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.
Itoh
T
T
Department of Pharmaceutics, School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan. itoht@pharm.kitasato-u.ac.jp
Ono
K
K
Koido
K I
KI
Li
Y H
YH
Yamada
H
H
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