P ≤ 0.004). 5-Methyl-THF was 19% lower (P < 0.001) and UMFA was 126% higher (P < 0.001) in supplement users than in nonusers. Women who consumed >400 μg FA/d had proportionally lower 5-methyl-THF and higher UMFA than did women who consumed ≤400 μg FA/d.Conclusions: FA-supplement use was associated with modestly higher breast-milk total folate. Detectable breast-milk UMFA was nearly ubiquitous, including in women who did not consume an FA supplement. Breast-milk UMFA was proportionally higher than 5-methyl-THF in women who consumed >400 μg FA/d, thereby suggesting that higher doses exceed the physiologic capacity to metabolize FA and result in the preferential uptake of FA in breast milk. Therefore, FA-supplement doses >400 μg may not be warranted, especially in populations for whom FA fortification is mandatory.

Folate binding protein: therapeutic natural nanotechnology for folic acid, methotrexate, and leucovorin

Blood serum proteins play a critical role in the transport, biodistribution, and efficacy of systemically-delivered therapeutics. Here, we have investigated the concentration- and ligand-dependent aggregation of folate binding protein (FBP), focusing in particular on folic acid, an important vitamin and targeting agent; methotrexate, an antifolate drug used to treat cancer and rheumatoid arthritis; and leucovorin which is used to decrease methotrexate toxicity. We employed atomic force microscopy to characterize, on a particle-by-particle basis, the volumes of the FBP nanoparticles that form upon ligand binding. We measured the distribution of FBP nanoparticle volumes as a function of ligand concentration over physiologically- and therapeutically-relevant ranges. At physiologically-relevant concentrations, significant differences in particle volume distributions exist that we hypothesize are consistent with different trafficking mechanisms for folic acid and methotrexate. In addition, we hypothesize leucovorin is trafficked and delivered like folic acid at therapeutically-relevant concentrations. We propose that changes in dosing procedures could improve the delivery and therapeutic index for methotrexate and other folic acid-targeted drug conjugates and imaging agents. Specifically, we suggest pre-binding the drugs to FBP may provide a better formulation for drug delivery of methotrexate for both cancer and rheumatoid arthritis. This would be analogous to pre-binding paclitaxel to albumin, which is already used in the clinic.

Challenges in the determination of unsubstituted food folates: impact of stabilities and conversions on analytical results

Tetrahydrofolate is the parent molecule of the folate coenzymes required for one carbon metabolism. Together with other unsubstituted folates such as dihydrofolate and folic acid, tetrahydrofolate represents the third pool of dietary folates following 5-methyltetrahydrofolate and formyl folates. Low intake of dietary folates and poor folate status are common problems in many countries. There is a critical need for reliable methods to determine folate in foods to accurately estimate folate intakes in populations. However, current values for folates in foods in databanks are often underestimated due to the high instability of several folate forms, especially tetrahydrofolate. The present review highlights the occurrence of unsubstituted folates in foods and their oxidation mechanisms and chemical behavior as well as interconversion reaction between tetrahydrofolate and 5,10-methylenetetrahydrofolate. The review shows also the important role of antioxidants in protecting folates during analysis and describes strategies to stabilize unsubstituted folates throughout all steps of the analytical procedure.

Structural and dynamic investigation of bovine folate receptor alpha (FOLR1), and role of ultra-high temperature processing on conformational and thermodynamic characteristics of FOLR1-folate complex

The folate receptor alpha (FOLR1) present in milk has widely been studied to investigate the effects of pasteurization, ultra-high temperature (UHT) processing and fermentation on net folate concentration. However, the folate binding mechanism with FOLR1, and effect of temperature on FOLR1-folate complex is poorly explored till now in bovine milk which is a chief resource of folate. Despite of enormous importance of folic acid and the routine intake of bovine milk, folic acid deficiency diseases are common in human race. To understand the folate deficiency in milk after processing, in absence of experimental structure, 3D model of bovine FOLR1 (bvFOLR1) was built followed by 40ns molecular dynamics (MD) simulation. The folate and its derivatives binding sites in bvFOLR1 were anticipated by molecular docking using AutoDock 4.2. Essential MD studies suggested the presence of a longer signal peptide (22 residues) and a short propeptide (7 residues) at the C-terminus that may cleaved during post-translational modification. MD analysis of bvFOLR1-folate complex at 298, 323, 353, 373 and 408K followed by binding energy (BE) calculation showed maximum binding affinity at ∼353K. However, at 373K and UHT (408K), the folate BE is significantly decreased with substantial conformational alteration. Heating at UHT followed by cooling within 298-408K range demoed no structural reformation with temperature reduction, and the folate was displaced from the active site. This study presented the disintegration of folate from bvFOLR1 during high temperature processing and revealed a lower folate concentration in UHT milk and dairy products.

Occurrence, stability, and determination of formyl folates in foods

The B-vitamin folate has specific tasks as a one-carbon (C1) group supplier in the building and repair of DNA and RNA as well as in the methylation of homocysteine to methionine. Folate occurs in all living cells as a dynamic pool of several interconvertible forms carrying different C1 groups. Along the food chain, this dynamic pool of folates constantly changes due to either enzymatic or chemical interconversions during food processing and storage. These interconversions make it difficult to determine individual folate forms in foods. The formyl folates, the second most predominant forms of food folates, after 5-methyltetrahydrofolate, are particularly prone to interconvert at low pH. Today, this knowledge is often neglected, leading to risks for analytical underestimation of formyl folates. The purpose of the review is to explore the stability and interconversions of formyl folates in foods as well as to analyze the pitfalls in the determination of formyl folates.

Structural and functional insights on folate receptor α (FRα) by homology modeling, ligand docking and molecular dynamics

Folate receptor α (FRα) is a cell surface, glycophosphatidylinositol (GPI)-anchored protein with a high affinity for its ligand partner, which is highly expressed in malignant cells and has been selected as a therapeutic target and marker for the diagnosis of cancer. No direct structural information is available from either X-ray diffraction or NMR on the post-translational structure of this disulfide-rich protein. Three-dimensional models of the FRα structure have been derived with the recent homology modeling packages, using the crystal structure of the riboflavin-binding protein (RfBP) as a template. Molecular dynamics trajectories have been exploited allowing successfully the formation of a full disulfide bridge network, which was expected based on the similarities between FRα and RfBP. After the selection of the best model, a folic acid molecule was docked "in silico" onto the putative binding site and its binding mode was compared with that of vintafolide, a much larger molecule designed as a chemotherapy agent targeting specifically FRα. In both cases, a 40ns molecular dynamics trajectory was calculated, providing suggestions regarding the key structural determinants driving the affinity and specificity of FRα for folic acid with respect to other folate homologues. Moreover, some other crucial experimental results related to the structure of the receptor are discussed, such as the expected location/accessibility of known immune epitopes, the set of N-linked glycosylation sites and the effect of point mutations on the impairment of folate binding. Our results may provide useful insights for studies related to folate-targeted drug delivery or cancer therapies involving folate uptake.

Folate-status response to a controlled folate intake in nonpregnant, pregnant, and lactating women

BACKGROUND

Folate dose-response studies in women of childbearing age who consumed a folic acid (FA)-containing multivitamin in the era of FA fortification are lacking.

OBJECTIVE

We sought to investigate folate-status response to a known folate dose comprising an FA-containing prenatal supplement (750 μg/d) plus natural food folate (400 μg/d) in third-trimester pregnant women, lactating women 5-15 wk postpartum, and nonpregnant women.

DESIGN

Pregnant (n = 26), lactating (n = 28), and nonpregnant (n = 21) women consumed the study folate dose under controlled intake conditions for 10-12 wk. Blood, urine, and breast milk were collected at baseline, study midpoint, and study end.

RESULTS

Study-end serum total folate concentrations averaged ~30 ng/mL and did not differ by physiologic group (P = 0.876). Study-end urinary folate excretion represented ~9-43% of total folate intake and ranged from 100 to 500 μg/d. Third-trimester pregnant women excreted less urinary folate than did lactating (P = 0.075) and nonpregnant (P < 0.001) women. Lactating women excreted less (P < 0.001) urinary FA than did nonpregnant women. Breast-milk total folate concentrations remained constant (P = 0.244; 61.8 ng/mL at study end), whereas breast-milk FA concentrations increased (P = 0.003) to 24.1 ng/mL at study end.

CONCLUSIONS

The consumption of the study folate dose yielded a supranutritional folate status regardless of the physiologic state. Based on urinary folate excretion, folate use was greatest to least: pregnant > lactating > nonpregnant women. Breast-milk folate species were responsive to maternal folate intake, and FA made up ~40% of breast-milk total folate at study end. These findings warrant revisiting prenatal supplement FA formulation in populations exposed to FA-fortification programs.