Studies on the stability of 3H-dopamine, 3H-apomorphine and 3H-ADTN: effects of sodium ascorbate and EDTA.

The rates of decomposition of 3H-dopamine (3H-DA), 3H-apomorphine and 3H-ADTN were determined in Tris buffer at pH 7.4 and in a Tris buffer containing a neostriatal membrane preparation representative of that used in binding experiments. In both the Tris buffer alone and in the neostriatal membrane preparation, 3H-DA was the most stable, 3H-ADTN was intermediate and 3H-apomorphine was the least stable. In the Tris buffer, the extent of decomposition of all three 3H-catechols was greatly retarded by sodium ascorbate. In contrast, in the neostriatal membrane preparation pronounced inhibitory effects of ascorbate were obtained only with 3H-ADTN. Even in the presence of high concentrations of sodium ascorbate (i.e., 0.5 mM), there was an extensive decomposition of 3H-apomorphine in the neostriatal membrane preparation. The data suggest that one exercise great caution in choosing appropriate conditions for binding experiments with these unstable ligands.