Uptake of 5-methyltetrahydrofolate into PC-3 human prostate cancer cells is carrier-mediated

Uptake of 5-methyltetrahydrofolate into the PC-3 human prostate cancer cells was linear for the first 60 min. There was no difference in the initial rate of uptake in cells incubated in folate-free medium for 24 or 48 hr compared to control cells grown in folate-containing medium. The initial rate of 5-methyltetrahydrofolate uptake showed little dependence on extracellular pH and it was independent of extracellular sodium ions. Transport of 5-methyltetrahydrofolate into PC-3 cells was saturable - K(m) = 0.74 micro M and V(max) = 7.78 nmol/10(9)cells/min and these kinetic constants were not different in cells incubated for 24 hr in folate-free medium (K(m) = 0.80 +/- 0.22, V(max) = 8.52 +/- 0.50; P = 0.09, N = 3). Uptake of 5-methyltetrahydrofolate was inhibited by structural analogs with the K(i) values being 0.50, 1.79, and 31.8 micro M for 5-formyltetrahydrofolate, methotrexate, and folic acid, respectively. Uptake of 5-methyltetrahydrofolate was inhibited by the energy poisons, sodium cyanide, sodium arsenate, p-chloromercuriphenylsulfonate, and sodium azide. Uptake was inhibited by increasing concentrations of sulfate and phosphate ions, suggesting that 5-methyltetrahydrofolate may be transported by an anion-exchange mechanism. These results show that 5-methyltetrahydrofolate is transported into PC-3 prostate cancer cells by a carrier-mediated process.

Transport of 5-formyltetrahydrofolate into primary cultured cerebellar granule cells

Transport of 5-formyltetrahydrofolate (5-FTHF) into primary cultured cerebellar granule cells (CGC) was studied. Uptake of 5-FTHF into CGC was saturable with K(m)=2.86 microM and V(max)=40.8 pmol/mg protein/45 min in pH 7.4 medium. Uptake of 5-FTHF in the astrocytes has a similar style in the time curve. Uptake of 5-FTHF is characterized by countertransport because adding unlabeled 5-FTHF in the medium resulted in the efflux of labeled 5-FTHF. Uptake of 5-FTHF was inhibited by the structural analogs 5-methyltetrahydrofolate, methotrexate and folic acid (K(i)=6.64, 7.69, and 19.38 microM, respectively). Uptake was significantly decreased by high concentrations of sodium azide and sodium arsenate but not by sodium cyanide. Uptake was also inhibited by p-chloromercuriphenylsulfonate and by the anions probenecid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Acute exposure of the cells to ethanol (100 mM) did not affect the uptake. It is concluded that CGC have a carrier-mediated system for the uptake of 5-FTHF and other folates.

Neither methionine nor nitrous oxide inactivation of methionine synthase affect the concentration of 5,10-methylenetetrahydrofolate in rat liver

5,10-Methylenetetrahydrofolate occupies a key position in folate-dependent one-carbon metabolism. It is involved directly in the biosynthesis of deoxythymidine, it can be converted to 10-formyltetrahydrofolate for purine synthesis and it may be reduced to 5-methyltetrahydrofolate for methylation of homocysteine to methionine. We have developed a HPLC method for measuring 5,10-methylenetetrahydrofolate in liver and we have used this method to investigate two conditions that perturb one-carbon metabolism: 1) administration of methionine and 2) administration of the anesthetic gas, nitrous oxide (N(2)O). Rats were given 1.3 mmol/kg of methionine, and folate coenzymes in liver were measured. As expected, giving methionine resulted in an apparent increase in the concentration of 10-formyl- and tetrahydrofolate and an apparent decrease in 5-methyltetrahydrofolate concentration at 30 and 60 min. After 120 min, the concentrations of these coenzymes appeared to revert to control values. There was no apparent change in the concentration of 5,10-methylenetetrahydrofolate. Exposing rats to an atmosphere containing N(2)O results in inactivation of methionine synthase and accumulation of 5-methyltetrahydrofolate at the expense of other folate coenzymes. In liver from rats breathing N(2)O, 5-methyltetrahydrofolate increased, whereas there was no change in 5- or 10-formyltetrahydrofolates (P > 0.7 and P > 0.8, respectively). Tetrahydrofolate was not detected in liver from the N(2)O group, whereas it constituted 24% of folates in the control group. The concentration of 5,10-methylenetetrahydrofolate was not significantly affected by N(2)O (P > 0.18). These results suggest that the concentration of 5,10-methylenetetrahydrofolate is tightly regulated in liver.

Transport of 5-formyltetrahydrofolate into primary cultured rat astrocytes

Transport of 5-formyltetrahydrofolate (5-FTHF) into primary cultured rat astrocytes was studied. Uptake of 5-FTHF into astrocytes was linear in the first 60 min and is saturable with K(m)=3.3 microM and V(max)=27.5 pmol/mg protein/45 min in pH 7.4 medium. Uptake of 5-FTHF displayed the characteristics of countertransport. Uptake of 5-FTHF was inhibited by the structural analogs 5-methyltetrahydrofolate, methtrexate, and folic acid (K(i)=3.8, 2.7, and 18.4 microM, respectively). Uptake was significantly decreased by sodium azide but was increased by high concentration of sodium cyanide and low concentration of sodium arsenate. Uptake was also inhibited by p-chloromercuriphenylsulfonate and by the anions probenecid and 4,4(')-diisothiocyanostilbene-2,2(')-disulfonic acid. Acute exposure of the cells to ethanol (100mM) inhibited the uptake for 90 min of the experimental duration. It is concluded that astrocytes have a system for the uptake of 5-FTHF and folate analogs which is carrier mediated, this system is sensitive to energy inhibitors and alcohol exposure.

Metabolic engineering in yeast demonstrates that S-adenosylmethionine controls flux through the methylenetetrahydrofolate reductase reaction in vivo

One-carbon flux into methionine and S-adenosylmethionine (AdoMet) is thought to be controlled at the methylenetetrahydrofolate reductase (MTHFR) step. Mammalian MTHFRs are inhibited by AdoMet in vitro, and it has been proposed that methyl group biogenesis is regulated in vivo by this feedback loop. In this work, we used metabolic engineering in the yeast Saccharomyces cerevisiae to test this hypothesis. Like mammalian MTHFRs, the yeast MTHFR encoded by the MET13 gene is NADPH-dependent and is inhibited by AdoMet in vitro. This contrasts with plant MTHFRs, which are NADH-dependent and AdoMet-insensitive. To manipulate flux through the MTHFR reaction in yeast, the chromosomal copy of MET13 was replaced by an Arabidopsis MTHFR cDNA (AtMTHFR-1) or by a chimeric sequence (Chimera-1) comprising the yeast N-terminal domain and the AtMTHFR-1 C-terminal domain. Chimera-1 used both NADH and NADPH and was insensitive to AdoMet, supporting the view that the C-terminal domain is responsible for AdoMet inhibition. Engineered yeast expressing Chimera-1 accumulated 140-fold more AdoMet and 7-fold more methionine than did the wild-type and grew normally. Yeast expressing AtMTHFR-1 accumulated 8-fold more AdoMet. This is the first in vivo evidence that the AdoMet sensitivity and pyridine nucleotide preference of MTHFR control methylneogenesis. (13)C labeling data indicated that glycine cleavage becomes a more prominent source of one-carbon units when Chimera-1 is expressed. Possibly related to this shift in one-carbon fluxes, total folate levels are doubled in yeast cells expressing Chimera-1.

Transport of methotrexate into LNCaP human prostate cancer cells

Uptake of methotrexate into the LNCaP human prostate cancer cells was linear for the first 60 min. The initial rate of methotrexate uptake was highest at extracellular pH 4.5 and decreased markedly until pH 7.0 to 8.0. Transport of methotrexate into LNCaP cells showed two components, one saturable -K(m) = 0.13 +/- 0.06 microM and V(max) = 1.20 +/- 0.16 pmol x 45 min(-1) x mg(-1) protein at low concentrations and the other apparently not saturable up to 10 microM. Uptake of methotrexate was inhibited by structural analogs with the K(i) values being 6.53, 12.4, and 85.6 microM for 5-formyltetrahydrofolate, 5-methyltetrahydrofolate, and folic acid, respectively. Uptake of methotrexate into LNCaP cells was not inhibited by the energy poisons in contrast to methotrexate uptake into PC-3 prostate cancer cells. Uptake was inhibited by increasing concentrations of sulfate and phosphate ions and by the organic anions probenecid and DIDS, suggesting that methotrexate may be transported by an anion-exchange mechanism. These results show that methotrexate is transported into LNCaP prostate cancer cells by a carrier-mediated process.

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.

Transport of methotrexate into PC-3 human prostate cancer cells

Transport of methotrexate (MTX) into human prostatic PC-3 cells was studied. Uptake of MTX vs concentration was saturable at pH 7.4 in cells grown in normal medium and in cells incubated for 24 h in folate-free medium (Km = 3.24 and 4.84 microM, respectively (P > 0.05, n = 3) and Vmax = 0.64 and 0.92 nmol x min(-1) x 10(-9) cells, respectively (P < 0.05, n = 3)). In contrast, uptake at pH 4.5 showed both a saturable component (Km = 1.03 microM, Vmax = 0.42 nmol x min(-1) x 10(-9) cells) and a nonsaturable, linear component. Uptake was inhibited by the structural analogs 5-methyltetrahydrofolate, 5-formyltetrahydrofolate, and folic acid (K(i) = 6.8, 10.9, and 89.6 microM, respectively). Uptake was inhibited by increasing concentrations of chloride ion, suggesting that MTX transport in PC-3 cells may be via an anion-exchange mechanism. Uptake was significantly decreased by high concentrations of sodium cyanide and sodium arsenate but not by sodium azide. Uptake was inhibited by the sulfhydryl inhibitor p-chloromercuriphenylsulfonate and by the anions probenecid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Uptake of MTX was independent of sodium ions in the medium. It is concluded that PC-3 human prostate cancer cells have a carrier-mediated system for the uptake of MTX and other folates.

Folate deficiency delays the onset but increases the incidence of leukemia in Friend virus-infected mice

Clinical studies have indicated that folate deficiency may enhance the development of various malignancies. In animal studies that examined the effect of folate deficiency on malignancies, conflicting results have been reported. In some studies, folate deficiency increased the development and growth of malignant tumors; in others, it decreased the development and growth of malignancies. We examined the effect of transient folate deficiency on the development of leukemia in mice infected with the anemia-inducing strain of Friend leukemia virus. Friend virus disease can be considered as a model for human acute leukemias that are preceded by a preleukemic period. These include leukemias that develop in patients who received previous chemotherapy and/or radiation therapy, as well as patients with chronic granulocytic leukemia or myelodysplasia. Folate deficiency around the time of Friend virus-infection delayed the onset but increased the incidence of leukemia. The rates of rearrangement of the Spi-1 (PU.1 ) oncogene by provirus integration and alteration of the p53 tumor-suppressor gene were the same in leukemia cell lines derived from folate-deficient mice as they were in cell lines from control mice. These results indicate that folate deficiency did not exert its enhancement of leukemogenesis through changes in either Spi-1 or p53, even though these two genes have been found to be the most frequently altered ones in Friend virus-induced leukemias. Our results suggest that folate deficiency may enhance the development of acute leukemia in patients who are at high risk for this disease.

Compartmentation of folate metabolism in rat pancreas: nitrous oxide inactivation of methionine synthase leads to accumulation of 5-methyltetrahydrofolate in cytosol

Folate-dependent one-carbon metabolism and methylation reactions have been implicated in the secretory function of the pancreas. Because vitamin B-12 deficiency perturbs folate metabolism, we determined the effects of nitrous oxide inactivation of methionine synthase on the compartmentation of folate metabolism in rat pancreas. Rats were exposed to an atmosphere of nitrous oxide and oxygen (80 and 20%, respectively) for 18 h; control rats breathed air. Folate coenzyme concentrations were determined by HPLC and Lactobacillus casei microbiological assay of the cytosolic and mitochondrial fractions of pancreas, which contained 62 and 46%, respectively, of the total folate. In pancreas of control rats, cytosolic folates were 5-methyltetrahydrofolate (31% of total folates), tetrahydrofolate (54%) and 5- and 10-formyltetrahydrofolate (6 and 8%, respectively). In the rats exposed to nitrous oxide, cytosolic 5-methyltetrahydrofolate concentrations were significantly greater (59% of total folates) and tetrahydrofolate concentrations were significantly lower (32%) than in controls; however, total cytosolic folate levels were unaffected by nitrous oxide exposure. In controls, mitochondrial folates were composed of 5-methyltetrahydrofolate (9% of total folates), tetrahydrofolate (60%) and 5- and 10-formyltetrahydrofolate (22 and 10%, respectively). Exposure to nitrous oxide led to significantly lower total mitochondrial folates (1.49 +/- 0.18 vs. 0.75 +/- 0.29 nmol/g, control vs. nitrous oxide, P < 0.05). This was due to a significantly lower concentration of tetrahydrofolate and 5-formyltetrahydrofolate, but not of 5-methyl- or 10-formyltetrahydrofolate. The activity of methionine synthase was 85% lower (P < 0.001) in pancreatic extracts of rats exposed to nitrous oxide than in controls. These results show that cytosolic folates accumulate in pancreas as the 5-methyl derivative at the expense of other reduced folates, as happens in liver. However, in contrast to results in liver, the mitochondrial folate concentration was lower in the pancreas of rats exposed to nitrous oxide, and this decline was limited to the 5-formyl- and tetrahydrofolate derivatives.

Transport of S-adenosylmethionine in isolated rat liver mitochondria

Mitochondria do not have the enzyme, methionine adenosyltransferase (ATP: L-methionine S-adenosyltransferase, EC 2.5.1.6), necessary for the biosynthesis of S-adenosylmethionine. Nevertheless, about 30% of total hepatic S-adenosylmethionine resides in the mitochondria and radiolabeled S-adenosylmethionine may be isolated from the mitochondria after administration of radiolabeled methionine. This leads to the hypothesis that a carrier-mediated system is responsible for S-adenosylmethionine transport from the cytosol into the mitochondria. We have characterized such a system in isolated rat liver mitochondria. Uptake of S-adenosylmethionine consisted of two components. One component was incorporation of the methyl group into phospholipids as shown by thin-layer chromatography. The second component represented uptake into the mitochondria since addition of excess unlabeled S-adenosylmethionine resulted in efflux of labeled substrate. This countertransport is characteristic of a carrier-mediated transport system. Uptake (corrected for incorporation into phospholipids) was saturable with an apparent Km = 8.9 microM and Vmax = 54.3 pmol x mg protein(-1) x min(-1). Uptake was not inhibited by methionine, adenosine, 5'-methylthioadenosine, carnitine, choline, betaine, quinine, or hemicholinium-3. Uptake was inhibited by sinefungin and by S-adenosylhomocysteine (Ki = 53.4 microM). Uptake of S-adenosylmethionine was not dependent on the electrical potential across the mitochondrial membrane. These results indicate that S-adenosylmethionine is taken up into mitochondria via a specific, carrier-mediated system.

Apoptosis of late-stage erythroblasts in megaloblastic anemia: association with DNA damage and macrocyte production

An in vitro model of folate-deficient erythropoiesis has been developed using proerythroblasts isolated from the spleens of Friend virus-infected mice fed an amino acid-based, folate-free diet. Control proerythroblasts were obtained from Friend virus-infected mice fed the same diet plus 2 mg folic acid/kg diet. Our previous studies showed that, after 20 to 32 hours of culture in folate-deficient medium with 4 U/mL of erythropoietin, the folate-deficient proerythroblasts underwent apoptosis, whereas control erythroblasts survived and differentiated into reticulocytes over a period of 48 hours. The addition of folic acid or thymidine to the folate-deficient medium prevented the apoptosis of the folate-deficient erythroblasts, thereby implicating decreased thymidylate synthesis as the main cause of apoptosis in the folate-deficient erythroblasts. In the study reported here, we examined intracellular folate levels, uracil misincorporation into DNA, p53 and p21 proteins, and reticulocyte formation in erythroblasts cultured in folate-deficient or control medium. In all experiments, the folate-deficient erythroblasts cultured in folate-deficient medium gave results that varied significantly from folate-deficient erythroblasts cultured in control medium or control erythroblasts cultured in either folate-deficient or control media. Folate-deficient erythroblasts cultured in folate-deficient medium had marked decreases in all coenzyme forms of folate that persisted throughout culture, increased uracil misincorporation into DNA, persistent accumulations of p53 and p21, and decreased reticulocyte production but increased size of individual reticulocytes. A model of folate-deficient erythropoiesis based on apoptosis of late stage erythroblasts is presented. This model provides explanations for the clinical findings in megaloblastic anemia.

Metabolic role of cytoplasmic isozymes of 5,10-methylenetetrahydrofolate dehydrogenase in Saccharomyces cerevisiae

Saccharomyces cerevisiae possesses two cytosolic 5,10-methylenetetrahydrofolate (CH2-THF) dehydrogenases that differ in their redox cofactor specificity: an NAD-dependent dehydrogenase encoded by the MTD1 gene and an NADP-dependent activity as part of the trifunctional C1-THF synthase encoded by the ADE3 gene. The experiments described here were designed to define the metabolic roles of the NAD- and NADP-dependent CH2-THF dehydrogenases in one-carbon interconversions and de novo purine biosynthesis. Growth studies showed that the NAD-dependent CH2-THF dehydrogenase is interchangeable with the NADP-dependent CH2-THF dehydrogenase when flow of one-carbon units is in the oxidative direction but that it does not participate significantly when flux is in the reductive direction. 13C NMR experiments with [2-13C]glycine and unlabeled formate confirmed the latter conclusion. Direct measurements of cellular folate coenzyme levels revealed substantial levels of 10-formyl-THF (CHO-THF), the one-carbon donor used in purine synthesis, in the purine-requiring ade3 deletion strain. Thus, CHO-THF is necessary but not sufficient for de novo purine synthesis in yeast. Disruption of the MTD1 gene in this strain resulted in undetectable CHO-THF, indicating that the NAD-dependent CH2-THF dehydrogenase was responsible for CHO-THF production in the ade3 deletion strain. Finally, we examined the ability of wild-type and catalytically-inactive domains of the cytoplasmic C1-THF synthase to complement the adenine auxotrophy of the ade3 deletion strain. Both the dehydrogenase/cyclohydrolase (D/C) domain and the synthetase domain could functionally replace the full-length protein, but, at least for the D/C domain, complementation was not dependent on catalytic activity. These results reveal a catalytic role for the NAD-dependent CH2-THF dehydrogenase in the oxidation of cytoplasmic one-carbon units and indicate that the cytoplasmic C1-THF synthase plays both catalytic and noncatalytic roles in de novo purine biosynthesis in yeast.

10-Formyltetrahydrofolate dehydrogenase: identification of the natural folate ligand, covalent labeling, and partial tryptic digestion

10-Formyltetrahydrofolate dehydrogenase (EC 1.5.1.6) was previously identified as a folate-binding protein in rat liver cytosol (R.J. Cook and C. Wagner, Biochemistry 21, 4427-4434, 1982) by virtue of the tetrahydrofolate polyglutamate tightly bound to the partially purified enzyme. In this current study we provide evidence to show that when liver cytosol was rapidly processed to identify the protein bound folate, large amounts of both 10-formyl- and 5-formyltetrahydrofolate were present. After overnight storage of the cytosol at 5 degrees C before processing, almost no formylfolates were present and the major protein-bound form was tetrahydrofolate. This suggests that 10-formyltetrahydrofolate polyglutamates are tightly bound to the enzyme in vivo and are converted to tetrahydrofolate forms during isolation by the hydrolase activity associated with the enzyme. Covalent binding of the stable folate analogue, 5-formyltetrahydrofolate, to the purified enzyme resulted in 2 mol bound per mole of enzyme subunit. This is consistent with earlier reports suggesting the enzyme is capable of carrying out both oxidative and hydrolytic conversion of 10-formyltetrahydrofolate to tetrahydrofolate at the same time. Partial tryptic digestion of the purified enzyme selectively inhibited dehydrogenase activity of the enzyme but did not affect the hydrolase or aldehyde dehydrogenase activities.

Apoptosis mediates and thymidine prevents erythroblast destruction in folate deficiency anemia

Deficiency of the vitamin folic acid causes pancytopenia by decreasing the production of new blood cells. Although impaired DNA synthesis and destruction of hematopoietic cells have been implicated, the mechanism by which folate deficiency decreases blood cell production is uncertain. An in vitro model of folate-deficient erythropoiesis was developed by using proerythroblasts isolated from folate-deficient mice that were infected with Friend leukemia virus. Proerythroblasts from folate-deficient mice had one-tenth the total folate as did proerythroblasts from control mice. The folate-deficient proerythroblasts underwent apoptosis, a form of programmed cell death, after 20-32 h in culture in folate-deficient medium. At the time of apoptosis the cells had differentiated into the later erythroblast stages and some had begun hemoglobin synthesis. Addition of either folic acid or thymidine, but not deoxycytidine or inosine, to the folate-deficient medium prevented the apoptosis and permitted proliferation and differentiation of the proerythroblasts into reticulocytes. The prevention of apoptosis by thymidine indicates (i) that decreased thymidylate synthesis plays a role in erythroblast apoptosis and the anemia of folate deficiency and (ii) that DNA cleavage is likely to be a primary event in the apoptosis of folate-deficient erythroblasts. Apoptosis of erythroblasts in the late stages of differentiation leads to decreased erythrocyte production and to anemia. The increased erythropoietin produced in response to the anemia increases the number of erythroid progenitor cells in the differentiation stages preceding those in which the cells undergo apoptosis. This population shift to earlier stage erythroblasts and proerythroblasts is characteristic of bone marrows of individuals with folate deficiency anemia.

Pancreatic one-carbon metabolism in early folate deficiency in rats

An amino acid-defined, folate-deficient diet was used to investigate the regulation of pancreatic glycine N-methyltransferase in vivo. This enzyme modulates the ratio of S-adenosylmethionine to S-adenosylhomocysteine and is inhibited by bound folate in vitro. Rats were fed either a folate-deficient diet, a folate-supplemented diet (pair-fed to the deficient group), or a folate supplemented diet ad libitum and measurements were made after 2, 3, and 4 wk. Folate concentrations were greatly reduced in the folate-deficient pancreas after only 2 wk and pancreatic glycine N-methyltransferase activity was elevated but the amount of immunologically measured enzyme protein was the same. The ratio of S-adenosylmethionine to S-adenosylhomocysteine was rapidly reduced in the deficient pancreas. This ratio was also reduced with age in the ad libitum control rats. The pancreas of deficient rats had more immature secretory granules and the ducts were devoid of secreted material.

5-Methyltetrahydrofolate transport in basolateral membrane vesicles from human liver

Transport of 5-methyltetrahydrofolate was studied in vesicles isolated from the basolateral membrane (BLM) of human liver. Uptake was mostly via transport into an osmotically active intravesicular space, with some membrane binding (approximately 20%). Transport was more rapid with an imposed pH gradient (pHout = 5.0, pHin = 7.5) as compared with either pHout = pHin = 5.0 or pHout = pHin = 7.5. Transport under the influence of a pH gradient showed a transient overshoot; uptake at 60 s was 4.2 times higher than at equilibrium (60 min). Transport in the presence of a pH gradient was saturable; apparent Km = 0.55 mumol/L and Vmax = 1.98 nmol.g protein-1.10 s-1. Transport was not saturable at pHout = pHin = 7.5. Transport was inhibited by the structural analogs 5-formyltetrahydrofolate, folic acid, and methotrexate, and was electroneutral in nature. These results suggest that 5-methyltetrahydrofolate transport in human BLM vesicles is via carrier-mediated cotransport with hydrogen ions.

Transport of folates and antifolates in liver

The characteristics and mechanisms of hepatic transport of folates and antifolate cancer drugs, for example, methotrexate, have been studied in perfused liver, isolated hepatocytes (in both freshly isolated cells and in primary cell culture), and membrane vesicles isolated from the basolateral membrane. Both naturally occurring folates and antifolates are taken up by the perfused liver and secreted into bile by apparently active processes, since these compounds are concentrated in liver and bile compared with the perfusate. Transport of the naturally occurring folate 5-methyltetrahydrofolate in isolated hepatocytes and basolateral membrane vesicles is via cotransport with hydrogen ions, is electroneutral, and is inhibitable by other reduced folates and by methotrexate. Transport of methotrexate is by a multispecific anion carrier, is electrogenic, and is not inhibitable by reduced folates (e.g., 5-methyl- and 5-formyltetrahydrofolate). Thus, the hepatocyte has separate systems for uptake of the naturally occurring, reduced folates and for the 4-amino-substituted antifolates.

Hepatic one-carbon metabolism in early folate deficiency in rats

Glycine N-methyltransferase (GNMT) is inhibited by 5-methyltetrahydrofolate polyglutamate in vitro. It is believed to play a regulatory role in the synthesis de novo of methyl groups. We have used the amino-acid-defined diet of Walzem and Clifford [(1988) J. Nutr. 118, 1089-1096] to determine whether folate deficiency in vivo would affect GNMT activity, as predicted by the studies in vitro. Weanling male rats were fed on the folate-deficient diet or a folate-supplemented diet pair-fed to the deficient group. A third group was fed on the folate-supplemented diet ad libitum. Development of folate deficiency rapidly resulted in decreased levels of S-adenosylmethionine (SAM) and elevation of S-adenosylhomocysteine (SAH). The ratios of SAM to SAH were 1.8, 2.7 and 1.5 in the deficient group for weeks 2, 3 and 4 of the experiment, and the values were 9.7, 7.1 and 8.9 for the pair-fed control group and 10.3, 8.8 and 8.0 for the control group ad libitum fed. The activity of GNMT was significantly higher in the deficient group than in either of the two control groups at each time period. This was not due to increased amounts of GNMT protein, but reflected an increase in specific enzyme activity. Levels of folate in both the cytosol and mitochondria were severely lowered after only 2 weeks on the diet. The distribution of folate coenzymes was also affected by the deficiency, which resulted in a marked increase in the percentage of tetrahydrofolate polyglutamates in both cytosol and mitochondria and a very large decrease in cytosolic 5-methyltetrahydrofolate. The increased GNMT activity is therefore consistent with decreased folate levels and decreased inhibition of enzyme activity.

Uptake of 5-formyltetrahydrofolate in isolated rat liver mitochondria is carrier-mediated

We have characterized the uptake of 5-formyltetrahydrofolate into isolated rat liver mitochondria. Uptake of 5-formyltetrahydrofolate, 5-methyltetrahydrofolate and folic acid was linear for the first 2 min of incubation. 5-Methyltetrahydrofolate and 5-formyltetrahydrofolate were accumulated in mitochondria to 3.7- and 4.3-fold, respectively. 5-Formyltetrahydrofolate transport showed a pH optimum at 5.5 and was saturable with an apparent Km = 2.80 mumol.L-1 and Vmax = 3.49 pmol.min-1.mg protein-1. Uptake of 5-formyltetrahydrofolate was inhibited by the structural analog 5-methyltetrahydrofolate and, to a much lesser extent, folic acid and methotrexate. Uptake was not inhibited by various substrates of known mitochondrial transporters, e.g., succinate, malate, citrate, glutamate and phosphate, nor was uptake inhibited by the sulfhydryl reagent, N-ethylmaleimide. Uptake was not stimulated by preloading of mitochondria with malate or aspartate. Uptake of 5-formyltetrahydrofolate was not affected by the proton ionophore carbonyl cyanide 4-trifluoromethoxyphenyl-hydrazone nor by collapsing the electrical gradient with valinomycin. These results indicate that 5-formyltetrahydrofolate uptake into mitochondria via a carrier-mediated system specific for the reduced, naturally occurring folates. The folates, once in the mitochondria, may be converted to the polyglutamylated forms, which will not readily diffuse out of the mitochondria.

Transport of methotrexate in basolateral membrane vesicles from rat liver

Transport of the antifolate cancer drug methotrexate was studied in vesicles isolated from the basolateral membrane of rat liver. Transport of methotrexate by basolateral membrane vesicles (BLMVs) was mostly via uptake into an osmotically active intravesicular space, with some binding (approximately 9%), as shown by initial uptake studies and by varying medium osmolarity with increasing concentrations of sucrose. Methotrexate transport was linear for the first 20 s of incubation. Transport was not affected by imposition of a Na+ gradient across the vesicular membrane. Transport of methotrexate displayed a broad pH optimum: at an intravesicular pH of 7.5, the initial rate of uptake was not significantly different at extravesicular pH values ranging from 5.5 to 7.5, but uptake was less at extravesicular pH of 5.0 or 8.0. Methotrexate transport was saturable: Km = 0.15 +/- 0.05 microM and Vmax = 11.4 +/- 1.1 pmol 10 s-1 mg-1 protein. Methotrexate uptake into BLMVs was not inhibited by 5-methyltetrahydrofolate nor by 5-formyltetrahydrofolate but was weakly inhibited by folic acid in a concentration-dependent manner. Uptake was also inhibited by anion-exchange inhibitor 4,4'-diisothio-cyanostilbene-2,2'-disulfonic acid (DIDS), and by the structurally unrelated anions ATP, ADP, Cl-, SO4(2-), and oxalate2-. Adenosine (no negative charge) had no effect on transport. When vesicles were preloaded with anions (ADP, SO4(2-), oxalate2-) such that an anion gradient existed from the intra- to the extravesicular compartment, and methotrexate uptake was measured, no stimulation of uptake was seen. Methotrexate uptake into rat liver BLMVs was electrogenic as shown by stimulation of the initial rate of uptake by a valinomycin-imposed K+ diffusion potential across the vesicular membrane. These results suggest that methotrexate is transported into the hepatocyte across the basolateral membrane by an electrogenic, multispecific anion carrier system.

Uptake of biotin by isolated rat liver mitochondria

This study examined the mechanism of biotin uptake by liver mitochondria. Mitochondria were isolated from rat liver by an established procedure and demonstrated normal respiratory control ratios. Uptake of biotin with time at incubation buffer pH 6.1 was rapid and linear and occurred with a distinct "overshoot" phenomenon that peaked at approximately 1 min of incubation. At incubation buffer pH 7.4, however, uptake of biotin with time was significantly slower and no overshoot was observed. Gradual lowering of incubation buffer pH from 7.9 to 6.1 caused a similar pattern of increase in uptake of low (0.024 microM) and high (8 and 30 microM) concentrations of biotin. At incubation buffer pH 6.1 and 7.4, uptake of biotin as a function of concentration (0.012-30 microM) was linear and occurred at rates of 3.62 and 1.90 pmol.mg protein-1.5 s-1, respectively. Addition to the incubation medium of high concentrations of unlabeled biotin, its related compounds (biocytin, desthiobiotin, biotin methyl ester, thioctic acid, and thioctic amide), or substrates of known mitochondrial transporters (succinate, pyruvate, glutamate, malate, and phosphate) failed to cause any significant inhibition in uptake of [3H]biotin by mitochondria incubated in buffer pH 6.1 and 7.4. Furthermore, preloading mitochondria with unlabeled biotin, biocytin, malate, or aspartate failed to cause any significant stimulation in biotin uptake. At incubation buffer pH 6.1, treatment of mitochondria with the protonophore FCCP caused significant inhibition in pH-dependent overshoot of biotin uptake. However, treatment of mitochondria with the potassium ionophore valinomycin caused significant stimulation in the pH-dependent overshoot of biotin uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Biotin transport in human liver basolateral membrane vesicles: a carrier-mediated, Na+ gradient-dependent process

The characteristics of biotin transport into human liver were examined using purified liver basolateral membrane vesicle (BLMV) preparations. Biotin uptake by BLMVs was mostly due to transport of the vitamin into the inside of vesicles. In the presence of an Na+ gradient (out greater than in), biotin transport with time was significantly higher than that in the presence of a K+ gradient and showed transient accumulation (overshoot). High concentrations of unlabeled biotin and related compounds caused significant cis inhibition in biotin transport in the presence of an Na+ (but not a K+) gradient. Transport of biotin as a function of concentration in the presence of an Na+ gradient included a saturable component, while it was lower and linear in the presence of a K+ gradient. Apparent Km and Vmax of the saturable Na+ gradient-dependent component were 1.22 mumol/L and 4.76 pmol.mg protein-1 x 10 s-1, respectively. Induction of a negative or positive intravascular potential using valinomycin-K diffusion methodology did not affect biotin transport into BLMVs. Also, neither the anion-exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid nor 4-acetamido-4-isothiocyanostilbene-2,2'-disulfonic acid caused significant inhibition in biotin transport. These results indicate that biotin transport into human liver occurs via a specialized, carrier-mediated transport system. This system is Na(+)-gradient dependent and transports the vitamin via an electroneutral process.

Transport of 5-methyltetrahydrofolate in basolateral membrane vesicles of rat liver

We examined 5-methyltetrahydrofolate transport across the basolateral membrane (BLM) of rat liver using isolated membrane vesicles. Uptake of 5-methyltetrahydrofolate by BLM vesicles (BLMV) was found by osmolarity and initial uptake studies to be mostly the result of transport of the substrate into an active, intravesicular space with some binding (approximately 25%) to membrane surfaces. Uptake was similar in the presence of an inwardly directed Na+ or K+ gradient, indicating that transport is not dependent on Na+. Uptake of 5-methyltetrahydrofolate was linear for the first 30 s and was more rapid when an initial pH gradient was imposed across the vesicular membrane [extravesicular pH (pHo) = 5.0, intravesicular pH (pHi) = 7.5]. Under pH gradient conditions, uptake at 3-5 min was about twofold higher than at equilibrium (60 min). The initial rate of uptake at pHo = pHi = 5.0 was more rapid than at pHo = pHi = 7.5, but in neither case was uptake above equilibrium observed. Uptake under pH gradient conditions and at pH 5.0 (no pH gradient) was saturable (apparent Michaelis constants of 0.58 and 0.36 microM, respectively; maximum uptake rates of 1.25 and 0.79 pmol.10 s-1.mg protein-1, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of biotin in basolateral membrane vesicles of rat liver

We examined biotin transport across the basolateral membrane (BLM) of rat liver using BLM vesicles (BLMV) technique. The purity and suitability for transport studies of liver BLMV were demonstrated by morphological (electron microscopy), enzymatic, and functional criteria. Orientation of liver BLMV was determined by freeze-fracture electron microscopy and by [3H]ouabain binding methodology and was found to be 65.3-69.7% in the right-side-out orientation. Uptake of biotin by liver BLMV was found by osmolarity and temperature studies to be mostly the result of transport of the substrate into an active intravesicular space with little binding to membrane surfaces. Transport of biotin was found to be Na+ gradient dependent with a distinct "over-shoot" phenomenon. Initial rate of transport of biotin as a function of concentration was found to include a saturable component in the presence of a Na+ gradient (out greater than in) but was linear and lower in the presence of a K+ gradient (no Na+). Kinetic parameters of the saturable Na+ gradient-dependent transport process were 0.39 microM and 1,807 fmol.mg protein-1.20 s-1 for the apparent Km and Vmax, respectively. In the presence, but not the absence, of a Na+ gradient (out greater than in), the addition of structural analogues to the incubation medium caused significant inhibition in the transport of 0.079 microM [3H]biotin. Induction, with the use of valinomycin and an inwardly directed K+ gradient, of a relatively positive intravesicular space caused significant inhibition in the initial rate of biotin transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Malacoplakia of the prostate sonographically mimicking carcinoma

Malacoplakia is an uncommon granulomatous inflammatory disorder that usually affects the urinary bladder and only rarely affects the prostate. Prostatic malacoplakia was detected in five patients by means of transrectal sonography and confirmed by means of ultrasound-directed transrectal biopsy. Hypoechoic peripheral zone lesions, which are suggestive of carcinoma, were found in all five patients, including two with capsular irregularity and one with a lobular prostatic contour suggesting stage C disease. The results of this study suggest that, with the increased use of sonographically directed transrectal biopsy, prostatic malacoplakia may be found to be more common than previously suspected.

Na+ and pH dependence of 5-methyltetrahydrofolic acid and methotrexate transport in freshly isolated hepatocytes

The dependence of the high-affinity transport systems for 5-methyltetrahydrofolic acid (5-CH3-H4PteGlu) and methotrexate on sodium ions and on pH was examined in freshly isolated rat hepatocytes. Previous studies indicated that transport of these folate derivatives was sodium-dependent. Experiments to determine the Km for sodium of 5-CH3-H4PteGlu transport showed no dependence on extracellular sodium. However, uptake was sodium-dependent when hepatocytes were preincubated for 30 min in sodium-free medium, a treatment which resulted in an increase in the transmembrane pH gradient (delta pH = pH out-pH in) and a decrease in the uptake of 5-CH3-H4PteGlu. Uptake of methotrexate displayed a linear dependence on extracellular sodium ions. Uptake of 5-CH3-H4PteGlu increased linearly as the transmembrane pH gradient decreased; i.e., as the medium became more acid with respect to the cytosol. Lineweaver-Burk and Scatchard plots of 5-CH3-H4PteGlu uptake indicated an apparent Km for H+ of about 24 nM, equivalent to a pH of 7.6. Hill-plots suggested a stoichiometry of 1:1 for the interaction of protons with the 5-CH3-H4PteGlu transport system. Both the Km and Vmax for 5-CH3-H4PteGlu transport were increased at pH 5.5 compared to pH 7.4, suggesting that extracellular protons increased the number of and/or the activity of the membrane carrier. In contrast, methotrexate transport was maximal at pH 7 where the transmembrane pH gradient was zero. These results suggest the possibility that 5-CH3-H4PteGlu may be cotransported along with H+ ions in hepatocytes, although they do not rule out a 'catalytic coupling' whereby protons interact with the carrier to stimulate substrate flux without concomitant H+ transport.

Effect of aging on intestinal biotin transport in the rat

The influence of aging of biotin intestinal transport was examined in Fisher 344 rats (3- 12- and 24-month old) using a brush border membrane vesicle (BBMV) technique. In all age groups examined, transport of biotin as a function of concentration was saturable in the presence of a Na+ gradient (out greater than in), but was lower and linear in the presence of a choline gradient (out greater than in.). The Vmax of the biotin transport process was found to be significantly (p less than 0.01) higher in 24-month-old rats compared to 3- and 12-month old rats. On the other hand, the apparent Km of the biotin transport system was similar in all age groups. We also measured plasma levels of biotin in the different age groups. The mean plasma levels of biotin in 24-month-old rats was found to be significantly higher than that of 3-month (p less than 0.005) and 12-month (p less than 0.025) old rats. These results demonstrate that aging is associated with an increase in biotin intestinal transport. This increase appears to be due to changes in the activity (and/or number) but not the affinity of the biotin transport system. Furthermore, an increase in plasma biotin levels was observed with aging, which might be a consequence to the increase in the vitamin's intestinal transport.

Small bowel intussusception: an unusual complication of retroperitoneal lymph node dissection

Modified bilateral retroperitoneal lymph node dissection is used widely in the staging and treatment of patients with nonseminomatous germ cell testis tumors. Complications are uncommon and include vascular injury, infertility and small bowel obstruction from fibrous adhesions. Small bowel intussusception following retroperitoneal lymph node dissection has not been reported previously. We report 2 cases of small bowel intussusception after retroperitoneal lymph node dissection, and discuss the etiology and possible preventive measures.

Aging: effect on hepatic metabolism and transport of folate in the rat

Effects of aging on hepatic folate metabolism and transport were assessed in male Fisher 344 rats. Total serum and hepatic folate levels were measured. Hepatic folates were measured by high-performance liquid chromatography and by Lactobacillus casei assay. Transport of 5-methyltetrahydrofolate (5-CH3-H4PteGlu) was measured in isolated hepatocytes. Serum folate declined with aging; however, neither the total folate level nor the distribution of hepatic folate coenzymes was affected by the aging process. The level of liver folate monoglutamates was not significantly different in any group. The initial rate of uptake of 5-CH3-H4PteGlu was significantly decreased in hepatocytes from the 24-mo-old rats, as was the ability to concentrate this folate from the medium. Aged rats maintain apparently normal levels of hepatic folates despite decreased serum levels and decreased ability to take up folates, suggesting that membrane transport of folates may not be a limiting factor in hepatic folate assimilation.

Wegener granulomatosis: prostatic involvement and bladder outlet obstruction

Renal involvement is present in about 90 percent of patients with Wegener granulomatosis and only sporadically is the remainder of the genitourinary tract affected. Prior to 1974, the mean survival rate was five months from the time of diagnosis. Since 1974, therapy with cytotoxic drugs, cyclophosphamide or azathioprine, has produced remissions in about 95 percent of patients. Herein, a case is reported that presented with bladder outlet obstructive symptoms, secondary to prostatic involvement with Wegener granulomatosis. After treatment with cyclophosphamide and prednisone, the patient's urinary obstructive symptoms promptly resolved.

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.

Effect of dietary methyl group deficiency on one-carbon metabolism in rats

Amino acid-defined diets deficient in methyl groups have been shown to result in a very high incidence of hepatocellular carcinoma. It has been suggested that this is a result of decreased levels of S-adenosylmethionine and the undermethylation of DNA. Accordingly, the enzyme glycine N-methyltransferase (GNMT, EC 2.1.1.20) may play a major role in maintaining the levels of S-adenosylmethionine in liver in response to changes in dietary methionine. The effect of methyl-deficient, amino acid-defined diets on GNMT activity and S-adenosylmethionine levels in rat liver was therefore investigated. When rats were fed a defined amino acid diet containing no choline in which homocysteine was substituted for the methionine of the control diet at an equimolar level, there was a rapid and marked decrease in growth rate in spite of the fact that the rats consumed 85% of the food eaten by control rats fed a nutritionally adequate, defined amino acid diet. The GNMT activity in livers of methyl-deficient rats decreased rapidly, but there was no difference in amount of GNMT protein as measured immunologically. In methyl-deficient rats, the levels of S-adenosylmethionine were maintained but the levels of S-adenosylhomocysteine were rapidly elevated compared to control values. These changes are consistent with the postulated role of GNMT in regulating methyl group metabolism.

Effect of dietary methyl group deficiency on folate metabolism in rats

The carcinogenic effects of methyl-deficient, amino acid-defined diets have been attributed to alterations in cellular methylation reactions. These diets contain no choline, and methionine is replaced by homocysteine. Hence, all methyl groups needed for methionine biosynthesis with subsequent formation of S-adenosylmethionine and polyamines must be formed de novo utilizing folate-dependent reduction of one-carbon units. In rats fed the methyl-deficient diet, there was a marked decrease in total liver folate levels. This decrease was apparent in the levels of the individual forms of folate: 10-HCO-H4folate, 5-HCO-H4folate, 5-CH3-H4folate and H4folate. The percent of the total folate pool made up by 5-CH3-H4folate did not change, however, until after the rats had been fed the methyl-deficient diet for 4 wk, and then an increase was seen. After the methyl-deficient rats were switched to a nutritionally adequate control diet containing methionine and choline, all values rapidly reversed. Increased use of folate for methyl group biosynthesis may be responsible for the loss of folates from the liver.

Effects of nitrous oxide inactivation of vitamin B12 and of methionine on folate coenzyme metabolism in rat liver, kidney, brain, small intestine and bone marrow

The effects of nitrous oxide inactivation of the vitamin B12-dependent enzyme, methionine synthetase (EC 2.1.1.13), and of methionine on folate coenzyme metabolism were determined in rat liver, kidney, brain, small intestine and bone marrow cells. Nitrous oxide exposure led to an increase in the proportion of 5-methyltetrahydrofolate at the expense of other reduced folates in all tissues examined. Administration of methionine at levels up to 400 mg/kg resulted in the normalization of folate coenzyme patterns in liver as a result of the increased levels of S-adenosylmethionine. In other tissues examined, methionine had no effect on the levels of S-adenosylmethionine or S-adenosylhomocysteine, or on the distribution of folate coenzymes. These results are consistent with the methyl trap hypothesis as the explanation of the relationship between vitamin B12 and folate metabolism, and provide direct evidence that the sparing effect of methionine on folate metabolism is a phenomenon restricted to the liver.

Lactobacillus casei microbiological assay of folic acid derivatives in 96-well microtiter plates

Microbiological assay is still widely used for estimating folic acid derivatives in serum and other biological samples. We describe here a modification of this procedure involving use of 96-well microtiter plates. This procedure, used with modern, computer-interfaced microtiter-plate readers and data-reduction software, greatly shortens the time and minimizes reagent costs for this assay. Under the conditions of our assay procedures, all folic acid derivatives tested gave equal growth response for Lactobacillus casei. Results for assays of rat liver extracts showed excellent agreement between the standard bioassay and the 96-well procedure.

Lactobacillus casei microbiological assay of folic acid derivatives in 96-well microtiter plates

Microbiological assay is still widely used for estimating folic acid derivatives in serum and other biological samples. We describe here a modification of this procedure involving use of 96-well microtiter plates. This procedure, used with modern, computer-interfaced microtiter-plate readers and data-reduction software, greatly shortens the time and minimizes reagent costs for this assay. Under the conditions of our assay procedures, all folic acid derivatives tested gave equal growth response for Lactobacillus casei. Results for assays of rat liver extracts showed excellent agreement between the standard bioassay and the 96-well procedure.

Primary signet-ring cell carcinoma of bladder

Patients presenting with rare primary signet-ring cell carcinoma of the bladder are often found to be in renal failure and to have distressing symptoms of bladder instability. Their clinical course most often is rapidly fatal, and radical cystectomy with urinary diversion offers the only hope of increasing longevity. We herein review the literature of this rare variant of bladder adenocarcinoma and present a case in which a unique form of urinary diversion reversed the uremic state and increased this patient's longevity.

Effect of human milk folate binding protein on folate intestinal transport

The present study examined the effect of human milk folate binding protein (FBP) on the intestinal transport of 5-methyltetrahydrofolate (5-CH3H4PteGlu). This was performed by examining the transport of radiolabeled 5-CH3H4PteGlu bound to FBP using everted sacs of rat intestine. In the jejunum at pH 6, transport of 27 nM bound 5-CH3H4PteGlu was linear with time for 30 min of incubation. Transport of 13 nM bound 5-CH3H4PteGlu was higher in the jejunum than in the ileum at both pH 6 (2.1 +/- 0.3 and 0.36 +/- 0.03 pmol/g wet wt/25 min, respectively) and pH 8 (1.9 +/- 0.3 and 0.32 +/- 0.02 pmol/g wet wt/25 min, respectively). In the jejunum, transport of 13 nM bound 5-CH3H4PteGlu at pH 6 was less than transport of an equimolar concentration of free 5-CH3H4PteGlu (2.1 +/- 0.3 and 5.1 +/- 0.5 pmol/g wet wt/25 min, respectively) but was similar at pH 8 (1.9 +/- 0.3 and 2.47 +/- 0.3 pmol/g wet wt/25 min, respectively). In the ileum transport of bound and free 5-CH3H4PteGlu was similar at pH 6 (0.36 +/- 0.03) and 0.41 +/- 0.06 pmol/g wet wt/25 min, respectively) and pH 8 (0.32 +/- 0.02 and 0.43 +/- 0.1 pmol/g wet wt/25 min, respectively). The transport process of bound 5-CH3H4PteGlu in the jejunum was energy, temperature, and Na+ dependent, but not pH dependent, and was competitively inhibited by sulfasalazine. Ninety-two percent of the transport substrate that appeared in the serosal compartment following incubation with bound 5-CH3H4PteGlu was found to be free (unbound) 5-CH3H4PteGlu. These results show that human milk FBP decreases the rate of transport of 5-CH3H4PteGlu in the jejunum and suggest that FBP-bound 5-CH3H4PteGlu may utilize the same transport system as free 5-CH3H4PteGlu. The results also suggest a role for human milk FBP in regulating the nutritional bioavailability of folate.

Effect of nitrous oxide inactivation of vitamin B12 on the levels of folate coenzymes in rat bone marrow, kidney, brain, and liver

The effects of nitrous oxide inactivation of the vitamin B12-dependent enzyme, methionine synthetase, on the distribution of folic acid derivatives in rat bone marrow cells, kidney, brain, and liver were determined. Methionine synthetase activity was decreased by about 90% in bone marrow cells, kidney, and brain and by 83% in liver. The proportion of 5-methyltetrahydrofolate (5-CH3-H4PteGlu) in N2O-exposed rats increased from 1.4- to 1.9-fold depending on the tissue examined. This increase was at the expense of a decrease in different folate derivatives in different tissues--in bone marrow cells, kidney, and liver 5-HCO-H4PteGlu, 10-HCO-H4PteGlu, and H4PteGlu decreased; in brain only H4PteGlu decreased significantly. Total endogenous folates, as measured by Lactobacillus casei after conjugase treatment, were unchanged in all tissues after nitrous oxide exposure. The results are interpreted as direct support of the methyl trap hypothesis as the explanation of the interrelationship of folate and vitamin B12 metabolism in bone marrow cells, kidney, and brain, as well as in liver.

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.

Use of glycerol-cryoprotected Lactobacillus casei for microbiological assay of folic acid

A simple procedure for preparing glycerol-cryoprotected Lactobacillus casei cultures has been developed. L. casei grown in medium supplemented with low concentrations of folic acid (0.3 micrograms/L) is diluted with an equal volume of glycerol (800 mL/L) and stored at -20 degrees C. Growth response of the glycerol-cryoprotected L. casei to low concentrations of folic acid exceeded that of cultures maintained by monthly agar stab transfer. Also, growth for the zero-folate blanks was considerably less for the cryoprotected cultures. Assay of folate in several rat tissues correlated well (r = 0.999) with the standard microbiological assay. The growth rate of the culture depends on the inoculum size, and a heavy inoculum of cryoprotected L. casei may be used to complete the assay after only an overnight incubation.

Use of glycerol-cryoprotected Lactobacillus casei for microbiological assay of folic acid

A simple procedure for preparing glycerol-cryoprotected Lactobacillus casei cultures has been developed. L. casei grown in medium supplemented with low concentrations of folic acid (0.3 micrograms/L) is diluted with an equal volume of glycerol (800 mL/L) and stored at -20 degrees C. Growth response of the glycerol-cryoprotected L. casei to low concentrations of folic acid exceeded that of cultures maintained by monthly agar stab transfer. Also, growth for the zero-folate blanks was considerably less for the cryoprotected cultures. Assay of folate in several rat tissues correlated well (r = 0.999) with the standard microbiological assay. The growth rate of the culture depends on the inoculum size, and a heavy inoculum of cryoprotected L. casei may be used to complete the assay after only an overnight incubation.

Properties of folic acid gamma-glutamyl hydrolase (conjugase) in rat bile and plasma

The properties of folic acid gamma-glutamyl hydrolase (conjugase) [EC 3.4.12.10] in rat bile and plasma were investigated. Conjugase activity in bile showed two pH optima; one at pH 4.5-5.0 and one at pH 6.7-7.5. The enzyme activity in plasma had a broad pH--profile with an optimum at pH 6.2-7.5. Conjugase activity from both bile and plasma was inhibited in the presence of the sulfhydryl reagent, p-hydroxymercuriphenylsulfonate, and stimulated in the presence of 2-mercaptoethanol. Conjugase activity in bile was inhibited by Zn2+ at pH 7.5 but not at pH 4.5 and was much more stable to heat at pH 4.5. No separation of the biliary conjugase activity measured at the two different pH values was obtained by Sephadex G-150 chromatography. Secretion of biliary conjugase was essentially constant over a 6-hour period when activity was assayed at pH 4.5 or pH 7.5. The enzyme in bile converted pteroyltriglutamate to a mixture of about 5% glutamate an 95% gamma-glutamylglutamate at either pH, whereas the enzyme in plasma produced 23% glutamate and 77% gamma-glutamylglutamate. The possible contribution of biliary conjugase to intestinal absorption of folate polyglutamates is discussed.

Effect of dietary and nitrous oxide-induced vitamin B-12 deficiency on uptake of 5-methyltetrahydrofolate by isolated rat hepatocytes

Prior exposure of rats to a mixture of nitrous oxide/oxygen (80/20) for 20 to 24 hours and feeding rats a diet deficient in vitamin B-12 and methionine for 23 weeks resulted in a 86 to 90% decrease in the activity of the vitamin B-12 dependent enzyme, 5-methyltetrahydrofolate-homocysteine methyltransferase (EC 4.2.99.10). In addition, deficient animals excreted some 15 times as much methylmalonic acid as controls indicating impaired functioning of the vitamin B-12-dependent enzyme, methylmalonyl CoA mutase (EC 5.4.99.2). Hepatocytes prepared from rats deficient in vitamin B-12 and methionine and from nitrous oxide-treated rats showed no impairment of the membrane transport of 5-methyltetrahydro-[G-3H]folic acid when compared to the respective controls. This observation was true both for the initial rate of entry and for the ability of these cells to accumulate this folate derivative. These findings support the "methyl trap" hypothesis and apparently rule out the "cellular transport" hypothesis for the folate-vitamin B-12 metabolic interrelationship.