Effect of metal chelating agents on the storage of norepinephrine in vitro by cerebral synaptic vesicles

The inhibitory effects of fourteen different metal chelating agents on the storage in vitro of ³H]dl-norepinephrine (NE) by the isolated synaptic vesicles from rat brain were investigated with a view to explore the possible importance of metal chelation phenomena. The chelators under examination belong to the classes of polyamines, aminocarboxylic and hydroxy carboxylic acids exhibiting a wide range of metal-binding strengths. Values of ic₅₀, viz., the amount of the chelator which will inhibit 50 per cent of the storage of ³H]dl-NE, were determined for the different compounds. The strengths of chelate formation of the “synaptosomal and vesicular metal ions”, viz. Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Mg²⁺ and Ca²⁺, with the above compounds were compared with their inhibition in vitro of the vesicular storage of ³H]dl-NE in order to determine if a correlation existed between the two. On the basis of regression analyses of the data on the stabilities of the individual metal chelates (log K_ML) and the inhibitory effects of the different chelators (log ic₅₀/NE), significant correlation coefficients were obtained with the Cu²⁺ and Zn²⁺ amine systems, i.e. 0.98 with P < 0.02, and 0.88 with P < 0.02 respectively. In regard to the metal-aminoacids systems, Mg²⁺ gave the most satisfactory correlation coefficient, i.e. 0.91, P < 0.01; with Fe(II) = 0.84, P < 0.02 and the mixed-metal system Cu + Zn + Fe + Mg + Ca = 0.87, P < 0.05. The existence of such correlation is considered significant from the point of view of the metal coordination hypothesis for the vesicular binding and storage of NE.