Effects of P-Aminophenyl Dichloroarsine on Reduced High-affinity [3H]Nicotine Binding Sites from Chick Brain: A Covalent, Yet Reversible, Agent for Neuronal Nicotinic Receptors.

Neuronal nicotinic acetylcholine receptor (nAChR) alpha-subunits contain a conserved disulphide that is essential for function. Here, we have examined the effects of sulphydryl redox reagents on [3H]nicotine binding to chick brain nAChR immunoisolated with the monoclonal antibody mAb35. The disulphide reducing agent, dithiothreitol (DTT), inhibited [3H]nicotine binding [50% inhibitory concentration (IC50)=146 microM] but this effect was reversed (93 +/- 1.5%) by subsequent reoxidation with 1 mM dithio-bis(nitrobenzoic acid) (DTNB). The trivalent arsenical, p-aminophenyl dichloroarsine (APA), which reacts with pairs of spatially close sulphydryls, was a potent inhibitor of reoxidation by DTNB (IC50=35 nM). However, application of the 'anti-arsenical', 2,3-dimercaptopropane sulphonic acid (DMPS), restored agonist binding after APA treatment (50% effective concentration=120 microM). Paradoxically, DMPS was also found to be a potent oxidizing agent of these receptors. Affinity alkylation of reduced nAChRs with bromoacetylcholine (BAC; 100 microM) irreversibly blocked nicotine binding (>90%). We propose (but have not proven) that APA interacts with the cysteines homologous to Cys192 - 193 in Torpedo AChRs, since APA pretreatment of reduced neuronal receptors protected against irreversible BAC alkylation, as shown by subsequent reversal of DMPS (2 mM; 20 min). This study illustrates the potent and reversible nature of the arsenical's covalent interaction with an isolated nAChR and suggests that modified arsenicals could be useful nAChR probes.