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PubMed1983

Influence of folate-binding protein from bovine milk on the absorption of folate in gastrointestinal tract of rat.

The absorption of [14C]pteroylglutamic acid (PteGlu) bound to the folate-binding protein of bovine milk was investigated in rat gastrointestinal tract in vivo and in situ. When bound [14C]PteGlu was given to rat intragastrically via oral intubation, a considerable amount of PteGlu was released from folate-binding protein under the acidic conditions in stomach, and it recombined with folate-binding protein in jejunum in vivo. Compared with free PteGlu, bound PteGlu was more gradually absorbed in small intestine, but finally the total amount of bound PteGlu absorbed was almost the same as that of free PteGlu. In all experiments in situ, bound PteGlu was only slightly absorbed in jejunum, where free PteGlu was rapidly absorbed under the same conditions. On the other hand, the absorption rates of the two forms of PteGlu were almost similar to each other in ileum. These results suggest that PteGlu bound to folate-binding protein is absorbed by a manner different from that of free PteGlu in rat gastrointestinal tract.

Animals↗
PubMed1986

Characterization of multiple forms of folate-binding protein from human leukemia cells.

Folate-binding proteins were isolated from the particulate fraction (44,000 X g pellet) and the soluble fraction (44,000 X g supernate) of the homogenate of a spleen obtained from a patient who had an acute leukemic (blast) transformation of chronic myelogenous leukemia. The folate-binding activity which was obtained from the particulate fraction by solubilization with 1% Triton X-100 could be resolved into two binding proteins (Mr 310,000 and 28,000) by gel filtration through Sephadex G-200 after incubation with excess [3H]pteroylglutamic acid (PteGlu). The folate-binding protein in the solubilized particulate fraction and the soluble folate-binding protein in the 44,000 X g supernatant cytoplasm were purified by affinity chromatography. Only a 32 kDa protein was identified by SDS-polyacrylamide gel electrophoresis in the final preparation of the purified folate-binding protein from the particulate, whereas two protein bands (Mr 42,000 and 32,000) were identified by SDS-polyacrylamide gel electrophoresis in the purified preparation of the soluble folate-binding protein. Both of these species were immunologically crossreacting. Both the purified folate-binding protein from the particulate fraction and the purified soluble form had higher affinity for oxidized folate than for the reduced folate cofactors, and both proteins had very low affinity for the antifolate compound, methotrexate. The amino-acid composition of the soluble folate-binding protein was similar with regard to the content of apolar amino acids to that reported for the membrane-derived folate-binding protein purified from milk and human placenta.

Amino Acids↗
PubMed1987

Purification, properties, and immunological characterization of folate-binding proteins from human leukemia cells.

A new matrix for affinity chromatography using pteroylglutamic acid coupled to an epoxy-activated matrix via hexanediamine resulted in negligible ligand leakage and permitted the purification of soluble and membrane-associated folate-binding proteins from human leukemia cells contained in a human spleen. Two species of membrane-associated folate-binding proteins were purified from the solubilized membrane fraction of the tissue using 2 M guanidine-HCl to elute the proteins from the affinity matrix. The higher molecular weight binding protein had an Mr of approximately 310,000 and the smaller species had an Mr of approximately 28,000 by gel filtration. By SDS-polyacrylamide gel electrophoresis the smaller species of membrane-associated protein had a molecular weight of 35,500, but the molecular weight of the larger membrane-associated species could not be determined by this method because of the high concentration of residual Triton X-100 in the sample which interfered with the silver staining of the gel. Two folate-binding proteins, which by SDS-polyacrylamide gel electrophoresis had molecular weights of 34,500 and 32,000, were purified from the 44,000 X g supernatant fraction of the tissue homogenate by acid elution from the affinity matrix. Despite the different cell components from which the soluble and membrane-associated folate-binding proteins were purified, the amino acid compositions were similar, especially with respect to the apolar amino acids. All these forms of folate-binding proteins had higher affinity for oxidized than for reduced folates, and very low affinity for 5-formyltetrahydrofolate and methotrexate. Although these proteins cross-react with one antiserum raised previously to a folate-binding protein from other human leukemia cells, they do not cross-react with the folate-binding proteins purified from two other sources of human leukemia cells, from human placenta, or from the human KB cell line.

Antibodies, Neoplasm↗
PubMed1988

Identification of 10-formyltetrahydrofolate dehydrogenase-hydrolase as a major folate binding protein in liver cytosol.

10-Formyltetrahydrofolate dehydrogenase (10-formyltetrahydrofolate:NADP+ oxidoreductase, EC 1.5.1.6) purified from pig liver contained bound tetrahydropteroylhexa-gamma-glutamate, a potent product inhibitor. Dehydrogenase purified from rat liver had chromatographic properties indistinguishable from those of a previously described major cytosolic folate binding protein of unknown function (Zamierowski, M.M. and Wagner, C. (1977) J. Biol. Chem. 252, 933-938; Cook, R.J. and Wagner, C. (1982) Biochemistry 21, 4427-4434). The dehydrogenase catalyzes the oxidative deformylation of 10-formyltetrahydrofolate to carbon dioxide and tetrahydrofolate. The tight binding of product to the enzyme suggests that oxidation of one-carbon moieties is regulated by the ratio of formyltetrahydrofolate to tetrahydrofolate in liver.

Animals↗
PubMed1992

Genomic organization of the human folate receptor genes on chromosome 11q13.

There has been interest in the high affinity folate receptor (FOLR) recently because of its high expression in the majority of ovarian tumors. The FOLR genes are part of a family that includes an adult gene, a fetal gene, and one or more pseudogenes, which have been localized to chromosome 11. As a step toward understanding why the adult FOLR gene product is expressed on tumors, we have determined the organization of all the human FOLR-related genes. YAC clones were isolated using the adult FOLR probe. The organization of the locus was determined by PFGE of YAC DNA and by YAC fragmentation. Four FOLR-related genes were found within 140 kb. The adult and fetal genes are not more than 23 kb apart, with the 3' end of the adult gene facing the 5' of the fetal gene. A physical map of over 900 kb of the surrounding region was also constructed. The chromosomal assignment of the FOLR locus was refined to 11q13.3-q13.5 telomeric of the FGF3 locus using fluorescence in situ hybridization.

Adult↗
PubMed1992

Folate binding protein from kidney brush border membranes contains components characteristic of a glycoinositol phospholipid anchor.

A number of cell surface proteins have been shown to be anchored to the plasma membrane by a covalently attached glycoinositol phospholipid (GPL) in amide linkage to the C-terminus of the mature protein. We applied several criteria to establish that folate binding protein (FBP) in brush border membranes of rat kidney contains a GPL anchor. Brush border membranes were isolated and labeled with [3H]folate, and the complex of FBP and [3H]folate was shown to be released to the supernatant by incubation with purified bacterial phosphatidylinositol-specific phospholipase C (PIPLC) but not by incubation with a purified bacterial phosphatidylcholine-specific phospholipase C. The FBP-[3H]folate complex both in crude extracts and after FBP purification by ligand-directed affinity chromatography interacted with Triton X-114 micelles, and prior incubation with PIPLC prevented this detergent interaction. Individual residues characteristic of GPL anchors were found to be covalently associated with FBP following polyacrylamide gel electrophoresis in sodium dodecyl sulfate. These included glucosamine and ethanolamine, which were radiolabeled by reductive methylation and identified by chromatography on an amino acid analyzer, and inositol phosphate, which was inferred by Western blotting with an anti-CRD antisera. This antisera gave positive immunostaining only after FBP had been cleaved by PIPLC, a reliable diagnostic of a GPL anchor. The relationship between GPL-anchored FBP in biological membranes and soluble FBP in biological fluids also is discussed.

Animals↗
PubMed1991

Optically detected magnetic resonance study of the interaction of an arsenic(III) derivative of cacodylic acid with EcoRI methyl transferase.

The interaction of the enzyme Escherichia coli RI methyl transferase (methylase) with an arsenic(III) derivative of cacodylic acid has been investigated by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy in zero applied magnetic field. The reactive derivative (CH3)2AsSR is formed by the reduction of cacodylate by a thiol. The As(III) derivative binds to the enzyme by mercaptide exchange with a cysteine (Cys) residue located close to a tryptophan (Trp) site. The arsenical binding selectively induces an external heavy-atom effect, perturbing the nearby Trp residue in the enzyme. Zero-field splittings (ZFS) and total decay rate constants of the individual triplet-state sublevels of the Trp residue in the presence and absence of perturbation by As(III) have been determined. The perturbed Trp shows a large reduction in the overall decay lifetime compared with unperturbed Trp residue, exhibiting a high selectively for the Tx sublevel. This selectivity suggests that the As atom lies in the xz plane of the principal magnetic axis system of Trp, but not directly along the z (out-of-plane) axis. The accessibility of this enzyme binding site to the arsenical is decreased upon forming a ternary complex of methylase with sinefungin and a DNA oligomer, d[GCGAA(BrU)(BrU)CGC], containing two 5-bromouracil (BrU) bases in place of thymine within the hexadeoxynucleotide recognition sequence. This result indicates that the arsenical binding site in methylase which produces the Trp heavy-atom effect is protected from this ligand by ternary complex formation or the enzyme undergoes a conformation change, removing the Cys from the Trp site. This protection is also observed in fluorescence quenching experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Arsenic↗
PubMed1991

Affinity labeling of folate transport proteins with the N-hydroxysuccinimide ester of the gamma-isomer of fluorescein-methotrexate.

Fluorescein-methotrexate, a derivative in which the fluorophore is linked via a diaminopentane spacer to either the alpha- or gamma-carboxyl group of the glutamate moiety in the drug [Gapski et al. (1975) J. Med. Chem. 18, 526-528], has been synthesized by an improved procedure and separated by DEAE-Trisacryl chromatography into the alpha- and gamma-isomers (alpha-F-MTX and gamma-F-MTX). Each isomer was characterized by mass spectrometry, elemental analysis, absorbance spectrum, TLC, and reversed-phase HPLC. Identity of the isomers was established by the following enzymatic criteria: (a) gamma-F-MTX (but not the alpha-isomer) was hydrolyzed at the pteroate-glutamate bond by carboxypeptidase G2 to yield 4-amino-4-deoxy-10-methylpteroate and gamma-glutamyldiaminopentane-fluorescein; and (b) gamma-F-MTX was a much better inhibitor of human dihydrofolate reductase than the alpha-isomer (Ki values of 0.079 and 4.6 nM). alpha- and gamma-F-MTX were comparable as inhibitors (Ki values of 1.6 and 0.6 microM) of the transport system for reduced folates and MTX in L1210 cells, but the transporter in Lactobacillus casei was inhibited only by the gamma-isomer (Ki = 4.3 microM). The gamma-isomer, therefore, was selected for covalent labeling of proteins. When L. casei folate transport protein (18 kDa) was treated with gamma-F-MTX that had been activated with N-hydroxysuccinimide (NHS), the protein was readily visualized as a fluorescent band on SDS-PAGE electrophoretograms. The probe was also able to detect the transporter in membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity Labels↗
PubMed1991

Synthesis and biological evaluation of a fluorescent analogue of folic acid.

A fluorescein derivative of the lysine analogue of folic acid, N alpha-pteroyl-N epilson-(4'-fluoresceinthiocarbamoyl)-L-lysine (PLF), was synthesized as a probe for dihydrofolate reductase (DHFR) and a membrane folate binding protein (m-FBP). Excitation of PLF at 282 nm and at 497 nm gave a fluorescence emission maximum at 518 nm. Binding of PLF to human DHFR or human placental m-FBP results in approximately a 20-fold enhancement in the magnitude of the fluorescence emission, suggesting that the ligand interacts with a hydrophobic region on these proteins. Additional evidence suggests that an energy transfer may occur between the pteridine and the fluorescein moieties. PLF binds to the active site of human DHFR since methotrexate (MTX) competes stoichiometrically and the denatured enzyme in the presence of PLF did not exhibit fluorescent enhancement. The dissociation constant for the fluorescein derivative with respect to human DHFR is 115 nM as compared to 111 nM for folic acid. The Ki value for the competitive inhibition of human DHFR by the fluorescent analogue of folic acid is 2.0 microM compared to 0.48 microM for folic acid. PLF was reduced to N alpha-(7,8-dihydropteroyl)-N epilson-(4'-fluoresceinthiocarbamoyl)-L-lysine (H2PLF) and assayed by the enzymatic conversion to the tetrahydro derivative. The Km value for human DHFR for the dihydrofolate analogue is 2.0 microM. The KD value for H2PLF to human DHFR is 47 nM as compared to 44 nM for dihydrofolate. The KD values for both H2PLF and PLF indicate that the fluorescein moiety does not significantly affect folate binding in enzyme binary complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding, Competitive↗
PubMed1982

Purification and partial characterization of rat liver folate binding protein: cytosol I.

The high molecular weight folate binding protein of rat liver cytosol has been purified to apparent homogeneity. Purification was achieved by using a combination of gel filtration, O-(diethylaminoethyl)cellulose chromatography, and affinity chromatography. This folate binding protein was initially identified during purification by an in vivo labeling procedure involving intraperitoneal injection of [3H]folic acid prior to sacrifice and subsequently by its ability to bind naturally reduced [3H]folate polyglutamates in vitro. A molecular weight of 210 000 was estimated by gel chromatography. This is distinct from the trifunctional formyl-methenyl-methylene synthetase of rat liver which has a molecular weight of 225 000. Sodium dodecyl sulfate electrophoresis revealed a single band with a molecular weight of about 100 000 which suggests the native protein is composed of two identical subunits. The partially purified protein contains bound tetrahydropteroylpentaglutamate.

Animals↗
PubMed1988

Characterization of the methotrexate transport pathway in murine L1210 leukemia cells: involvement of a membrane receptor and a cytosolic protein.

A radioiodinated photoaffinity analogue of methotrexate, N alpha-(4-amino-4-deoxy-10-methyl-pteroyl)-N epsilon-(4-azidosalicylyl)-L- lysine (APA-ASA-Lys), was recently used to identify the plasma membrane derived binding protein involved in the transport of this folate antagonist into murine L1210 cells [Price, E. M., & Freisheim, J. H. (1987) Biochemistry 26, 4757-4763]. The labeled protein has an apparent molecular weight of 46K-48K when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but no such labeling occurs in a methotrexate transport-defective cell line (L1210/R81). Labeling of the total cytosolic protein from disrupted cells, followed by electrophoresis and autoradiography, showed, among other proteins, a 21K band, corresponding to dihydrofolate reductase (DHFR), in both the parent and R81 cells and a 38K band only in the parent cells. However, when whole cells were UV irradiated at various times at 37 degrees C following addition of radiolabeled APA-ASA-Lys, the 38K protein and DHFR were the only cytosolic proteins labeled in the parent cells, while the intact R81 cells showed no labeled cytosolic protein, since the photoprobe is not transported. Further, when the parent cells were treated with a pulse of radiolabeled photoprobe, followed by UV irradiation at different times at 37 degrees C, the probe appeared sequentially on the 48K membrane protein and both the 38K cytosolic protein and dihydrofolate reductase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Affinity labeling of the folate-methotrexate transporter from Leishmania donovani.

An affinity labeling technique has been developed to identify the folate-methotrexate transporter of Leishmania donovani promastigotes using "activated" derivatives of the ligands. These "activated" derivatives were synthesized by incubating folate and methotrexate with a 10-fold excess of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) for 10 min at ambient temperature in dimethyl sulfoxide. Preincubation of intact cells with nonradioactive "activated" folate or methotrexate at a concentration of 40 microM inhibited the capacity of wild-type cells to transport submicromolar concentrations of unmodified ligand. When intact wild-type (DI700) Leishmania donovani or preparations of their membranes were incubated with a 0.4 microM concentration of either "activated" [3H]folate or "activated" [3H]methotrexate, the radiolabeled ligands were covalently incorporated into a polypeptide with a molecular weight of approximately 46,000, as demonstrated by SDS-polyacrylamide gel electrophoresis. No affinity labeling of a 46,000-dalton protein was observed when equimolar concentrations of "activated" radiolabeled ligands were incubated with intact cells or membranes prepared from a methotrexate-resistant mutant clone of Leishmania donovani, MTXA5, that is genetically defective in folate-methotrexate transport capability [Kaur, K., Coons, T., Emmett, K., & Ullman, B. (1988) J. Biol. Chem. 263, 7020-7028]. However, some labeling of a 46,000-dalton protein was observed when MTXA5 cells were incubated with higher concentrations of "activated" ligands. Time course studies indicated that maximal labeling of the 46,000-dalton protein occurred within 5-10 min of incubation of intact cells with "activated" ligand.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity Labels↗
PubMed1989

Homologous membrane folate binding proteins in human placenta: cloning and sequence of a cDNA.

A preparation of folate binding protein purified from human placental membranes in the presence of a variety of protease inhibitors followed by deglycosylation with N-glycanase gave a sharp band at Mr approximately 28,000 following SDS-polyacrylamide gel electrophoresis. The deglycosylated protein bound [3H]folic acid as tightly as the native protein. Peptides obtained following digestion of the purified protein with staphylococcal V8 protease and HPLC purification were sequenced. Polyclonal antibodies against the protein preparation were affinity purified and used to screen a placental cDNA expression library. A full-length cDNA for a placental folate binding protein was thus obtained and the corresponding protein sequence deduced. This result, taken together with the peptide sequence data, indicates the expression of at least two homologous folate binding proteins in placenta, one of which appears to be identical with the folate binding protein from human milk and nasopharyngeal epidermoid carcinoma (KB) cells; the cDNA sequence obtained corresponds to the other protein. The deduced protein sequence is characterized by a putative 16-residue amino-terminal signal peptide that is cleaved, resulting in a 239-residue polypeptide. The mature protein exhibits two potential sites for N-linked glycosylation at Asn-99 and Asn-179, eight potential intramolecular disulfide bonds, and a stretch of hydrophobic residues at the carboxyl terminus that could form a transmembrane domain. The protein bears a 68% sequence homology with the KB cell folate binding protein and may represent a fetal folate transport protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1992

A high-affinity folate binding protein in proximal tubule cells of human kidney.

High-affinity 3H-folate binding in solubilized brush border membranes of human kidney cortex display characteristics such as apparent positive cooperativity typical of specific folate binding. The folate binding activity and the activity of the brush border membrane-marker enzyme, gamma-glutamyltransferase, were eightfold higher in brush border membranes compared to crude kidney homogenate. Ultrogel AcA 44 chromatography revealed a major (Mr approximately 100 kDa) and a minor (Mr approximately 25 kDa) folate binding protein in brush border membranes. The large molecular size form may represent a membrane-derived hydrophobic folate binding protein inserted in Triton X-100 micelles. This notion was supported by the identical molecular weights of the 100 kDa and 25 kDa folate binding peaks determined by sodium dodecylsulfate polyacrylamide gel electrophoresis and immunoblotting. The folate binding protein in renal brush border membranes cross reacted with rabbit antibodies against 25 kDa human milk folate binding protein, and showed immunoprecipitation in the presence of these antibodies. Paraffin embedded sections of kidney cortex showed immunostaining of cells in the convoluted proximal tubules after exposure to rabbit antiserum (1:8000 dilution). Glomeruli and distal tubules showed no immunostaining.

Carrier Proteins↗
PubMed1981

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

An improved affinity-chromatographic method for the preparation of folate-binding protein from cow's milk is described. Under dissociating conditions the protein appeared homogeneous in the ultracentrifuge, with a molecular weight of 35 000 +/- 1500, but it was heterogeneous on electrophoresis and ion-exchange chromatography and evidently consisted of several glycoproteins with similar molecular weights that all bound folic acid. Overall, the protein contained a high proportion of half-cystine (18 residues/molecule) and 10.3% of carbohydrate. At saturation it bound approx. 1 mol of folate/mol of protein at pH 7.2. Equilibrium-dialysis measurements of the binding of folic acid and 5-methyltetrahydrofolate to the purified protein gave non-linear Scatchard plots, the shapes of which depended on pH. The results were interpreted in terms of ligand binding to a polymerizing system in which the affinity of ligand for monomer was greater than its affinity for polymer. When the protein concentration was similar to that in cow's milk, dissociation constants (Kd) for folate and 5-methyltetrahydrofolate were 3 nM and 5 nM respectively at pH 7.2 and 37 degrees C, whereas Kd for the binding of folate to monomer was about 50 pM. The properties of the binding protein are discussed in relation to its possible role in folate absorption in the gut.

Amino Acids↗
PubMed1989

Folate (pteroylglutamate) uptake in human red blood cells, erythroid precursors and KB cells at high extracellular folate concentrations. Evidence against a role for specific folate-binding and transport proteins.

Membrane-associated folate (pteroylglutamate, PteGlu)-binding proteins (FBPs) play an important role as PteGlu-transport proteins in malignant and normal human cells. Since high extracellular folate (PteGlu) concentrations (EFC) profoundly influenced uptake and toxicity of the anti-PteGlu methotrexate in malignant KB cells, we studied human cells to determine additional mechanisms for PteGlu uptake when the EFC was varied. At low EFC (less than 10 nM), the predominant mechanism for folate uptake in mature erythrocytes was through binding to externally oriented FBPs which were quantitatively insignificant (4-6 orders of magnitude lower) and of no apparent physiological relevance when compared with KB cells. However, the predominant mechanism of PteGlu accumulation at high EFC [10-250 nM] in intact erythrocytes and sealed right-side-out (RSO) ghosts was not FBP-mediated and non-specific. This conclusion was based on the findings that radiolabelled PteGlu uptake: (i) continued even in the presence of a 1000-fold excess of unlabelled PteGlu and was linear and not saturable up to 250 nM; (ii) was two-fold higher at pH 4.5 than 7.5; (iii) was less than 2-fold increased at 37 degrees C compared with 4 degrees C; and (iv) was unaffected after trypsin-mediated proteolysis of greater than 75% FBPs. The [3H]PteGlu and 125I-PteGlu (histamine derivative) accumulated intracellularly through the non-specific PteGlu-uptake mechanism was unaltered biochemically and in a soluble compartment. Raising the EFC 500-fold higher than controls during erythropoiesis in vitro resulted in reversal of the expected anti-(placental folate-receptor)-antiserum-induced megaloblastic changes in orthochromatic normoblasts derived from burst-forming unit-erythroid colonies. Furthermore, at EFC greater than 0.1 microM, KB-cell accumulation of [3H]PteGlu was also predominantly through a mechanism that did not involve specific FBPs. Thus, at high EFC, a major component of PteGlu transport in human cells is not mediated through FBPs and is likely to be a passive diffusion process.

Carrier Proteins↗
PubMed1990

Affinity labelling of the folate-binding protein in pig intestine.

A specific transport system for folate and a high-affinity folate-binding protein have been identified in pig intestinal brush-border membranes. To determine if the binding protein plays a role in folic acid (PteGlu) uptake in to the cell, the inactivation of folate binding and transport by N-hydroxysuccinimide esters of folic acid (NHS-PteGlu) was compared. In addition, the number of brush-border proteins modified by the affinity reagent was assessed. Brush-border vesicles were incubated with various concentrations of NHS-PteGlu or NHS-methotrexate. Transport and binding of [3H]PteGlu by the vesicles were measured at 37 and 4 degrees C respectively by using the vacuum-filtration technique. NHS-methotrexate and NHS-PteGlu specifically inhibited PteGlu transport. Incubating the vesicles with 1 microM-NHS-PteGlu inactivated [3H]PteGlu transport by 60% and binding by 80%. Half-maximal inhibition of both transport and binding was observed at similar concentrations of the affinity reagent (0.05 and 0.07 microM-NHS-PteGlu respectively). Treating the vesicles with radiolabelled NHS-PteGlu followed by gel electrophoresis and autoradiography revealed a specifically labelled protein with an Mr of 56,000. These results indicate that the intestinal folate-binding and transport proteins are identical and that the function of the folate-binding protein is to transport folate into the cell.

Affinity Labels↗