title: Evidence for the existence of a carrier-mediated folate uptake mechanism in human colonic luminal membranes.
abstract: Colonic bacteria synthesize significant amounts of folate. The current studies were undertaken to examine the presence of a possible folate transporter and its characterization in apical membranes of the human colon. Apical membrane vesicles were purified from mucosal scrapings of proximal organ donor colons using a differential centrifugation and divalent cation precipitation technique. [3H]folate (PteGlu) uptake was measured by a rapid filtration technique. Our results demonstrate that [3H]PteGlu uptake into these vesicles 1) was significantly increased with decreasing pH of the incubation buffer, 2) was markedly inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid but not by acetazolamide, amiloride, bumetanide, furosemide, and diphenyl 2-carboxylic acid, 3) was sensitive to temperature and osmolarity of the incubation medium, 4) was inhibited by structural analogs methotrexate and 5-methyltetrahydrofolate, 5) exhibited trans-stimulation phenomenon, 6) was potential insensitive, and 7) exhibited saturation kinetics with an apparent Michaelis constant for PteGlu of 8.2 +/- 2.4 microM and a maximal enzyme reaction rate of 19.8 +/- 2.9 pmol.mg protein-1.10 s-1. Studies on distal colonic membranes also demonstrated the presence of a folate transporter with similar kinetic characteristics. These results demonstrate the existence of a pH-dependent, DIDS-sensitive, electroneutral carrier-mediated mechanism for folate absorption in the human colon.
authors.affiliation: Department of Medicine, University of Illinois at Chicago 60612, USA.
authors.forename: P K
authors.initials: PK
authors.lastname: Dudeja
authors.forename: S A
authors.initials: SA
authors.lastname: Torania
authors.forename: H M
authors.initials: HM
authors.lastname: Said
mesh_terms: D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D003106:Colon / Q000378:metabolism*; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007413:Intestinal Mucosa / Q000378:metabolism*; D007700:Kinetics; D008353:Mannitol / Q000494:pharmacology; D011956:Receptors, Cell Surface; D014316:Tritium
chemical_list: D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D011956:Receptors, Cell Surface; D014316:Tritium; D008353:Mannitol; D005492:Folic Acid; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid