Abstract
Binding of [ 3H]serotonin and [ 3H]dopamine to serotonin-binding proteins (SBP) from soluble extracts of bovine frontal cortex is increased by Fe 2+. This group recently attributed this effect of Fe 2+ to its ability to enhance the oxidation of [ 3H]serotonin and [ 3H]dopamine in the presence of dissolved molecular oxygen, and to the ability of the fonned oxidation products to bind covalently to cysteine residues of SBP. In this study it is shown that the binding of both ligands is potently inhibited by dopamine as well as by several catecholamine-and serotonin-related neurotoxins: adrenochrome, 5,6-dihydroxytryptamine, 5,7-dihydroxytryptamine, 6-hydroxydopamine and 6,7-dihydroxy-l,2,3,4-tetrahydroisoquinoline. In contrast, serotonin can only potently inhibit part (36%) of the [ 3H]dopamine binding, while 1,2,3,4-tetrahydroisoquinoline is only a weak competitor for both ligands. Potent inhibition by the toxins is associated with the presence of electrophilic centres at the aromatic ring, either of the products themselves (adrenochrome) or of their oxidation products (all other competitors). These findings suggest that “SBP” represent an important target for the Fe 2+-mediated binding of [ 3H]-serotonin, [ 3H]dopamine and related neurotoxins.
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