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.