Effects of blocking sulfhydryl groups and of reducing disulfide bonds on the acetylcholine-activated permeability system of the electroplax

Abstract

The depolarization of the single-cell preparation of the electroplax of Elecrophorus electricus by acetylcholine, carbamylcholine, and trimethylbutylammonium ion is inhibited by prior treatment of the electroplax with $\text{p-}\text{chloromercuribenzoate}$ (PCMB) or with 1,4-dithiothreitol (DTT). The inhibition by either agent is characterized by an increase in the concentration of carbamylcholine eliciting a half-maximal response with no great change in the maximum response. The inhibition due to PCMB is reversed by subsequent treatment with thiol compounds, and that due to DTT is reversed by oxidizing agents and by cysteine (but not by other thiol compounds). Washing alone has no effect on the inhibition due to either agent. Treatment of the electroplax with $\text{N-}\text{ethylmaleimide}$ (NEM) after DTT blocks the reversal of the inhibition by oxidizing agents and by cysteine, added subsequently. NEM alone under the same conditions has no direct effect on the depolarization. It is concluded that PCMB is reacting with sulfhydryl groups and DTT with disulfide bonds, present in one or more component of the acetylcholine-activated permeability system at the junctional regions of the innervated membrane of the electroplax. The evidence is taken as support for the view that some of the components of this system are proteins.