Itoh T, Ono K, Koido K I, Li Y H, Yamada H
Department of Pharmaceutics, School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan.
Chirality. 2001;13(3):164-9. doi: 10.1002/1520-636X(2001)13:3<164::AID-CHIR1015>3.0.CO;2-Z.
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.
使用兔小肠刷状缘膜囊泡（BBMV）检测叶酸转运体的立体选择性。甲氨蝶呤（MTX）及其对映体（D-氨甲蝶呤）用作该转运体的模型底物。在存在H⁺梯度的情况下，叶酸（FA）和MTX被主动摄取到BBMV中。FA和MTX的初始摄取呈浓度依赖性，FA和MTX的Km值分别为1.5和1.6 microM。FA和MTX以竞争性方式相互抑制摄取，Ki值与相应的Km值相似，表明FA和MTX共享叶酸转运体。D-氨甲蝶呤也竞争性抑制FA摄取，其Ki值比MTX大约60倍，表明D-异构体（D-氨甲蝶呤）对叶酸转运体的亲和力远低于L-异构体（MTX）。本研究中观察到的立体选择性程度与先前报道的人类口服给药后氨甲蝶呤对映体之间血浆浓度的差异一致。
