Stereoselectivity of the folate transporter in rabbit small intestine: Studies with amethopterin enantiomers

Stereoselectivity of the folate transporter was examined using rabbit intes tinal brush border membrane vesicles (BBMV). Methotrexate (MTX) and the ant ipode (D-amethopterin) were used as model substrates of the transporter. Fo lic acid (FA) and MTX were actively taken up into BBMV in the presence of a n H+ gradient. Initial uptake of FA and MTX was concentration-dependent wit h Km values of 1.5 and 1.6 muM for FA and MTX, respectively. FA and MTX mut ually inhibited uptake in a competitive manner, with Ki values being simila r to the corresponding Km values, demonstrating that FA and MTX share the f olate transporter. D-Amethopterin also inhibited FA uptake competitively, w ith 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 d ifferences in plasma concentration between amethopterin enantiomers followi ng oral administration in humans. Chirality 13.164-169, 2001. (C) 2001 Wile y-Liss, Inc.