Determination of folate by use of radioactive folate and binding proteins

Sorting of the human folate receptor in MDCK cells

The human folate receptor (hFR) is a glycosylphosphatidy-linositol (GPI) linked plasma membrane protein that mediates delivery of folates into cells. We studied the sorting of the hFR using transfection of the hFR cDNA into MDCK cells. MDCK cells are polarized epithelial cells that preferentially sort GPI-linked proteins to their apical membrane. Unlike other GPI-tailed proteins, we found that in MDCK cells, hFR is functional on both the apical and basolateral surfaces. We verified that the same hFR cDNA that transfected into CHO cells produces the hFR protein that is GPI-linked. We also measured the hFR expression on the plasma membrane of type III paroxysmal nocturnal hemoglobinuria (PNH) human erythrocytes. PNH is a disease that is characterized by the inability of cells to express membrane proteins requiring a GPI anchor. Despite this defect, and different from other GPI-tailed proteins, we found similar levels of hFR in normal and type III PNH human erythrocytes. The results suggest the hypothesis that there may be multiple mechanisms for targeting hFR to the plasma membrane.

Mitochondria are a major site for folate and thymidylate synthesis in plants

The subcellular distributions of folate and folate-synthesizing enzymes were investigated in pea leaves. It was observed that the mitochondrial folate pool (approximately 400 micron) represented approximately 50% of the total pool. Furthermore, all the enzymes involved in tetrahydrofolate polyglutamate synthesis were present in the mitochondria. In marked contrast, we failed to detect any significant activity of these enzymes in chloroplasts, cytosol, and nuclei. The presence of the tetrahydrofolate synthesis pathway in mitochondria is apparently a general feature in plants since potato tuber mitochondria also contained a high folate concentration (approximately 200 micron) and all the enzymes required for tetrahydrofolate polyglutamate synthesis. The specific activities of tetrahydrofolate-synthesizing enzymes were rather low (1.5-15 nmol h-1 mg-1 matrix protein), except for dihydrofolate reductase (180-500 nmol h-1 mg-1 matrix protein). Dihydrofolate reductase was purified to homogeneity. The enzyme had a native molecular mass of approximately 140 kDa and was constituted of two identical 62-kDa subunits. Interestingly, this mitochondrial protein appeared to be a bifunctional enzyme, also supporting thymidylate synthesis. The cell distribution of thymidylate synthase was also investigated. No significant activity was observed in cell fractions other than mitochondria, indicating that plant cell mitochondria are also a major site for thymidylate synthesis.

Influence of smoking on folate intake and blood folate concentrations in a group of elderly Spanish men

OBJECTIVE

To investigate the relationship between smoking habits and folate status in a group of elderly Spanish men.

DESIGN

We studied folate intake, serum and erythrocyte folate concentrations, hematological and biochemical data in 72 elderly males (comparing 24 smokers data with 44 nonsmokers ones).

RESULTS

Percentage deficiencies for folate intake, serum and erythrocyte levels were higher in smokers. Serum folate levels were significantly higher in nonsmokers. Smokers had higher mean corpuscular volume than did nonsmokers.

CONCLUSIONS

Smoking could be harmful to the intake and folate status. For smokers who will not give up the habit, improved dietary intake or a folate supplement is advisable.

Megaloblastic hematopoiesis in vitro. Interaction of anti-folate receptor antibodies with hematopoietic progenitor cells leads to a proliferative response independent of megaloblastic changes

We tested the hypothesis that anti-placental folate receptor (PFR) antiserum-mediated effects on hematopoietic progenitor cells in vitro of increased cell proliferation and megaloblastic morphology were independent responses. We determined that (a) purified IgG from anti-PFR antiserum reacted with purified apo- and holo-PFR and specifically immunoprecipitated a single (44-kD) iodinated moiety on cell surfaces of low density mononuclear cells (LDMNC); (b) when retained in culture during in vitro hematopoiesis, anti-PFR IgG (in contrast to PFR-neutralized anti-PFR IgG and nonimmune IgG) consistently led to increased cloning efficiency of colony forming unit-erythroid (CFU-E), burst forming unit-E (BFU-E), CFU-granulocyte macrophage (CFU-GM), and CFU-GEM megakaryocyte (CFU-GEMM), and objectively defined megaloblastic changes in orthochromatic normoblasts from CFU-E- and BFU-E-derived colonies; (c) when anti-PFR antiserum was removed after initial (less than 1 h) incubation with LDMNC, a cell proliferation response was induced, but megaloblastic changes were not evident. (d) Conversely, delay at 4 degrees C for 24 h before long-term plating with antiserum resulted in megaloblastosis without increased cell proliferation; (e) however, 500-fold molar excess extracellular folate concentrations completely abrogated the expected anti-PFR antiserum-induced megaloblastic changes, without altering cell proliferative responses. Thus, although cell proliferative and megaloblastic changes are induced after short-term and prolonged interaction of antibody with folate receptors on hematopoietic progenitors, respectively, they are independent effects.

Influence on immunoreactive folate-binding proteins of extracellular folate concentration in cultured human cells

The influence of extracellular folate concentration on cellular levels of the folate transport protein and its soluble product was studied directly in cultured human nasopharyngeal carcinoma (KB) cells. As determined by radioimmunoassay, levels of the folate transport protein and the soluble folate-binding protein were 58 +/- 17 (mean +/- SD) and 5 +/- 2 pmol/mg cell protein, respectively, in KB cells maintained in standard medium (containing 2,300 nM folic acid). These levels significantly increased to 182 +/- 34 and 26 +/- 6 pmol/mg cell protein, respectively, in KB cells serially passaged in low folate medium (containing 2-10 nM 5-methyltetrahydrofolate). Increases in folate-binding protein levels occurred more rapidly in KB cells serially passaged in very low folate medium containing less than 2 nM folate and were prevented by the addition of 100 nM 5-methyltetrahydrofolate or 0.1-1 microM 5-formyltetrahydrofolate to this medium. When KB cells which had been passaged in low folate medium were passaged back into either standard medium or low folate medium supplemented with reduced folates, the levels of both folate-binding proteins fell linearly towards the levels in KB cells continuously maintained in standard medium. The folate transport protein was identified in and underwent similar changes in human and mouse mammary tumor cells. These studies indicate that the folate transport system is probably regulated by the extracellular folate concentration through changes in intracellular metabolite levels.

Haematological and tissue abnormalities in chicks caused by acute and subclinical folate deficiency

1. Haematological, histological and ultrastructural findings in young chicks fed on a purified diet severely deficient in folate are reported. 2. Growth of the birds was greatly depressed and they had a macrocytic anaemia. Other haematological changes included abnormal nuclear formations in erythrocytes, numerous mitoses and hypersegmented granulocytes. 3. Megaloblasts were observed in bone marrow and their fine structure is described for the first time in an avian species. 4. Morphological changes occurred also in the liver. The parenchyma had damaged sinusoidal endothelium, inflammatory cells and no glycogen. Mitochondria were damaged and many were associated with unusual crystalline structures. 5. Chickens fed on a semi-purified diet of low folate content showed no growth depression or clinical signs of deficiency but had abnormal haematological values and morphological changes that resembled those seen in birds fed on the purified diet. 6. These abnormalities responded to dietary supplements of pteroylmonoglutamic acid in a dose-related manner and may be useful in diagnosing subclinical folate deficiency.

Effect of perturbation of specific folate receptors during in vitro erythropoiesis

Although antisera to specific placental folate receptors inhibits the uptake of 5-methyltetrahydrofolate into cultured malignant human cells, little is known of the functional significance of folate receptors in normal human cells. Human bone marrow cells were therefore assayed for erythropoietic burst-forming units in the presence of an antihuman placental folate receptor serum and preimmune serum to determine the role of such a receptor in erythroid differentiation. When marrow cells were assayed in the presence of anti-receptor antiserum, there was (i) a threefold increase in erythropoietic burst formation and a twofold increase in the number of cells per erythroid burst; (ii) morphological evidence for nuclear/cytoplasmic dissociation of orthochromatic normoblasts composing erythroid bursts (megaloblastic erythropoiesis); (iii) intracellular folate deficiency with a 70% reduction of intracellular folate in antiserum treated cells as compared with control cells; and (iv) complete reversal of antiserum-induced changes on preincubation of antiserum with purified human placental folate receptor. These data support the conclusion that folate receptors on marrow cells provide an important function in the cellular uptake of folates during in vitro erythropoiesis. This process of folate uptake also appears to play a pivotal role in the differentiation and proliferation of erythroid progenitor cells.

The stimulation of yeast mitochondrial protein synthesis by low molecular weight cytoplasmic factors: requirement for intact mitochondria and lack of effect by folate derivatives

Protein synthesis by GTP-supplemented yeast mitochondria is stimulated by a fraction of molecular weight less than 2,000 isolated from yeast high-speed supernatant (S-150). The low molecular weight fraction works independently of the respiratory chain as the stimulation effect is not cyanide-sensitive. Stimulation of mitochondrial protein synthesis by cytoplasmic factors is dependent upon the method of mitochondrial isolation. The low molecular weight stimulatory factor(s) are not reduced folate derivatives which supply formyl groups required for initiation of mitochondrial protein synthesis.