The Mechanism of 5-Methyltetrahydrofolate Transport by Human Erythrocytes

Abstract

A B S T R A C T The mechanism involved inThus peripheral erythrocytes incorporate 5-methyltetrahydrofolate by a saturable, temperature-dependent, substrate-specific process which is influenced by counter-transport. This mechanism is qualitatively similar to the carrier-mediated transport of folate compounds previously described in other cell types. Therefore, human erythrocytes should be useful for detailed characterization of this membrane carrier system.

Identification of high affinity folate binding proteins in human erythrocyte membranes.

“…9 (3, 5-7, 12, 15, 24-29). However, studies on folate uptake systems in several other intact mammalian cells including L-1210 cells (33)(34)(35), PHA stimulated lymphocytes (36), and human bone marrow cells (37) and human erythrocytes (10) suggest that transport was equally efficient and maximal for 5-methyltetrahydrofolate, 5-formyltetrahydrofolate and amethopterin, while that for PteGlu was poor.…”

“…Therefore, it is not known ifother unique folate transport proteins in these cells can bind and internalize various reduced folates more efficiently than PteGlu. Along these lines, we are currently investigating the basis for the apparent dichotomy between the high binding constants of erythrocyte FBPs for PteGlu and 5-methyltetrahydrofolate and the marked differences reported in the affinity of the transport system in intact human erythrocytes (10). It should nevertheless be pointed out that in our recent studies with human KB cells, membrane-associated FBPs bound PteGlu and 5-methyltetrahydrofolate with high affinity, and intracellular transport of both folates were comparable and mediated by these FBPs (8,24,38 Recently, hemoglobin has been shown to possess a folate binding site (40).…”

“…Earlier, antiserum to purified placental folate receptors (PFR) identified moieties with shared antigenic determinants in mature human erythrocytes (3). Since human erythrocytes can take up folates (9)(10)(11) and retain them intracellularly (1), these cells seemed a logical choice to investigate for the presence of PFBPs. Such studies have not been performed with these or other homogeneous plasma membranes from normal human cells.…”

Rescue of Embryonic Lethality in Reduced Folate Carrier-deficient Mice by Maternal Folic Acid Supplementation Reveals Early Neonatal Failure of Hematopoietic Organs

“…While folate deficiency in humans is associated with megaloblastic anemia (1), cardiovascular disease (4,5), and neoplasia (6), aplasia anemia was observed in RFC1-null mice, a phenomenon that is unexplained but may be related to the severity of the folate deficiency (37). It is of interest that RFC1 ϩ/Ϫ mice manifest no apparent pathological changes, although the level of RFC1 protein in RFC ϩ/Ϫ embryonic fibroblasts was decreased as compared with that of wild-type cells, and transport was reduced by ϳ30%.…”

Rationale Grundlagen und Praxis des Citrovorumfaktor(Leucovorin^®)-Schutzes nach hochdosierter Methotrexat^®-Therapie

“…Außerdem wur den Weichteilsarkome, akute lymphatische Leukämien, NichtHodgkin-Lymphome, Bronchialkarzinome, Tumoren im HNO-Bereich, zerebrale Tumoren und einige gynäkologische Tumoren mit diesem Regime behandelt [Literatur bei 25]. Für eine Resistenz gegen MTX kommen drei verschiedene Mechanismen in Frage [5]: Außer der Induktion der Aktivität der Dihydrofolatreduktase (Zielenzym der MTX-Wirkung) oder der nur einmal beschrie benen Synthese eines durch MTX nicht hemmbaren Iso enzyms der Dihydrofolatreduktase ist der Grund für die Resistenz vorwiegend eine Störung im Membrantransportsy stem der Zellen, über das sowohl das MTX wie auch die redu zierten aktivierten Folsäurederivate aufgenommen werden [6,8,19,23,29]. Am Modell von menschlichen lymphoblastoiden Zellen ist der Folat-Carrier in der Zellmembran temperatur-und energieabhängig mit niedrigen MTX-Dosen (unter 20 pM) sättigbar.…”