The preparation and properties of folate-binding protein from cow's milk

Folate-conjugated hydrophobicity modified glycol chitosan nanoparticles for targeted delivery of methotrexate in rheumatoid arthritis

BACKGROUND

Targeted delivery to the Rheumatoid arthritis (RA) which is characterized by destruction and degeneration of bones due to chronic inflammation is of great need. RA being a chronic autoimmune disorder might result in severe disability and morbidity. A targeted delivery system is designed to deliver methotrexate (MTX) for RA.

METHODS

Here, we synthesized folic acid (FA) conjugated hydrophobically modified glycol chitosan (GC) self-assembled nanoparticles (FA-GC-SA) for the targeted delivery of MTX to RA. The FA conjugation and hydrophobic modification of GC by stearic acid (SA) was confirmed by Fourier-transform infrared spectroscopy (FTIR). The FA-GC-SA was exploited for developing targeted nanoparticles encapsulating MTX by the ionic gelation method. The particles were characterized and evaluated for their targeting potential in in vitro cell culture studies. Further their in vivo efficacy in arthritis induced rats using collagen was also evaluated.

RESULTS

FTIR confirms the successful modification of GC-SA and FA-GC-SA. The FA-GC-SA-MTX of size 153 ± 9 nm were prepared with high encapsulation efficiency of MTX. The FA-GC-SA-MTX size was further confirmed by transmission electron microscopy (TEM). In vitro cell studies revealed the superior efficacy of FA-GC-SA-MTX in cell cytotoxicity. Also, significantly higher cellular uptake of FA functionalized FA-GC-SA-MTX was observed in comparison to non-functionalized GC-SA-MTX attributed to folate receptors (FRs) mediated endocytosis. In vivo results confirms the potential of FA-GC-SA-MTX which reduces reduces the pro-inflammatory cytokines, paw thickness, and arthritis score in collagen induced rats.

CONCLUSION

The results shows that FRs targeted FA-GC-SA-MTX has superior efficacy in the treatment of RA.

Folate Conjugated Double Liposomes Bearing Prednisolone and Methotrexate for Targeting Rheumatoid Arthritis

PURPOSE

This investigation was aimed to explore the targeting potential of folate conjugated double liposomes (fDLs) bearing combination of synergistic drugs (Prednisolone and Methotrexate) for effective management of the rheumatoid arthritis (RA).

METHODS

To overcome the drawbacks of monotherapy, a combination of prednisolone (PRD) (an anti-inflammatory agent) and methotrexate (MTX) (a disease modifying anti-rheumatoid agent, DMARDs) was chosen for dual targeting approach. fDLs were prepared in two steps i.e. development of inner liposomes (ILs) using thin film casting method followed by encapsulation of ILs within folate conjugated outer liposomes (double liposomes; fDLs). Developed liposomes were characterized for various physicochemical parameters and in vivo performance.

RESULTS

fDLs were prepared using FA-PEG-4000-NH-DSPE conjugate. These double liposomes were having 429.3 ± 3.6 nm in size with 0.109 PDI, 8.01 ± 0.3 mV zeta potential (ζ) and 66.7 ± 3.9% and 45.3 ± 1.7% entrapments of PRD and MTX, respectively. After 24 h, the concentrations of PRD in blood were observed to be 8.66 ± 3.11 (ILs) and 15.13 ± 0.81% (DLs) while concentration of MTX were found to be 10.89 ± 0.69 and 2.34 ± 3.15% when given as ILs and fDLs, respectively. The concentration of both drugs in inflamed joint was observed to be higher than that in the non-inflamed joints.

CONCLUSIONS

The folate conjugated double liposomes possess superior targeting efficiency than conjugated and unconjugated single liposomes.

In vitro bioaccessibility of added folic acid in commercially available baby foods formulated with milk and milk products

Milk contains a certain amount of folate binding proteins. The binding capacity varies in acidic conditions and affects the bioavailability of folic acid. Folic acid is commonly added into baby foods to ensure adequate intake of infants. The aim of this study was to determine the bioaccessibility of added folic acid in baby foods formulated with milk and milk products under different gastric pH values by an in vitro digestive system. The bioaccessibility of folic acid ranged between 56-71 and 35-49% in infant formula samples, between 59-78 and 31-67% in cereal-based baby foods, and between 42-67 and 38-57% in follow-on baby milk at gastric pH 1.5 and pH 4, respectively. Our results demonstrate that the bioaccesibility of folic acid that is added to baby food is affected by gastric pH. Therefore, it was observed that the bioaccesibility of folic acid was lower in the higher gastric pH.

High pressure processing and post-high pressure storage induce the change of polyglutamyl folate and total folate from different legumes

The aim of this work was to study the effect of high pressure processing (HPP) and post-HPP cold storage on the distribution of polyglutamyl and monoglutamyl folate and the absolute concentration of total folate in green beans, yardlong beans and winged beans using a validated ultra-high performance liquid chromatography-tandem mass spectrometry method. The results showed that HPP led to the deglutamylation of polyglutamyl folate to monoglutamyl folate in all of the investigated beans. The degree of deglutamylation was increased with enhancing processing pressure and extending holding time. During HPP, significant loss of total folate was observed under 600 MPa/10 min treatment. Uniquely 300 MPa/5 min and 450 MPa/5 min could significantly release more folate from yardlong beans and green beans matrix. During the following cold-storage, the deglutamylation keep progressing. For those untreated beans, no significant deglutamylation and total folate loss was observed during cold storage for yardlong beans and green beans while there is slight change for the total folate in winged beans. For those HPP treated beans, total folate loss followed the first order kinetics over the storage. The rate constant of degradation was positively proportional to the applied pressure, holding time and the proportion of monoglutamyl folate. This research provided a reference for understanding the deglutamylation of polyglutamyl folate and folate loss during HPP treatment and further shelf life.

Tackling serum folate test in European countries within the health technology assessment paradigm: request appropriateness, assays and health outcomes

Several authors have recently claimed an excess in serum folate test ordering, suggesting phasing out it from clinical use. According to studies performed in countries undergoing folic acid fortification policies, it is indeed no more cost-effective to test folate in the face of deficiency prevalence <1%. In this paper, we sought to evaluate request appropriateness, analytical issues, and cost-effectiveness of serum folate determination for clinical purposes in the European context, considering if evidence retrieved in fortified countries may be generalized. Studies performed in non-fortified countries have generally reported a suboptimal folate intake and suggest a remarkable prevalence of folate deficiency. Our internal data suggest that ~20%–25% of the subjects undergoing serum folate test are at risk for deficiency. However, a reliable evaluation of the risk for deficiency implies the knowledge of all issues related to the total testing process of folate measurement as well as the identification of the appropriate population in which to perform the test. The cost-effectiveness of the test is maximized when the request is oriented to subjects suggestive/at risk for deficiency, becoming low if the test is used as a screening tool or for monitoring of vitamin intake/supplementation. Because the individual folate status has a key role in ensuring normal development, physiologic growth, and maintenance of optimal health, the evaluation of its serum levels has to be retained in the clinical use in non-fortified countries, boosting for more appropriate request, and evidence from countries following fortification policies should be cautionary interpreted.

The complex interplay between ligand binding and conformational structure of the folate binding protein (folate receptor): Biological perspectives

This review analyzes how interplay between folate binding and changes in folate binding protein (FBP) conformation/self-association affects the biological function of FBP. Concentration-dependent, reversible self-association of hydrophobic apo-FBP at pI=7.4 is associated with decreased affinity for folate, probably due to shielding of binding sites between interacting hydrophobic patches. Titration with folate removes apo-monomers, favoring dissociation of self-associated apo-FBP into apo-monomers. Folate anchors to FBP through a network of hydrogen bonds and hydrophobic interactions, and the binding induces a conformational change with formation of hydrophilic and stable holo-FBP. Holo-FBP exhibits a ligand-mediated concentration-dependent self-association into multimers of great thermal and chemical stability due to strong intermolecular forces. Both ligand and FBP are thus protected against biological/physicochemical decomposition. In biological fluids with low FBP concentrations, e.g., saliva, semen and plasma, hydrophobic apo-monomers and hydrophilic holo-monomers associate into stable asymmetrical complexes with aberrant binding kinetics unless detergents, e.g., cholesterol or phospholipids are present.

The interrelationship between ligand binding and thermal unfolding of the folate binding protein. The role of self-association and pH

The present study utilized a combination of DLS (dynamic light scattering) and DSC (differential scanning calorimetry) to address thermostability of high-affinity folate binding protein (FBP), a transport protein and cellular receptor for the vitamin folate. At pH7.4 (pI=7-8) ligand binding increased concentration-dependent self-association of FBP into stable multimers of holo-FBP. DSC of 3.3μM holo-FBP showed Tm (76°C) and molar enthalpy (146kcalM(-1)) values increasing to 78°C and 163kcalM(-1) at 10μM holo-FBP, while those of apo-FBP were 55°C and 105kcalM(-1). Besides ligand binding, intermolecular forces involved in concentration-dependent multimerization thus contribute to the thermostability of holo-FBP. Hence, thermal unfolding and dissociation of holo-FBP multimers occur simultaneously consistent with a gradual decrease from octameric to monomeric holo-FBP (10μM) in DLS after a step-wise rise in temperature to 78°C≈Tm. Stable holo-FBP multimers may protect naturally occurring labile folates against decomposition or bacterial utilization. DSC established an interrelationship between diminished folate binding at pH5, especially in NaCl-free buffers, and low thermostability. Positively charged apo-FBP was almost completely unfolded and aggregated at pH5 (Tm 38°C) and holo-FBP, albeit more thermostable, was labile with aggregation tendency. Addition of 0.15M NaCl increased thermostability of apo-FBP drastically, and even more so that of holo-FBP. Electrostatic forces thus seem to contribute to a diminished thermostability at low pH. Fluorescence spectroscopy after irreversible thermal unfolding of FBP revealed a weak-affinity folate binding.

Ligand binding induces a sharp decrease in hydrophobicity of folate binding protein assessed by 1-anilinonaphthalene-8-sulphonate which suppresses self-association of the hydrophobic apo-protein

High affinity folate binding protein (FBP) regulates as a soluble protein and as a cellular receptor intracellular trafficking of folic acid, a vitamin of great importance to cell growth and division. We addressed two issues of potential importance to the biological function of FBP, a possible decrease of the surface hydrophobicity associated with the ligand-induced conformation change of FBP, and protein-inter-protein interactions involved in self-association of hydrophobic apo-FBP. The extrinsic fluorescent apolar dye 1-anilinonaphthalene-8-sulphonate (ANS) exhibited enhanced fluorescence intensity and a blueshift of emission maximum from 510-520 nm to 460-470 nm upon addition of apo-FBP indicating binding to a strongly hydrophobic environment. Neither enhancement of fluorescence nor blueshift of ANS emission maximum occurred when folate-ligated holo-FBP replaced apo-FBP. The drastic decrease in surface hydrophobicity of holo-FBP could have bearings on the biological function of FBP since changes in surface hydrophobicity have critical effects on the biological function of receptors and transport proteins. ANS interacts with exposed hydrophobic surfaces on proteins and may thereby block and prevent aggregation of proteins (chaperone-like effect). Hence, hydrophobic interactions seemed to participate in the concentration-dependent self-association of apo-FBP which was suppressed by high ANS concentrations in light scatter measurements.

Isotope Dilution–LC-MS/MS Reference Method Assessment of Serum Folate Assay Accuracy and Proficiency Testing Consensus Mean

BACKGROUND

Current methods for measuring folates in clinical laboratories are competitive folate binding protein assays. These assays show a considerable lack of agreement that has implications for the comparability of data across studies as well as for long-term population studies. The development of isotope dilution–liquid chromatography–tandem mass spectrometry (ID-LC-MS/MS) reference methods permitted the evaluation of method accuracy and consistency over time.

METHODS

We measured 3 pools of human serum by ID-LC-MS/MS, calculated values for total folate, and distributed the same pools to participants in a national External Quality Assessment scheme. We used linear regression to compare the all-laboratory and method data with reference method values. The exercise was repeated after 18 months to assess the stability of the all-laboratory and method data.

RESULTS

The distributed serum pools had mass spectrometry values for folate species typical of those found in healthy individuals from populations not receiving dietary folic acid fortification. There was good agreement of the all-laboratory data set with the reference method (y =0.86x + 0.91 μg/L) at both time points. Linear regression demonstrated that the Abbott Architect showed the closest agreement with the reference method. The Roche Elecsys method was nonlinear and showed a calibration offset of 2.6 μg/L (4.57 nmol/L).

CONCLUSIONS

Calibration of serum folate assays traceable to higher-order reference methods increases method accuracy and improves consistency. The all-laboratory consensus mean proved sufficiently accurate and stable to be used as the target for monitoring laboratory performance.

Chemical characterization of milk concentrated by ultrafiltration

Whole milks concentrated 1·5–4-fold and acidified and citrated milks concentrated 2·8-fold by ultrafiltration at 50 °C were analysed for chemical changes relevant to further processing, storage or nutrition. Fat and protein were entirely retained in the concentrate. The retention of water-soluble vitamins, Ca, Mg, phosphate and trace minerals depended on the proportion bound to the protein. Ascorbic acid was rapidly destroyed during concentration. Because of the differential retention of nitrogenous components, protein comprised a progressively higher proportion of the total N as the milk became more concentrated. No denaturation of whey protein or disruption of casein micelles was detected during concentration of whole milk, but some solubilization of the casein occurred after citration. Reduction of fat globule size occurred early in the concentration process, damage to the fat globule membrane was indicated and the milk became more susceptible to lipolysis. Apart from a tendency for preacidified or precitrated concentrates to gel, no change in the susceptibility of the milks to heat damage was detected.

Construction of anti-EGFR immunoliposomes via folate–folate binding protein affinity

A novel method for synthesis of anti-EGFR immunoliposomes using folate-folate binding protein (FBP) affinity is described. An anti-EGFR antibody (cetuximab or C225) was covalently linked to FBP via a thioether bond. Liposomes incorporating a lipophilic folate derivative (folate-PEG-cholesterol) were prepared by polycarbonate membrane extrusion. Anti-EGFR immunoliposomes were then obtained by combining FBP-C225 and folate-liposomes and evaluated for uptake and cytotoxicity in EGFR-overexpressing U87 human glioblastoma cells. Anti-EGFR immunoliposomes constructed via folate-FBP affinity exhibited excellent stability under physiological pH, and quickly released the bound FBP-C225 upon low pH (pH 3.5) treatment. Flow cytometry and fluorescence microscopy showed similar receptor-specific binding and internalization for both folate-FBP affinity-coupled and covalently coupled C225-immunoliposomes, but not for the non-targeted IgG-immunoliposomes. C225-immunoliposomes loaded with anticancer drug doxorubicin were more cytotoxic than non-targeted immunoliposomes in EGFR-overexpressing U87 glioma cells. Folate-FBP affinity is a potential method for construction of immunoliposomes and may have applications in synthesis of targeted drug carriers in general.

Characteristics of Reduced Fat Milks As Influenced By the Incorporation of Folic Acid

Folic acid plays an important role in the prevention of neural tube defects (e.g., spina bifida and anencephaly), heart defects, facial clefts, urinary abnormalities, and limb deficiencies. Milk and milk products serve as a potential source for folic acid fortification because of the presence of folate-binding proteins that seem to be involved in folate bioavailability. Although milk is not a good source of folic acid, fortification could help in the prevention of the above-mentioned defects. The objective of this study was to examine the physicochemical characteristics of reduced fat milks fortified with folic acid. Reduced fat milks were prepared using 25, 50, 75, and 100% of the recommended dietary allowance of 400 microg of folic acid. Treatments included addition of folic acid at these levels before and after pasteurization. Color, pH, fat, protein, viscosity, folic acid concentration, folate-binding protein concentration, folate-binding protein profile, standard plate count, and coliform counts were determined on d 1, 7, 14, and 21. A consumer acceptability test was conducted on d 7. Data from the consumer panel were analyzed using ANOVA (PROC GLM) with means separation to determine the differences among treatments. Data obtained from the color, pH, fat, protein, viscosity, folic acid concentration, folate-binding protein concentration, standard plate count, and coliform counts were analyzed using the GLM with a repeated measure in time. Significant differences were determined at P < 0.05 using Tukey's Studentized Range Test. There were no differences in the electrophoretic mobility of folate-binding protein in the samples. The concentration of folic acid was significantly higher in reduced fat milks fortified with folic acid after pasteurization compared with the treatments in which folic acid was added before pasteurization. The consumer panelists did not find any significant differences in flavor, appearance, or texture of folic acid fortified reduced fat milks compared with that of the control. Fortification of reduced fat milks with folic acid can be accomplished without adversely affecting the product characteristics.

Endothelial cells and macrophages, partners in atherosclerotic plaque progression

Heart disease and stroke, the main cardiovascular diseases (CVD), have become global epidemics in our days. High levels of cholesterol and other abnormal lipids are among the main risk factors of atherosclerosis, the number one killer in the world. However, recent advances in CVD treatment together with improvements in surgical techniques have increased the quality of life and reduced premature death rates and disabilities. Nevertheless, they still add a heavy burden to the rising global costs of health care. The medical priorities highlight not only the need for early recognition of the warning signs of a heart attack, but also the need for early biomarkers for prevention. Two active partners in the development and progression of atherosclerotic plaques are the macrophages and endothelial cells that influence each other and modify the microenvironment composition of the plaque leading to either rapid progression or regression of individual lesions in patients. In this review we address two specific aspects related to atherosclerosis: i) the way in which folic acid and folic acid conjugates may be helpful to identify activated macrophages and ii) the high potential of proteomic analysis to evidence and identify the multiple changes induced in activated vascular cells.

Bioavailability of folic acid from fortified pasteurised and UHT-treated milk in humans

OBJECTIVE

The aim of this study was to investigate whether milk fortified with folic acid enhances the folate status of humans and whether the presence of folate-binding proteins (FBP) in pasteurised milk affects the bioavailability of folic acid from fortified milk. In untreated and pasteurised milk, folate occurs bound to FBP, while FBP is (partly) denatured in ultra-high-temperature (UHT)-treated milk. The effect of FBP on folate bioavailability is still unclear.

DESIGN, SUBJECTS AND SETTING

Healthy, free-living subjects (n=69) aged 18-49 y participated in a 4-week double-blind, placebo-controlled dietary intervention study.

INTERVENTION

In addition to a fully controlled diet, the subjects consumed each day 500 ml of pasteurised or UHT milk, either fortified or not with 200 mug folic acid.

RESULTS

Consumption of fortified milk increased folate concentrations in serum and in red blood cells (RBC) by 6.6-7.0 nmol/l (P<0.001) and 32-36 nmol/l (P<0.01), respectively. Similarly, plasma homocysteine concentrations were lowered 0.88-0.89 micromol/l (P=0.001) in subjects who consumed fortified milk. The bioavailability of folic acid from pasteurised milk relative to that of folic acid from UHT milk was 74-94% (NS), depending on the parameter used.

CONCLUSIONS

Milk fortified to supply an additional 200 microg of folic acid/s substantially increased folate status, and decreased plasma total homocysteine concentrations in young, healthy subjects. Milk is therefore a suitable matrix for fortification to enhance the folate status in humans. No significant effect of endogenous FBP was found on the bioavailability of folic acid from milk.

Affinity and rate constants for interactions of bovine folate-binding protein and folate derivatives determined by optical biosensor technology. Effect of stereoselectivity

The interactions between bovine folate-binding protein (FBP) and different folate derivatives in pure diastereoisomeric forms were studied at pH 7.4 by a surface plasmon resonance technology (Biacore). The results show that folic acid had the most rapid association rate (k(a) = 1.0 x 10(6) M(-)(1) s(-)(1)), whereas (6S)-5-HCO-5,6,7,8-tetrahydrofolic acid had the most rapid dissociation rate (k(d) = 3.2 x l0(-)(3) s(-)(1)). The equilibrium dissociation constant (K(D)), calculated from the quotient of k(d)/k(a), showed that the two forms of folates not occurring in nature, that is, folic acid and (6R)-5-CH(3)-5,6,7,8-tetrahydrofolic acid, had the highest affinities for FBP, 20 and 160 pmol/L, respectively. The results thus show that there were great differences in the interactions between folate-binding protein and the major forms of folate derivatives. The nutritional implications of these differences are discussed.

Schizophyllan-folate conjugate as a new non-cytotoxic and cancer-targeted antisense carrier

Schizophyllan having folate-appendages was synthesized from native schizophyllan through NaIO(4)-oxidation and the subsequent reductive amination in aqueous ammonia followed by amido-coupling with folic acid. The resulting folate-appended schizophyllan can form stable complex with poly(dA), show specific affinity toward folate binding protein, and mediate effective antisense activity in cancer cells.

Quantitative description of the interaction between folate and the folate-binding protein from cow's milk

A detailed study has been carried out on the dependence of folate binding on the concentration of FBP (folate-binding protein) at pH 5.0, conditions selected to prevent complications arising from the pre-existing self-association of the acceptor. In contrast with the mandatory requirement that reversible interaction of ligand with a single acceptor site should exhibit a unique, rectangular hyperbolic binding curve, results obtained by ultrafiltration for the FBP-folate system required description in terms of (i) a sigmoidal relationship between concentrations of bound and free folate and (ii) an inverse dependence of affinity on FBP concentration. These findings have been attributed to the difficulties in determining the free ligand concentration in the FBP-folate mixtures for which reaction is essentially stoichiometric. This explanation also accounts for the similar published behaviour of the FBP-folate system at neutral pH, which had been attributed erroneously to acceptor self-association, a phenomenon incompatible with the experimental findings because of its prediction of a greater affinity for folate with increasing FBP concentration.

Specific Antitumor Targetable β-Cyclodextrin−Poly(ethylene Glycol)−Folic Acid Drug Delivery Bioconjugate

The tumor targeting properties of a new drug carrier synthesized by bioconjugation of folic acid (FA) to beta-cyclodextrins through a poly(ethylene glycol) (PEG) spacer (CD-PEG-FA) were investigated. Surface plasmon resonance demonstrated that CD-PEG-FA specifically interacts with immobilized folate binding protein (FBP) while the naked beta-cyclodextrins do not display any specific interaction. In vitro studies demonstrated that CD-PEG-FA was devoid of cell toxicity. [(3)H]-folic acid/CD-PEG-FA competition binding investigations performed with folate receptor overexpressing human epidermal carcinoma KB cells showed that CD-PEG-FA had about 14 times lower tumor cell binding capacity than free folic acid. The carrier cell trafficking properties were investigated using rhodamine-B as fluorescent probe, which possesses 3000 and 4580 M(-)(1) inclusion constants for CD-PEG-FA and beta-cyclodextrins, respectively. Cell-associated fluorescence measurements showed that CD-PEG-FA does not promote the rhodamine-B uptake into non-folate receptor expressing human lung carcinoma MCF7 cells while 19% higher accumulation in KB cells was found with respect to rhodamine-B loaded beta-cyclodextrins. Confocal laser scanning microscopy indicated the presence of cytosolic red fluorescent spots after 2 h of incubation of KB cells with rhodamine-B included CD-PEG-FA. The fluorescent dye resided primarily in small spots, namely, endosomes and multivesicular bodies. At 1 h after pulsed incubation, wider red fluorescent cellular structures appeared as a fusion of previous structures.

Effect of folate-binding protein on intestinal transport of folic acid and 5-methyltetrahydrofolate across Caco-2 cells

BACKGROUND

Milk products are a potential matrix for fortification with synthetic folic acid or natural 5-methyltetrahydrofolate (5-CH3-H4folate) to enhance the daily folate intake. In milk, folate occurs bound to folate-binding proteins (FBP). Our previous studies with an in vitro gastrointestinal model showed that 70% of the initial FBP content of the milk product was retained in the duodenal lumen. While folic acid remained bound to FBP after gastric passage, 5-CH3-H4folate was mainly present as free folate in the duodenal lumen.

AIM OF THE STUDY

To investigate the effect of FBP on the absorption of folic acid and 5-CH3-H4folate from the intestinal lumen.

METHODS

The transport of [3H]-folic acid and [14C]-5-CH3-H4folate across enterocytes was studied in the presence or absence of bovine FBP using monolayers of Caco-2 cells grown on semi-permeable inserts in a two-compartment model. The apparent permeability coefficients (P(app)) of folic acid and 5-CH3-H4folate were determined and compared with the permeability of reference compounds for low (mannitol) and high (caffeine) permeability.

RESULTS

The transport from the apical to the basolateral side of the Caco-2 cells was higher (P < 0.05) for folic acid (P(app) = 1.7*10(-6) cm/s) than for 5-CH3-H4folate (P(app) = 1.4*10(-6) cm/s) after 2 h incubation to 1 microM folic acid or 5-CH3-H4folate test solutions (pH 7). The permeability of folic acid and 5-CH3-H4folate across Caco-2 monolayers appeared to be higher (P < 0.05) than that of mannitol (P(app) = 0.5*10(-6) cm/s) but lower (P < 0.05) than that of caffeine (P(app) = 34*10(-6) cm/s). The addition of FBP to the medium led to a lower (P < 0.05) intestinal transport and cellular accumulation of folic acid and 5-CH3-H4folate.

CONCLUSIONS

Compared to the reference compounds, folic acid and 5-CH3-H4folate showed a moderate permeability across Caco-2 cells, which indicates that folate absorption from the intestinal lumen is not likely to be complete. The intestinal transport of folic acid and 5-CH3-H4folate was found to be dependent on the extent of binding to FBP at the luminal side of the cells.

Direct optical biosensor analysis of folate-binding protein in milk

An automated, rapid, sensitive, and label-free biosensor-based assay for folate-binding protein (FBP) in bovine milk utilizing surface plasmon resonance optical detection is described. The active concentration of FBP is estimated from its specific interaction with a pteroyl-l-glutamic (folic) acid derivative immobilized on the sensor surface in a direct binding assay format. Milk, colostrum, and milk powders are prepared for analysis by dilution into buffer. Analysis conditions, including ligand immobilization, flow rate, contact time, and regeneration, have been defined, and nonspecific binding considerations were evaluated. Performance parameters include a working range for FBP in buffer of 0-200 ng/mL, a method detection limit of 0.13 microg/mL in fluid milk, overall instrument response RSD(R) of 0.64%, a mean interassay RSD(R) of 7.3% for skim milk powder, and surface stability over ca. 200 samples. The technique was applied to the measurement of active FBP content of consumer milks, the heat classification of skim milk powders manufactured under a wide range of thermal processing protocols, and change during early bovine lactation.

The binding of folic acid and 5-methyltetrahydrofolate to folate-binding proteins during gastric passage differs in a dynamic in vitro gastrointestinal model

Despite its low natural folate concentration, milk is responsible for 10-15% of the daily folate intake in countries with a high dairy consumption. Milk products can be considered as a potential matrix for folate fortification, e.g., with synthetic folic acid, to enhance the daily intake of folate. In untreated milk, the natural folate, 5-methyltetrahydrofolate (5-CH3-H4folate), is bound to folate-binding proteins (FBP). In this study, the extent of binding to FBP for folic acid and 5-CH3-H4folate was investigated in a dynamic in vitro model simulating human gastric passage. Protein binding of folic acid and 5-CH3-H4folate was characterized using gel-exclusion chromatography. Before gastric passage, folic acid and 5-CH3-H4folate were bound mainly to FBP (76-79%), whereas 7% was free. Folic acid remained bound to FBP to a similar extent after gastric passage. For 5-CH3-H4folate, the FBP-bound fraction gradually decreased from 79 to 5% and the free fraction increased from 7 to 93%. Although folic acid enters the proximal part of the intestine bound to FBP, 5-CH3-H4folate appears to be present mainly as free folate in the duodenal lumen. The stability of FBP was similar in both folate/FBP mixtures, i.e., 70% of the initial FBP content was retained after gastric passage. This study indicated that FBP are partly stable during gastric passage but have different binding characteristics for folic acid and 5-CH3-H4folate in the duodenal lumen. This could result in different bioavailability from folic acid- and 5-CH3-H4folate-fortified milk products.

Bioaccessibility of Folic Acid and (6S)-5-Methyltetrahydrofolate Decreases after the Addition of Folate-Binding Protein to Yogurt as Studied in a Dynamic In Vitro Gastrointestinal Model

Milk products are only moderate sources of folate. Nevertheless, they are of interest due to their content of folate-binding proteins (FBP), which in some studies have been reported to increase folate bioavailability. The effect of FBP on folate bioavailability has been widely discussed. The aim of this study was to investigate the bioaccessibility of folic acid and (6S)-5-methyltetrahydrofolate (5-CH3-H4folate) from fortified yogurt using a dynamic in vitro gastrointestinal model (TIM). In addition, the effect of FBP on folate bioaccessibility and the stability of FBP added to yogurt during gastrointestinal passage were investigated. Folate bioaccessibility was 82% from yogurt fortified with folic acid and 5-CH3-H4folate. The addition of FBP to yogurt decreased (P < 0.05) folate bioaccessibility. The lowering effect of FBP was more pronounced in yogurt fortified with folic acid (34% folate bioaccessibility) than from yogurt fortified with 5-CH3-H4folate (57% folate bioaccessibility). After gastrointestinal passage, 17% of the FBP in yogurt fortified with 5-CH3-H4folate and 34% of the FBP in yogurt fortified with folic acid were recovered. No difference in folate bioaccessibility was found between folate-fortified yogurt and folate-fortified pasteurized milk (P = 0.10), whereas the lowering effect of FBP was (P < 0.05) greater in yogurt compared with pasteurized milk. In conclusion, based on the high bioaccessibility of folic acid and 5-CH3-H4folate, yogurt without active FBP can be considered to be an appropriate food matrix for folate fortification.

Folic acid and 5-methyltetrahydrofolate in fortified milk are bioaccessible as determined in a dynamic in vitro gastrointestinal model

Dairy products are a potential matrix for folate fortification to enhance folate consumption in the Western world. Milk folate-binding proteins (FBP) are especially interesting because they seem to be involved in folate bioavailability. In this study, folate bioaccessibility was investigated using a dynamic computer-controlled gastrointestinal model [TNO gastrointestinal model (TIM)]. We used both ultrahigh temperature (UHT)-processed milk and pasteurized milk, differing in endogenous FBP concentrations and fortified with folic acid or 5-methyltetrahydrofolate (5-CH(3)-H(4)folate). To study FBP stability during gastrointestinal passage and the effect of additional FBP on folate bioaccessibility, FBP-fortified UHT and pasteurized milk products were also tested. Folate bioaccessibility and FBP stability were measured by taking samples along the compartments of the gastrointestinal model and measuring their folate and FBP concentrations. Folate bioaccessibility from folic acid-fortified milk products without additional FBP was 58-61%. This was lower (P < 0.05) than that of the 5-CH(3)-H(4)folate-fortified milk products (71%). Addition of FBP reduced (P < 0.05) folate bioaccessibility from folic acid-fortified milk (44-51%) but not from 5-CH(3)-H(4)folate-fortified milk products (72%). The residual FBP levels in the folic acid- and 5-CH(3)-H(4)folate-fortified milk products after gastrointestinal passage were 13-16% and 0-1%, respectively, of the starting amounts subjected to TIM. In conclusion, milk seems to be a suitable carrier for folate, because both folic acid and 5-CH(3)-H(4)folate are easily released from the matrix and available for absorption. However, our results suggest that folic acid remains partly bound to FBP during passage through the small intestine, which reduces the bioaccessibility of folic acid from milk in this model.

Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers' milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

Tetrahydrofolates are greatly stabilized by binding to bovine milk folate-binding protein

The dietary supply of folates and their measurement are both affected, potentially, by the instability of some folates. Labile folates appear to be stabilized by binding to folate-binding protein (FBP); this paper reports measurements of that stabilization. The degradation rates of the very labile tetrahydrofolate (H(4)folate) and moderately labile 5-methyltetrahydrofolate (5-CH(3)H(4)folate) were measured with the compounds free or bound to either soluble or immobilized bovine milk FBP. Complexation increased stability from 2- to > 1000-fold, depending on buffer and temperature conditions. H(4)folate at 4 degrees C and pH 6.7 appeared to be quite stable for > 100 d when bound to soluble FBP but had a half-life of < 1 h when free. Stabilization of milk folates may be a role of FBP and would improve the bioavailability of milk folate to newborns and other consumers.

Isolation of complementary deoxyribonucleic acids encoding putative secreted and membrane-bound folate binding proteins from endometrium of swine

Two distinct forms of endometrial folate binding protein (FBP) cDNAs were isolated using reverse transcription-polymerase chain reaction and 3' and 5' rapid amplification of cDNA ends (RACE) procedures. On the basis of the absence or presence of an intact glycophosphatidylinositol linkage site in the C terminus of the predicted amino acid sequences, the two forms appear to encode secreted and membrane-bound forms of FBP. The cDNAs for the putative secreted and membrane forms encoded 252- and 249-amino acid proteins, respectively, that were 73% identical with each other and were 66-82% identical with other known FBPs. However, the nucleotide sequences within the 5' untranslated region and from codons 224 and 223 of the secreted and membrane forms, respectively, to the 3' ends of each RNA, were divergent. The divergence in the 3' ends of the two cDNAs was exploited to determine changes in concentrations of each mRNA in the endometrium during the estrous cycle and early pregnancy. Northern blots of endometrial total RNA probed with a putative secreted FBP specific probe indicated that mRNA concentrations do not change during early pregnancy. In contrast, blots probed with a putative membrane FBP specific probe indicated that mRNA concentrations increase dramatically from Day 15 to Day 24 of pregnancy. Finally, N-terminal amino acid sequencing of FBP purified from Day 15 pregnant uterine flushings matched the secreted form of FBP mRNA. These data are consistent with a role for putative secreted and membrane-bound forms of FBPs in the transport of folate to the developing swine conceptus during early pregnancy.

Effect of milk processing on the concentration of folate-binding protein (FBP), folate-binding capacity and retention of 5-methyltetrahydrofolate

The main objective of this study was to investigate the effects of pasteurisation, UHT processing and fermentation on the concentration of folate-binding proteins (FBP) and their folate binding capacity in comparison with the retention of the most predominant folate from, 5-CH3THF. The amount of folate-binding protein (FBP) was analysed using enzyme-linked immunosorbent assay (ELISA). Unprocessed milk and pasteurised milk were found to contain similar amounts, 211 and 168 nmol/l, of FBP, respectively. UHT-processed milk and Yoghurt naturelle, both processed at temperatures above 90 degrees C, contained only 5.2 and 0.2 nmol/l FBP, respectively. As an indication of the protein-binding capacity free and protein-bound folates were analysed after charcoal treatment using the radio-protein binding assay method (RPBA). These results indicated that all folates in unprocessed milk and pasteurised milk were protein-bound, while folates in UHT-processed milk and Yoghurt naturelle occurred freely which is supported by our findings on FBP. High-performance liquid chromatography analysis indicated that unprocessed milk, pasteurised milk, UHT-processed milk and Yoghurt naturelle contained 44.8 +/- 2.1 (n = 10), 41.1 +/- 0.9 (n = 10), 36.1 +/- 1.8 (n = 10) and 35.6 +/- 9.1 micrograms/l (n = 10) 5-methyltetrahydrofolates (5-CH3THF), respectively, after deconjugation. Corresponding values for total milk folates analysed using radio-protein binding assay were 80.4 +/- 0.9 (n = 10), 64.2 +/- 2.7 (n = 10), 48.2 +/- 1.8 (n = 10) and 54.0 +/- 8.2 micrograms/l (n = 10), respectively. Hence, both methods indicated significant (P < 0.05) losses of 5-CH3THF as a result of pasteurisation, UHT processing and fermentation, compared with unprocessed milk. In spite of apparent discrepancies in folate concentrations obtained using the two different methods, these results support the equimolar ratio of FBP and folates in unprocessed and pasteurised milk when data on 5-CH3THF, obtained using HPLC were corrected for differences in recovery. Thus, heat processing of milk not only reduced the amount of 5-CH3 THF significantly, but also changed the concentration of FBP and the folate-binding capacity of FBP, which may have implications on the bioavailability of milk folates.

High-affinity folate binding in human choroid plexus. Characterization of radioligand binding, immunoreactivity, molecular heterogeneity and hydrophobic domain of the binding protein

High-affinity [3H]folate binding in solubilized human choroid plexus homogenate displayed characteristics, e.g. apparent positive co-operativity, which are typical of specific folate binding. The highest folate-binding activity per g of protein was associated with the 27000 g membrane pellet where the membrane-marker enzyme gamma-glutamyltransferase had its main localization. Ultrogel AcA 44 chromatography revealed two major folate-binding proteins (molecular masses greater than 110 kDa and approx. 100 kDa) and one minor one (molecular mass approx. 25 kDa) and approx. 100 kDa) and one minor one (molecular mass approx. 25 kDa) in the Triton X-100-solubilized membrane pellet. After exposure of the membrane pellet to phosphatidylinositol-specific phospholipase C there was only one large 25 kDa peak of folate binding. This could suggest that the folate-binding protein is anchored to the membrane by a glycosylphosphatidylinositol moiety, which can be inserted into Triton X-100 micelles and thus can give rise to forms of large molecular size on gel filtration. This notion was supported by the identical molecular masses of the greater than 110 kDa and 25 kDa folate-binding peaks determined by SDS/PAGE and immunoblotting. The folate-binding protein in choroid plexus cross-reacted with rabbit antibodies against the 25 kDa human milk folate-binding protein, and paraffin-embedded sections of choroid plexus showed immunostaining after exposure to rabbit anti-(human milk folate-binding protein) serum (1:8000 dilution).

Complementary DNA for the folate binding protein correctly predicts anchoring to the membrane by glycosyl-phosphatidylinositol

Membrane bound and soluble forms of a high-affinity folate binding protein have been found in kidney, placenta, serum, milk, and in several cell lines. The two forms have similar binding characteristics for folates, are immunologically cross-reactive and based upon limited amino acid sequence data, are nearly identical. Based upon pulse-chase experiments, a precursor-product relationship has been suggested. The membrane form has been shown to mediate the transport of folate in cells grown in physiological concentrations of folate. A function for the soluble form has not yet been identified. We constructed a cDNA library from a human carcinoma cell line, Caco-2, which expresses the membrane form abundantly. The library was screened and a near full-length cDNA for the folate binder was isolated. Transfection of COS cells with the cDNA inserted in an expression vector resulted in marked overexpression of a membrane-associated folate binder as assessed by direct binding of radiolabeled folate and by indirect immunofluorescence. The deduced amino acid sequence is not consistent with a typical membrane spanning domain but rather with a signal for anchoring via a glycosyl-phosphatidylinositol linkage. Release of the binder with a phosphatidylinositol-specific phospholipase C strongly supports this hypothesis.

Influence of goat's-milk folate-binding protein on transport of 5-methyltetrahydrofolate in neonatal-goat small intestinal brush-border-membrane vesicles

1. The influence of goat's-milk folate-binding protein (FBP) on the uptake of 5-methyltetrahydrofolate (MTHF) by brush-border-membrane vesicles prepared from the small intestine of the 6-d-old goat was investigated using a rapid-filtration assay. 2. Uptake of MTHF by the membrane vesicles was strongly enhanced by FBP within the pH range 4.5-6.5, with an optimum at pH 5-5.5. 3. Both the initial rate of MTHF uptake and uptake of MTHF at equilibrium were markedly increased in the presence of FBP. 4. Uptake of MTHF by brush-border-membrane vesicles was maximal when the molar ratio FBP:MTHF was 1.0-2.5. 5. The relation between pH and 125I-labelled FBP binding to the membranes was similar to that for uptake of MTHF, with an optimum at pH 5. 6. In experiments in which the osmotic pressure of the incubation medium was progressively increased with cellobiose, 125I-labelled FBP was found to be taken up primarily by binding to the brush-border-membrane surface. 7. Uptake of 125I-labelled FBP was time-dependent and saturable, with a Km of 0.39 (SE 0.07) microM and Vmax of 6.73 (SE 0.92) micrograms/mg protein. 8. Experiments in which various milk proteins (cow FBP, goat FBP, alpha-lactalbumin, beta-lactoglobulin, bovine serum albumin and lactoferrin) were allowed to compete in turn with 125I-labelled FBP for uptake by brush-border-membrane vesicles indicated that high-affinity binding was probably specific to FBP, although lactoferrin reduced uptake possibly by non-specific coating of the mucosal surface. 9. It was concluded that a folate transport mechanism mediated by the FBP in milk exists at the intestinal brush border of neonatal goats. It is suggested that this may reinforce the developing endogenous transport system.

Neonatal role of milk folate-binding protein: studies on the course of digestion of goat's milk folate binder in the 6-d-old kid

Groups of kids were reared from birth to 5 d on goat's milk. On the 6th day five of the kids received by bottle a morning feed of goat's milk with [3H]folic acid added to saturate the folate-binding proteins (FBP) (Expt 1); three kids received raw goat's milk containing only the endogenous folate and hence a large surplus folate-binding capacity (FBC) (Expt 2). The contents of the stomach, duodenum, jejunum and ileum were recovered by washing out 1.5 h after feeding (Expt 1) or at 0.5, 1 and 3.5 h after feeding (Expt 2). Recovery of [3H]folic acid 1.5 h after feeding (Expt 1) in all segments was 58.4%, mainly in a soluble form, most of this being in the stomach (37.0%) and ileum (14.3%). No surplus FBC was found in any gut segment. Sephadex G-75 chromatography of the soluble fractions of the contents of the various gut segments showed that [3H]folic acid remained bound to FBP throughout the stomach and small intestine. The bound [3H]folic acid exhibited a molecular weight of 81 000 in stomach contents, similar to that in the milk feed, presumably representing an aggregated form of the FBP, whereas in the intestinal contents its molecular weight was 39 000 indicating dissociation to monomer due to dilution in the recovery process. In Expt 2, the total recovery of free FBP in all four gut segments was 67, 54 and 23% respectively at 0.5, 1 and 3.5 h after the milk feed, and the distribution of FBP along the gut at 1 h was similar to that of [3H]folic acid-labelled FBP at 1.5 h in Expt 1.(ABSTRACT TRUNCATED AT 250 WORDS)