Stereoselectivity of the folate transporter in rabbit small intestine: studies with amethopterin enantiomers.
Stereoselectivity of the folate transporter was examined using rabbit intestinal brush border membrane vesicles (BBMV). Methotrexate (MTX) and the antipode (D-amethopterin) were used as model substrates of the transporter. Folic acid (FA) and MTX were actively taken up into BBMV in the presence of an H+ gradient. Initial uptake of FA and MTX was concentration-dependent with Km values of 1.5 and 1.6 microM for FA and MTX, respectively. FA and MTX mutually inhibited uptake in a competitive manner, with Ki values being similar to the corresponding Km values, demonstrating that FA and MTX share the folate transporter. D-Amethopterin also inhibited FA uptake competitively, with a Ki value approximately 60-fold greater than that of MTX, showing that the affinity of the D-isomer (D-amethopterin) to the folate transporter is much less than that of the L-isomer (MTX). The extent of stereoselectivity observed in the present study is consistent with the previously reported differences in plasma concentration between amethopterin enantiomers following oral administration in humans.
Department of Pharmaceutics, School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan. itoht@pharm.kitasato-u.ac.jp
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D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D003080:Cold Temperature; D005492:Folic Acid / Q000378:metabolism / Q000493:pharmacokinetics*; D005493:Folic Acid Antagonists / Q000493:pharmacokinetics*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007408:Intestinal Absorption; D007421:Intestine, Small / Q000378:metabolism* / Q000648:ultrastructure; D007583:Jejunum / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D008727:Methotrexate / Q000493:pharmacokinetics*; D008871:Microvilli / Q000378:metabolism; D009994:Osmolar Concentration; D011817:Rabbits; D013237:Stereoisomerism; D013379:Substrate Specificity
D002352:Carrier Proteins; D005493:Folic Acid Antagonists; D005492:Folic Acid; D008727:Methotrexate