Interaction between 6-hydroxydopamine and transferrin: "Let my iron go''

The dopamine analogue 6-hydroxydopamine (6-OHDA) is selectively toxic to ca techolaminergic neurons. Because of its selectivity for neuroblastic cells in the sympathetic nervous system lineage, 6-OHDA has been suggested as a c hemotherapeutic agent for targeted treatment of patients with neuroblastoma , We tested the hypothesis that the toxicity of 6-OHDA is caused by its int eraction with serum ferric transferrin (Fe-TF) resulting in release of iron . We further hypothesized that this iron, through its redoxcycling by 6-OHD A, triggers generation of reactive oxygen species. 6-OHDA-induced release o f iron from Fe-TF was demonstrated by: (1) low-temperature EPR spectroscopi c evidence for decay of the characteristic Fe-TF signal (g = 4.3) and appea rance of the high-spin signal from iron chelated by 6-OHDA oxidation produc ts; (2) spectrophotometric detection of complexing of iron with the Fe2+ ch elator ferrozine; (3) redox-cycling of ascorbate yielding EPR-detectable as corbate radicals; and (4) generation of hydroxyl radicals as evidenced by E PR spectroscopy of their adduct with a spin trap, 5,5'-dimethylpyrroline ox ide (DMPO) (DMPO-OH). Our low-temperature EPR studies showed that in human plasma, 6-OHDA caused iron release only under nitrogen gas but not under ai r or oxygen. The absence of a 6-OHDA effect in plasma under aerobic conditi ons was most likely due to its ferroxidase activity [with consequent reupta ke of Fe(III) by apoTF] and catalytic oxidation of 6-OHDA by ceruloplasmin. Modeling of these plasma activities by a stable nitroxide radical, 2,2,6,6 -tetramethyl-1-piperidinyloxy (TEMPOL), resulted in protection of plasma Fe -TF against iron release under nitrogen. Parenteral administration of 6-OHD A to mice resulted in iron release from Fe-TF as evidenced by transformatio n of the Fe-TF low-temperature EPR signal that was indistinguishable from t hat seen in in vitro models. In addition, administration of the iron chelat or deferoxamine (DFO) to mice prior to administration of toxic doses of 6-O HDA resulted in a decrease in activity impairment of mice as compared to th at seen with 6-OHDA alone. These findings underscore the physiological and pharmacological relevance of 6-OHDA-mediated iron release from Fe-TF and su ggest that iron chelators (DFO) may be used for prevention of 6-OHDA toxici ty.