Differential stereospecificities and affinities of folate receptor isoforms for folate compounds and antifolates.
Two membrane folate receptor (MFR) isoforms are present in human tissues i.e. MFR-1 (e.g. placenta) and MFR-2 (e.g. placenta, KB cells, CaCo-2 cells). MFR-1 was expressed in COS-1 cells and the resulting protein had the same polypeptide molecular weight as the native protein. The affinities of (6S) and (6R) diastereoisomers of N5-methyltetrahydrofolate, N5-formyltetrahydrofolate, and 5,10-dideazatetrahydrofolate as well as folic acid and methotrexate to MFR-1, MFR-2 and placental MFR (MFR-1 plus MFR-2) were determined in terms of the Ki values for their competitive inhibition of the binding of [3H]folic acid to these proteins. The results indicated a striking difference in the stereospecificity of MFR-1 and MFR-2 for reduced folate coenzymes; MFR-2 preferentially bound to the physiological (6S) diastereoisomers and MFR-1 bound preferentially to the unphysiological (6R) diastereoisomers, while dideazatetrahydrofolate did not show significant stereospecificity for MFR-1. Furthermore, MFR-2 displayed significantly (2- to 100-fold) greater affinities for all the compounds tested compared to MFR-1. Purified placental MFR, a natural source of MFR-1 which contains variable amounts of MFR-2, showed intermediate Ki values for the compounds tested compared with MFR-1 and MFR-2 and stereospecificities similar to MFR-1. These observations demonstrate striking differences in the ligand binding sites of MFR-1 and MFR-2 which could potentially be exploited in the design of MFR isoform specific antifolates.
Wang
X
X
Department of Biochemistry and Molecular Biology, Medical College of Ohio, Toledo 43699-0008.
Shen
F
F
Freisheim
J H
JH
Gentry
L E
LE
Ratnam
M
M
D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D004247:DNA / Q000235:genetics; D005260:Female; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000378:metabolism*; D005493:Folic Acid Antagonists / Q000378:metabolism*; D000882:Haplorhini; D006801:Humans; D007624:KB Cells; D007700:Kinetics; D002955:Leucovorin / Q000378:metabolism; D008727:Methotrexate / Q000378:metabolism; D010920:Placenta / Q000648:ultrastructure; D011956:Receptors, Cell Surface; D013237:Stereoisomerism; D013763:Tetrahydrofolates / Q000378:metabolism
D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists; D011956:Receptors, Cell Surface; D013763:Tetrahydrofolates; C045894:lometrexol; D004247:DNA; D005492:Folic Acid; D002955:Leucovorin; C005984:5-methyltetrahydrofolate; D008727:Methotrexate