Document Detail

Anion specificity of the jejunal folate carrier: effects of reduced folate analogues on folate uptake and efflux.
MedLine Citation:
PMID:  3172179     Owner:  NLM     Status:  MEDLINE    
We previously reported that 3H-folate uptake by rabbit jejunal brush-border membrane (BBM) vesicles was markedly stimulated by an outwardly directed OH- gradient (pHin 7.7, pHout 5.5), inhibited by anion exchange inhibitors (DIDS, SITS, furosemide), and saturable (folate Km = 0.19 microM) suggesting carrier-mediated folate/OH- exchange (or H+/folate cotransport). In the present study, the anion specificity of this transport process was examined. Under conditions of an outwardly directed OH- gradient, DIDS-sensitive folate uptake was cis inhibited (greater than 90%) by reduced folate analogues: dihydrofolate (IC50 = 0.40 microM), folinic acid (IC50 = 0.50 microM), 5-methyltetrahydrofolate (IC50 = 0.53 microM), and (+)amethopterin (IC50 = 0.93 microM). In contrast, 10 microM (-)amethopterin had only a modest effect on folate uptake (18% inhibition) suggesting stereospecificity of the folate/OH- exchanger. The nonpteridine compounds which are transported by the folate carrier in L1210 leukemic cells (phthalate, thiamine pyrophosphate, and PO4(-3] did not inhibit jejunal folate uptake. Furthermore, folate uptake was not inhibited by SO4(-2) (4 mM) or oxalate (4 mM) thereby distinguishing this carrier from the previously described intestinal SO4(-2)/OH- and oxalate/Cl- exchangers. After BBM vesicles were loaded with 3H-folate, the initial velocity of 3H-folate efflux was stimulated by unlabeled folate in the efflux medium. The transstimulation of 3H-folate efflux by unlabeled folate was furosemide (or DIDS) inhibitable and temperature sensitive. Half-maximal stimulation of furosemide-sensitive 3H-folate efflux was observed with 0.25 +/- 0.05 microM unlabeled folate, a concentration similar to the Km for folate uptake. These data suggest that folate-stimulated 3H-folate efflux is mediated by the folate/OH- exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)
C M Schron; C Washington; B L Blitzer
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  The Journal of membrane biology     Volume:  102     ISSN:  0022-2631     ISO Abbreviation:  J. Membr. Biol.     Publication Date:  1988 Jun 
Date Detail:
Created Date:  1988-11-22     Completed Date:  1988-11-22     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0211301     Medline TA:  J Membr Biol     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  175-83     Citation Subset:  IM    
Department of Internal Medicine, University of Cincinnati College of Medicine, Ohio 45267.
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MeSH Terms
Anions / metabolism*
Biological Transport, Active
Carrier Proteins / metabolism*
Folic Acid / analogs & derivatives,  metabolism*
Ion Exchange
Jejunum / metabolism*,  ultrastructure
Microvilli / metabolism
Receptors, Cell Surface*
Grant Support
Reg. No./Substance:
0/Anions; 0/Carrier Proteins; 0/Receptors, Cell Surface; 0/folate-binding protein; 59-30-3/Folic Acid

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