Stereoselectivity of noradrenaline uptake into synaptic vesicles of the rat brain

To investigate the stereoselectivity of the ATP-Mg2+-dependent uptake of noradrenaline, synaptic vesicles were isolated from the rat brain by differential centrifugation and incubated with 3H-(+/-)-, 3H-(-)- or 14C-(+)-noradrenaline in the absence and in the presence of ATP-Mg2+. The Km values of the ATP-Mg2+-dependent uptake were found to be different for the two isomers (mumol/l): 3H-(+/-)-noradrenaline 14.9 +/- 2.2 x 10(-1), 3H-(-)-noradrenaline 7.7 +/- 0.5 x 10(-1), 14C-(+)-noradrenaline 17.3 +/- 3.7 x 10(-1), whereas the Vmax of the racemate was identical with those of the two isomers (pmol/mg protein/min): 3H-(+/-)-noradrenaline 5.5 +/- 0.4, 3H-(-)-noradrenaline 4.9 +/- 0.1, 14C-(+)-noradrenaline 5.1 +/- 0.4. Moreover, (+)-noradrenaline inhibited competitively the ATP-Mg2+-dependent uptake of 3H-(+/-)-noradrenaline (Ki 19.2 +/- 1.0 x 10(-1) mumol/l) and 3H-(-)-noradrenaline (Ki 17.7 +/- 1.8 x 10(-1) mumol/l), the Ki values being nearly identical with the Km of the ATP-Mg2+-dependent uptake of 14C-(+)-noradrenaline. It is concluded that the ATP Mg2+-dependent uptake of noradrenaline into synaptic vesicles of the rat brain is stereoselective and that both isomers share the same transport system.