Specific interactions between the human serotonin transporter and serotonin analogs at the solution/air interface

Purified serotonin transporter protein (SERT) was spread at the buffer solution/air interface. The monolayers appeared to be stable and exhibited an inflection point at πm = 15 mN m−1 and Am = 3302 A2 which has been considered as the maximum pressure below which the protein preserved its initial conformation. Specific interactions between SERT and serotonin (5-HT) or its analogs (5-HTP, 5-HTOL, 5-HIAA, indalpine) have been assessed by measuring the increase in the initial surface pressure of a SERT monolayer (πi = 7.5 mN m−1) on injection of its ligands into the equeous subphase. The strongest interaction was that observed with indalpine; this was attributed to the presence of an easily accessible amine function in position 3 of the indole ring of this molecule. Since no significant interaction between SERT and serotonin was observed, it has been inferred that the SERT conformation at the solution/air interface did not allow this interaction to occur due to the inaccessibility of the corresponding specific site.