Inhibitory Effects of Ascorbic Acid on the Binding of [3H]Dopamine Antagonists to Neostriatal Membrane Preparations : Relationship to Lipid Peroxidation

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Abstract
Ascorbic acid, sodium ascorbate, and isoascorbic acid (the stereo-isomer of ascorbic acid) inhibited the stereospecific binding of [3H]spiroperidol to neostriatal membrane preparations. Greater inhibitory effects were obtained at intermediate concentrations of the three ascorbic acid analogs (i.e., 0.06 and 0.6 mM) than at higher (6 mM) or lower (0.006 mM) concentrations. In parallel experiments, the three ascorbic acid analogs induced lipid peroxidation, which was also greater at the two intermediate than at higher or lower concentrations. Several known inhibitors of lipid peroxidation, including propyl gallate, butylated hydroxyanisole, butylated hydroxytoluene, α-naphthol, and cobalt chloride, as well as the iron chelating agents EDTA and DETAPAC (diethylenetriaminepentaacetic acid) were able to counteract the effects of the ascorbic acid analogs on both lipid peroxidation and on [3H]spiroperidol binding. These data strongly suggest that an iron-catalyzed lipid peroxidation is responsible for the observed inhibitory effects on binding. In other experiments, neostriatal membrane preparations that were preincubated with ascorbic acid (0.6 mM) and subsequently washed still had greatly diminished capacity to bind [3H]spiroperidol, indicating that ascorbic acid need not be physically present during the binding assay in order to affect binding. This experimental procedure also appears to be a way in which [3H]spiroperidol binding sites can be inactivated and washed free of the inactivating agent.