Ravna Aina Westrheim, Sylte Ingebrigt, Dahl Svein G
Department of Pharmacology, Institute of Medical Biology, University of Tromsø, Norway.
J Pharmacol Exp Ther. 2003 Oct;307(1):34-41. doi: 10.1124/jpet.103.054593. Epub 2003 Aug 27.
The selective serotonin reuptake inhibitors (SSRIs) and cocaine bind to the neural serotonin (5-HT) transporter (SERT) and thus inhibit presynaptic reuptake of 5-HT and elevate its concentration in the synaptic cleft. Cocaine also binds to the dopamine transporter (DAT) and to the noradrenaline transporter (NET) and inhibits presynaptic reuptake of dopamine and noradrenaline. SERT, DAT, and NET belong to the sodium/neurotransmitter symporter family, which is predicted to have a molecular structure with 12 transmembrane alpha-helices (TMHs) and intracellular amino- and carboxy terminals. We used an electron density projection map of the Escherichia coli Na+/H+ anti-porter, and site-directed mutagenesis data on DAT and SERT to construct 3-dimensional molecular models of SERT, DAT and NET. These models were used to simulate the molecular interaction mechanisms of the SSRI, S-citalopram, its less potent enantiomer, R-citalopram and of cocaine with the transporters. In the SERT model, a single amino acid (Tyr95) in TMH1 determined the transporter selectivity of S-citalopram for SERT over DAT and NET. A dipole-dipole interaction was formed between the hydroxy group of Tyr95 in SERT and the nitril group of S-citalopram, but could not be formed by S-citalopram in DAT and NET where the corresponding amino acid is a phenylalanine. The lower binding affinity of R-citalopram may be due to sterical hindrance at the binding site. The tropane ring of cocaine interacted with Tyr95 in SERT and with the corresponding phenylalanines in NET and DAT. This may explain why cocaine, but not S-citalopram, has high binding affinity to all three transporters.
选择性5-羟色胺再摄取抑制剂（SSRI）和可卡因可与神经5-羟色胺（5-HT）转运体（SERT）结合，从而抑制5-HT的突触前再摄取，并提高其在突触间隙中的浓度。可卡因还可与多巴胺转运体（DAT）和去甲肾上腺素转运体（NET）结合，并抑制多巴胺和去甲肾上腺素的突触前再摄取。SERT、DAT和NET属于钠/神经递质同向转运体家族，预计其分子结构具有12个跨膜α螺旋（TMH）以及细胞内氨基末端和羧基末端。我们利用大肠杆菌Na+/H+反向转运体的电子密度投影图，以及DAT和SERT的定点诱变数据，构建了SERT、DAT和NET的三维分子模型。这些模型用于模拟SSRI西酞普兰（S-citalopram）、其活性较低的对映体R-西酞普兰以及可卡因与转运体的分子相互作用机制。在SERT模型中，TMH1中的单个氨基酸（Tyr95）决定了S-西酞普兰对SERT的转运体选择性高于DAT和NET。SERT中Tyr95的羟基与S-西酞普兰的腈基之间形成了偶极-偶极相互作用，但在DAT和NET中，相应氨基酸为苯丙氨酸，S-西酞普兰无法形成这种相互作用。R-西酞普兰较低的结合亲和力可能是由于结合位点的空间位阻。可卡因的托烷环与SERT中的Tyr95以及NET和DAT中的相应苯丙氨酸相互作用。这或许可以解释为什么可卡因对所有三种转运体都具有高结合亲和力，而S-西酞普兰却没有。
