High-pressure liquid chromatographic separation of the naturally occurring folic acid monoglutamate derivatives

Determination of folate/folic acid level in milk by microbiological assay, immuno assay and high performance liquid chromatography

A concurrent determination of folate versus folic acid in milk by microbiological assay (MA) withLactobacillus rhamnosusas the assay organism, Enzyme Linked Immuno Sorbent Assay (ELISA) by competitive binding rapid ELISA kit (RIDASCREEN®) and high-pressure-liquid chromatography (HPLC) was done for detection of the folate form and its level. MA gave total folate content asLb. rhamnosusshowed similar response to most folate isomers formed by the tri-enzyme treatment in comparison with the other two methods which specifically estimated the folic acid. In case of ELISA, specificity was apparently limited to folic acid and dihydro folic acid and thereby showed a lower response for other folate derivatives. Estimation by HPLC with UV detector was highly specific and hence only folic acid could be detected without any cross reactivity. Among the different methods HPLC was observed to be the most sensitive method for determination of folic acid and hence can efficiently determine the folic acid fortification level while MA remained highly efficient, sensitive and reproducible method for estimation of total folate indicating its potential use for dietary folate estimation.

High-performance liquid chromatographic measurement of 5,10-methylenetetrahydrofolate in liver

The folate coenzyme 5,10-methylenetetrahydrofolate is an important folate metabolite which cannot be determined directly by HPLC near neutral pH because it dissociates to formaldehyde and tetrahydrofolate. A method for the determination of 5,10-methylenetetrahydrofolate in liver is described. This method involves (1) determination of liver 5-methyltetrahydrofolate; (2) chemical reduction of liver 5,10-methylenetetrahydrofolate (stabilized at pH 10) to 5-methyltetrahydrofolate; and (3) determination of total liver 5-methyltetrahydrofolate. Subtraction of (1) from (3) gives the concentration of 5,10-methylenetetrahydrofolate in liver.

Mimosine is a cell-specific antagonist of folate metabolism

Iron deficiency and iron chelators are known to alter folate metabolism in mammals, but the underlying biochemical mechanisms have not been established. Although many studies have demonstrated that the iron chelators mimosine and deferoxamine inhibit DNA replication in mammalian cells, their mechanism of action remains controversial. The effects of mimosine on folate metabolism were investigated in human MCF-7 cells and SH-SY5Y neuroblastoma. Our findings indicate that mimosine is a folate antagonist and that its effects are cell-specific. MCF-7 cells cultured in the presence of 350 microm mimosine were growth-arrested, whereas mimosine had no effect on SH-SY5Y cell proliferation. Mimosine altered the distribution of folate cofactor forms in MCF-7 cells, indicating that mimosine targets folate metabolism. However, mimosine does not influence folate metabolism in SH-SY5Y neuroblastoma. The effect of mimosine on folate metabolism is associated with decreased cytoplasmic serine hydroxymethyltransferase (cSHMT) expression in MCF-7 cells but not in SH-SY5Y cells. MCF-7 cells exposed to mimosine for 24 h have a 95% reduction in cSHMT protein, and cSHMT promoter activity is reduced over 95%. Transcription of the cSHMT gene is also inhibited by deferoxamine in MCF-7 cells, indicating that mimosine inhibits cSHMT transcription by chelating iron. Analyses of mimosine-resistant MCF-7 cell lines demonstrate that although the effect of mimosine on cell cycle is independent of its effects on cSHMT expression, it inhibits both processes through a common regulatory mechanism.

Effects of dietary zinc deficiency on homocysteine and folate metabolism in rats

In rats, zinc deficiency has been reported to result in elevated hepatic methionine synthase activity and alterations in folate metabolism. We investigated the effect of zinc deficiency on plasma homocysteine concentrations and the distribution of hepatic folates. Weanling male rats were fed ad libitum a zinc-sufficient control diet (382.0 nmol zinc/g diet), a low-zinc diet (7.5 nmol zinc/g diet), or a control diet pair-fed to the intake of the zinc-deficient rats. After 6 weeks, the body weights of the zinc-deficient and pair-fed control groups were lower than those of controls, and plasma zinc concentrations were lowest in the zinc-deficient group. Plasma homocysteine concentrations in the zinc-deficient group (2.3 +/- 0.2 micromol/L) were significantly lower than those in the ad libitum-fed and pair-fed control groups (6.7 +/- 0.5 and 3.2 +/- 0.4 micromol/L, respectively). Hepatic methionine synthase activity in the zinc-deficient group was higher than in the other two groups. Low mean percentage of 5-methyltetrahydrofolate in total hepatic folates and low plasma folate concentration were observed in the zinc-deficient group compared with the ad libitum-fed and pair-fed control groups. The reduced plasma homocysteine and folate concentrations and reduced percentage of hepatic 5-methyltetrahydrofolate are probably secondary to the increased activity of hepatic methionine synthase in zinc deficiency.

Bioactivity of orally administered unnatural isomers, [6R]-5-formyltetrahydrofolate and [6S]-5,10-methenyltetrahydrofolate, in humans

It has been assumed that humans cannot utilize 5,6,7,8-tetrahydrofolates with the unnatural configuration at carbon 6, since these folates are enzymatically and microbiologically inactive. We hypothesized that orally administered unnatural [6R]-5-formyltetrahydrofolate or [6S]-5,10-methenyltetrahydrofolate is bioactive in humans. Subjects were given independent oral doses of these unnatural folates and of a natural [6S]-5-formyltetrahydrofolate. Plasma, before and after the dose for 4 h, and 2 h urine were collected. Areas under the curve for the change in plasma folate concentrations were measured microbiologically and urinary folates were measured using HPLC. Based on findings of plasma and urinary folates, the unnatural folates were estimated to be 14-50% active as compared to [6S]-5-formyltetrahydrofolate. The major plasma and urinary folate was [6S]-5-methyltetrahydrofolate in all experiments. In urine, a [6S]-5-formyltetrahydrofolate peak was observed only after a [6S]-5-HCO-H4folate dose and peaks of unnatural [6S]-10-formyltetrahydrofolate and 5-formyltetrahydrofolate were identified after a [6R]-5-formyltetrahydrofolate dose. A possible pathway that explains our findings is discussed. This pathway includes the oxidation of the unnatural [6S]-10-formyltetrahydrofolate to 10-formyl-7,8-dihydrofolate which can be further metabolized by 5-amino-4-imidazolecarboxamide-ribotide transformylase producing dihydrofolate. Dihydrofolate can then be metabolized to [6S]-5-methyltetrahydrofolate by well-established metabolism.

Folate and homocysteine metabolism in copper-deficient rats

To investigate the effect of copper deficiency on folate and homocysteine metabolism, we measured plasma, red-cell and hepatic folate, plasma homocysteine and vitamin B-12 concentrations, and hepatic methionine synthase activities in rats. Two groups of male Sprague-Dawley rats were fed semi-purified diets containing either 0. 1 mg (copper-deficient group) or 9.2 mg (control group) of copper per kg. After 6 weeks of dietary treatment, copper deficiency was established as evidenced by markedly decreased plasma and hepatic copper concentrations in rats fed the low-copper diet. Plasma, red-cell, hepatic folate, and plasma vitamin B-12 concentrations were similar in both groups, whereas plasma homocysteine concentrations in the copper-deficient group were significantly higher than in the control group (P<0.05). Copper deficiency resulted in a 21% reduction in hepatic methionine synthase activity as compared to the control group (P<0.01). This change most likely caused the increased hepatic 5-methyltetrahydrofolate and plasma homocysteine concentrations in the copper-deficient group. Our results indicate that hepatic methionine synthase may be a cuproenzyme, and plasma homocysteine concentrations are influenced by copper nutriture in rats. These data support the concept that copper deficiency can be a risk factor for cardiovascular disease.

Determination of folate patterns in mouse plasma, erythrocytes, and embryos by HPLC coupled with a microbiological assay

Folates are important cofactors in one-carbon metabolism. Disturbances in folate homeostasis and metabolism may be related to an increased risk of cardiovascular disease and carcinogenesis and may lead to congenital malformations, namely neural tube defects. Determination of these compounds in biological samples is often a problem due to the existence of numerous folate metabolites, their relative instability, and the low contents in serum and most tissues. As existing methods have distinct limitations, we developed a method, which facilitates the separation as well as the sensitive detection of eight folates by coupling HPLC with a microbiological assay. After a simple sample preparation, including deproteinization and enzymatic hydrolysis of folate polyglutamates, extracts were chromatographed, fractions were collected on microtiter plates, and folates were quantitated using the Lactobacillus casei assay. The raw data were processed using a computing system after reconstructing the HPLC chromatogram with the bacterial growth data. Using the described method, the eight physiologically occurring folate monoglutamates could be simultaneously determined. The detection limits were 2-20 fmol per injection. The application of the method was demonstrated with the analysis of the folate pattern in milligram or sub-milligram quantities of plasma, erythrocyte, and embryos of pregnant mice during organogenesis.

High performance liquid chromatographic determination of folic acid and its photodegradation products in the presence of riboflavin

A high performance liquid chromatographic procedure was developed to determine folic acid and its photodegradation products, p-aminobenzoic acid, pterine-6-carboxylic acid, p-aminobenzoyl-L-glutamic acid, and pteroic acid in the presence of riboflavin. The method involves reversed phase, paired-ion chromatography on mu-BondaPak C18 column using a UV detector (254 nm), and isocratic solvent system (at ambient temperature) comprising 0.017 M monobasic potassium phosphate, tetrabutyl ammonium hydroxide solution (20%, aqueous) and methanol (870:15:250, v/v). The range of quantitation for the individual compounds was found to be: p-aminobenzoic acid, 0.01 - 1.25 x 10(-5) M; pterine-6-carboxylic acid, 0.01-2.0 x 10(-5) M; p-aminobenzoyl-L-glutamic acid, 0.02-2.0 x 10(-5) M; pteroic acid, 0.02-2.5 x 10(-5) M; folic acid, 1.0-5.0 x 10(-5) M; riboflavin, 1.0-5.0 x 10(-5) M. Linear regression analysis of the data demonstrates adequate performance of the method in terms of accuracy and precision (R. S. D. 3%). The method is specific, rapid and convenient and has been applied to photodegradation studies of folic acid in the presence and absence of riboflavin.

5-Formyltetrahydrofolate regulates homocysteine remethylation in human neuroblastoma

The metabolic role of 5-formyltetrahydrofolate is not known; however, it is an inhibitor of several folate-dependent enzymes including serine hydroxymethyltransferase. Methenyltetrahydrofolate synthetase (MTHFS) is the only enzyme known to metabolize 5-formyltetrahydrofolate and catalyzes the conversion of 5-formyltetrahydrofolate to 5,10-methenyltetrahydrofolate. In order to address the function of 5-formyltetrahydrofolate in mammalian cells, intracellular 5-formyltetrahydrofolate levels were depleted in human 5Y neuroblastoma by overexpressing the human cDNA encoding MTHFS (5YMTHFS cells). When cultured with 2 mM exogenous glycine, the intracellular serine and glycine concentrations in 5YMTHFS cells are elevated approximately 3-fold relative to 5Y cells; 5YMTHFS cells do not contain measurable levels of free methionine and display a 30-40% decrease in cell proliferation rates compared with 5Y cells. Medium supplemented with pharmacological levels of exogenous folinate or methionine ameliorated the glycine induced growth inhibition. Analysis of the folate derivatives demonstrated that 5-methyltetrahydrofolate accounts for 30% of total cellular folate in 5Y cells when cultured with 5 mM exogenous glycine. 5YMTHFS cells do not contain detectable levels of 5-methyltetrahydrofolate under the same culture conditions. These results suggest that 5-formyltetrahydrofolate inhibits serine hydroxymethyltransferase activity in vivo and that serine synthesis and homocysteine remethylation compete for one-carbon units in the cytoplasm.

Identification of 5,10-methylenetetrahydrofolate in rat bile

5,10-Methylenetetrahydrofolate (5,10-CH2-H4PteGlu) was identified as a major active reduced folate in rat bile using high-performance liquid chromatography with electrochemical detection (HPLC-ED). The identification of the folate derivative was based on the similarities in the retention-time profiles, electrochemical properties, UV absorption characteristics and demethylenation profiles of the bile folate and the synthetic standard. An HPLC-ED method was developed for the simultaneous determination of reduced folates including 5,10-CH2-H4PteGlu, tetrahydrofolate (H4PteGlu), 10-formyltetrahydrofolate (10-HCO-H4PteGlu) and 5-methyltetrahydrofolate (5-CH3-H4PteGlu) in rat bile. All peaks of the reduced folates in bile were separated using this method with a total retention time of less than 15 min. The detection limit was 0.01 ng/injection for H4PteGlu, 10-HCO-H4PteGlu and 5-CH3-H4PteGlu, and 0.02 ng/injection for 5,10-CH2-H4PteGlu at a signal-to-noise ratio of 3 and an injection volume of 100 microliters. Recoveries of synthetic folates from rat bile were higher than 90%. The distribution percentages of 5,10-CH2-H4PteGlu, H4PteGlu, 10-HCO-H4PteGlu and 5-CH3-H4PteGlu in rat bile were 29.6 +/- 7.2, 17.7 +/- 3.5, 24.4 +/- 6.5 and 28.2 +/- 7.1%, respectively, and total secretion rate of the bile reduced folates was 1514 +/- 663 ng/h (mean +/- S.D., n = 9).

[Dietary folates--a timely review. Stability, physiological significance, bioavailability, analytical determination methods, effect of food handling]

Because of the unequal and, in some instances, low stability of different folate vitamers against extreme conditions the analytical determination of folate and the estimation of folate losses in food processing and preparation cause considerable difficulties. HPLC allows determination of the native folate derivative patterns. As the bioavailability of folates is influenced by a variety of factors and different methods were employed for assessing bioavailability there is a considerable inconsistency in the results of these studies. Folates labeled with radioactive or stable isotopes provide new approaches to metabolic and bioavailability studies.

Effect of nitrous oxide inactivation of vitamin B12-dependent methionine synthetase on the subcellular distribution of folate coenzymes in rat liver

The effects of nitrous oxide inactivation of the vitamin B12-dependent enzyme, methionine synthetase (EC 2.1.1.13), on the subcellular distribution of hepatic folate coenzymes was determined. In controls, cytosolic folates were 5-methyltetrahydrofolate (45%), 5- and 10-formyltetrahydrofolate (9 and 19%, respectively), and tetrahydrofolate (27%). Exposure of rats to an atmosphere containing 80% nitrous oxide for 18 h resulted in a marked shift in this distribution pattern to 5-methyltetrahydrofolate, 84%; 5- and 10-formyltetrahydrofolate, 2.1 and 9.1%, respectively; and tetrahydrofolate, 4.7%. Activity of the cytosolic enzyme, methionine synthetase, was reduced by about 84% as compared to that of air breathing controls. In controls, mitochondrial folates were 5-methyltetrahydrofolate (7.3%), 5- and 10-formyltetrahydrofolate (11.5 and 33.1%, respectively), and tetrahydrofolate (48.1%). This distribution did not change after exposure to nitrous oxide. These results show that the effects of nitrous oxide inactivation of vitamin B12 are confined to the cytosol, at least in the short term, and suggest that there is little, if any, transport of free folates between the cytosolic and mitochondrial compartments.

A rapid and specific HPLC-electrochemical method for the determination of endogenous 5-methyltetrahydrofolic acid in plasma using solid phase sample preparation with internal standardization

A rapid and specific HPLC-electrochemical method for determining endogenous 5-methyltetrahydrofolic acid (5MeTHF) in plasma is described. Quantitative solid phase extraction of 5MeTHF and internal standard, beta-hydroxyethyltheophylline, was carried out using proprietary phenyl bonded-silica columns (Bond Elut Phenyl cartridges, 1.0 mL capacity). Chromatographic separation was achieved using a mobile phase consisting of 15% (v/v) methanol in 0.05 M KH2PO4, pH 3.5 at a flow rate of 2.0 mL/min in conjunction with a Waters Assoc. radially compressed Nova-Pak phenyl column (10 cm x 8 mm, 4 microns bonded silica). The internal standard was measured by UV detection at 254 nm. A Bioanalytical Systems Inc. LC-17 glassy carbon oxidative flow cell with a potential held at +0.35 V vs Ag/AgCl using the LC-4A amperometric controller allowed levels of 1-2 ng/mL 5MeTHF to be measured in 500 microL of plasma. Daily appraisal of the ratio produced by authentic materials clearly demonstrated that quantitation using dual detection was not subject to problems of differential response. Inter-day variation of the differential detector response is cited. Comparison of the Lactobacillus casei bioassay with HPLC demonstrates good agreement between methods but at the same time highlights the drawback of using such non-specific methods to measure samples where more than one folylmonoglutamate may be present. Antoxidant free storage for three months at -70 degrees C in darkness resulted in no deterioration of 5MeTHF. A comparison of the means and range of values for plasma folate obtained using HPLC, L. casei bioassay and the radiometric binding assay is reported.

A rapid and specific extraction procedure for folates determination in rat liver and analysis by high-performance liquid chromatography with fluorometric detection

The native fluorescence of the main biological derivatives of folic acid is maximal at acidic pH. The intensities obtained with 5-methyltetrahydrofolic acid, tetrahydrofolic acid, and formyltetrahydrofolic acid are in the ratio 10/5/1. After separating the metabolites by reversed phase high-performance liquid chromatography, the fluorometric detection limit varied between 0.4 and 20 pmol per injection. Rat liver extract (about 2 g/50 ml of 1% ascorbate solution) could be analyzed after incubation at 37 degrees C overnight, deproteinizing with acetone, and purification and concentration in an ion exchange column. The concentrations obtained varied from 1.27 to 7.96 nmol/g depending on the derivative. Recovery varied from 89 to 113%.

Resolution of the stereoisomers of leucovorin and 5-methyltetrahydrofolate by chiral high-performance liquid chromatography

Leucovorin (5-formyltetrahydrofolate, LV) is a reduced folate that has been in clinical use for many years as a rescue agent following methotrexate (MTX) therapy. Commercially available LV is a 1:1 mixture of [6R]-and [6S]-isomers. Due to the lack of a specific method for directly separating and quantitating the stereoisomers of LV, it has been difficult to precisely define the pharmacokinetic and biological characteristics of each stereoisomer. We have now developed a novel HPLC method to completely separate [6S]-LV and [6S]-5-methyltetrahydrofolate (MeTHF) from their respective [6R]-isomers using bovine serum albumin (BSA)-bonded silica as the chiral stationary phase. Baseline separation was achieved using 5 and 25 mM sodium phosphate buffers (pH 7.4) as the mobile phase with resolution factors of 1.65 for LV and 2.31 for MeTHF, respectively. The purity of each isomer prepared by this HPLC method is greater than 99%. The stereoisomers were identified by examining their ability to protect CEM cells from MTX (0.04 microM)-induced inhibition of growth. In the LV chromatogram, the first eluted peak provided complete protection from MTX growth inhibition when LV concentrations of 0.1 microM and above were used, whereas the last eluted peak failed to reverse MTX toxicity at concentrations up to 1.0 microM. Chemically pure synthetic [6R]-and [6S]-LV standards confirmed that the first eluted, biologically active peak is the [6S]-isomer. For MeTHF, only the last eluted peak effectively protects cells from MTX growth inhibition and is therefore believed to be the [6S]-isomer. This new HPLC method will serve as a useful tool to elucidate the clinical and cellular pharmacology of the stereoisomers of LV and MeTHF.

Direct determination of folate monoglutamates in plasma by high-performance liquid chromatography using an automatic precolumn-switching system as sample clean-up procedure

A simple and rapid technique for the simultaneous isolation and analysis of folate monoglutamates (folic acid, 7,8-dihydrofolic acid, 5,6,7,8-tetrahydrofolic acid and 5-formyl-, 5-methyl- and 10-formyl-5,6,7,8-tetrahydrofolic acids) was developed using reversed-phase high-performance liquid chromatography with an automatic precolumn-switching system. The plasma or the dissolved diet samples were directly injected onto a short precolumn flushed with 50 mM phosphate buffer. The folate vitamers absorbed on the precolumn were backflushed onto the analytical column with a 25 mM phosphate buffer containing 5% methanol and then detected by UV absorption at 280 nm. A linear response was found between the injected sample amounts and the integrated areas for all vitamers analysed. The detection limit was 1-10 pmol and the precision ranged from 1.6 to 10%, depending on the metabolite studied. The recovery rates of folates in plasma were 90-95%. Decomposition of the unstable folates was avoided. Our method was applied to the analysis of mouse plasma and animal diets.

High-performance liquid chromatographic determination of the distribution of naturally occurring folic acid derivatives in rat liver

Procedures which allow extraction and quantitation of labile, reduced folic acid derivatives in rat liver have been developed. These procedures entail extraction of hepatic folates at 100 degrees C in 2% (w/v) sodium ascorbate, 0.2 M 2-mercaptoethanol, pH 7.85. The extract was treated with conjugase to hydrolyze folate polyglutamates and reverse-phase, ion-pair high-performance liquid chromatography was used to separate the resulting monoglutamates which were measured by microbiological assay using Lactobacillus casei. Experiments with HPLC-purified standard derivatives, so treated, showed excellent stability of tetrahydropteroylglutamic acid (H4PteGlu), 10-formyl-H4PteGlu, 5-formyl-H4PteGlu, 5-methyl-H4PteGlu, and pteroylglutamic acid (PteGlu). Under these conditions, approximately 56% of H2PteGlu was recovered unchanged while about 27% was converted to PteGlu; 5,10-methylene-H4PteGlu was quantitatively recovered as H4PteGlu. These procedures were applied to the task of measuring the distribution of naturally occurring folate cofactors in rat liver. These results indicated that rat liver folates have the following compositions: 5-methyl-H4PteGlu, 37.2%; H4PteGlu, 32.7%; 10-formyl-H4PteGlu, 22.6%; and 5-formyl-H4PteGlu, 7.7%. Experiments with [3H]PteGlu injection showed that all hepatic folates had the same specific radioactivity as determined by radioassay and L. casei assay, indicating that L. casei exhibited the same growth response to all the folates detected in rat liver.

Evaluation of ascorbic acid in protecting labile folic acid derivatives

The use of ascorbic acid as a reducing agent to protect labile, reduced derivatives of folic acid has been evaluated by high-performance liquid chromatographic separations and Lactobacillus casei microbiological assay of eluate fractions. Upon heating for 10 min at 100 degrees C, solutions of tetrahydropteroylglutamic acid (H4PteGlu) in 2% sodium ascorbate gave rise to 5,10-methylene-H4PteGlu and 5-methyl-H4PteGlu. H2PteGlu acid gave rise to 5-methyl-H4PteGlu and PteGlu. 10-Formyl-H4PteGlu gave rise to 5-formyl-H4PteGlu and 10-formyl-PteGlu. 5-Formyl-H4-PteGlu gave rise to a small amount of 10-formyl-PteGlu. 5-Methyl-H4PteGlu and PteGlu appeared stable to these conditions. These interconversions were not seen when solutions of these folate derivatives were kept at 0 degrees C in 1% ascorbate. These observations indicate that elevated temperatures are necessary for the interconversions of folates in ascorbate solutions. Assays of ascorbic acid solutions indicated the presence of formaldehyde (approximately equal to 6 mM). This was confirmed by the identification of 3,5-diacetyl-1,4-dihydrolutidine by UV, visible, and fluorescence spectroscopy and by thin-layer chromatography of chloroform extracts of the reaction mixture of ascorbic acid solutions, acetylacetone, and ammonium acetate. These results indicate that solutions of sodium ascorbate used at elevated temperatures are not suitable for extracting tissue for the subsequent assay of the individual folic acid derivatives.

High-performance liquid chromatographic separation of physiological folate monoglutamate compounds. Investigation of absorption and conversion of pteroylglutamic acid in the small intestine of the rat in situ

A new high-performance liquid chromatographic method to investigate many folate monoglutamate compounds was developed. This method employed a Cosmosil 5-Ph column eluted with 10 mM potassium phosphate buffer (pH 7.0) containing 1% methanol at a flow-rate of 1.0 ml/min. Under these conditions, almost all of the physiological folate monoglutamate compounds were effectively separated and determined within 22 min. By applying this method to investigate the absorption and conversion of pteroylglutamic acid in the rat small intestine, it was demonstrated that pteroylglutamic acid was converted into 5-methyltetrahydrofolic acid during absorption, and that the conversion was a saturable process, whereas unchanged pteroylglutamic acid was absorbed almost linearly in proportion to the initial amount administered.

Analysis of folate cofactor levels in tissues using high-performance liquid chromatography

A method for the isolation and concentration of the monoglutamate forms of folate cofactors from tissues and for their subsequent separation and quantitation using HPLC coupled with uv detection at 284 nm is described. A chromatographic procedure utilizing Dowex 50 has been developed for the separation of the folate monoglutamates from a large portion of the nonfolate-related material following digestion of the polyglutamated forms with a highly purified preparation of rat liver conjugase. This chromatographic procedure combined with concentration of the Dowex eluate by lyophilization eliminates uv-absorbing material, which interferes with the detection and quantitation of the folate cofactors and makes possible uv measurement of the individual folates. Reverse-phase paired-ion chromatography on mu Bondapak C18 coupled with uv detection allows direct quantitation of the folates in the nanogram range.

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.