Dou_1994_J.Biol.Chem_269_20410

Interactions of dithiols with p-aminophenyldichloroarsine (APA) and with Torpedo nicotinic receptors were studied using two approaches. First, the stability of dithiol-APA complexes in solution was studied based on quenching thiol reactions with dithiobis-(nitrobenzoic acid). A peptide corresponding to a portion of the Torpedo alpha-subunit and various 1,2-dithiols such as 2,3-dimercaptopropanesulfonic acid (DMPS), 2,3-dimercaptosuccinic acid formed stable complexes with APA, while 1,4-dithiols, such as dithiothreitol (DTT) and 2,5 dimercapto-N,N,N'N'-tetradipamide (DTA) did not. The Kd of APA association with DTT in Tris buffer is 2 microM. These data suggest that APA has greater affinity for reduced nicotinic receptors than for either DTT or DTA, a prediction that was experimentally confirmed, since these reagents do not reverse the effects of APA on nicotinic receptors. Second, application of DMPS and BAL, but not 2,3-dimercaptosuccinic acid, to DTT-treated receptors both reversed the effects of APA-receptor complexes and prevented alkylation by bromoacetylcholine, suggesting that DMPS and BAL "oxidize" reduced nicotinic receptors. The presence of air is required for this "oxidizing" effect, but no clear mechanism was discovered, since prevention of formation of the reactive oxygen species superoxide, hydrogen peroxide, or hydroxyl radicals failed to block oxidation. These data suggest that oxygen reacts with dithiols to produce unknown reactive species that directly oxidize reduced nicotinic receptors, although other interpretations are still possible.