Stereochemical requirements of the saturable uptake carrier for abscisic acid in carrot suspension culture cells

The saturable uptake carrier in Daucus carota (carrot) suspension culture cells was used as a test system to measure the ability of the separate enantiomers of a range of analogues to inhibit the uptake of (+)-[14C]abscisic acid (ABA). A quantitative measure of the ability of the analogues to compete with [14C]ABA (0.41 μM) for the component of absorption mediated by a saturable uptake carrier was determined. The K1 of each analogue was calculated by linear regression of a Dixon plot of 1/(saturable component of uptake of (+)-[14C]ABA) against the concentrations of the added analogue. The K1 values for several analogues revealed a tolerance by the carrier of deviations in the structure of the ABA molecule in the region C1'–C3'. This tolerance may be exploitable in the design of azido derivatives of ABA specifically to label the carrier-protein by photolytic activation. All the analogues which competed with ABA for saturable uptake sites were capable of forming an anion at C1. This is in agreement with the earlier results of Astle and Rubery (1983) who showed that the carrier imports ABA−. (+)- and (−)-ABA competed equally with (+)-[14C] ABA for saturable uptake sites and, again, equally against uptake of (−)-[14C]-ABA. However, the latter unnatural enantiomer was absorbed at a slower rate. A model to account for this was proposed in which (−)-ABA and active analogues compete equally with (+)-ABA at a ‘docking site’ on the carrier. The unnatural (−)-enantiomer was taken into the cells much less rapidly than the (+)-, consequently it is deduced that the steric requirements of a subsequent, carrier-mediated ‘import event’ are more severe than those of the ‘docking site’. The carrier-mediated component of absorption of ABA by these cells is considered, there fore, to be biphasic.