N-substituted maleimide inactivation of the response to taste cell stimulation

Abstract

N-Ethylmaleimide (NEM) irreversibly inactivates the response of gustatory cells to stimulation by NaCl, sucrose and hydrogen ions. The rate of inactivation can be measured by monitoring the decay of NaCl-stimulated summated electrophysiological activity at the chorda tympani nerve in the presence of NEM. The observed pseudo first-order rate constants are linear with NEM concentration, and the second-order rate constant is 0.38 M⁻¹ sec⁻¹. Other N-substituted maleimides, such as N-methylmaleimide and N-butylmaleimide, which have ether:water partition coefficients similar to NEM, inactivate the NaCl-stimulated response at rates comparable to NEM. However, the hydrophobic derivative, 4-(N-maleimido)phenyltrimethylammonium has a significantly lower ether:water partition coefficient and is essentially ineffective as an inactivator of the NaCl response. These results, together with the observation that the inactivation rate is independent of pH between 4.5 and 7.0, indicate the inactivation site is either intracellular or buried within the cell membrane at a locus inaccessible to most extracellular fluids. The rate of inactivation of the sucrose and HCl responses were measured indirectly and found to be comparable to the NaCl-stimulated inactivation rate, indicating the inhibited event is common to the transduction of the response for all of the stimuli examined. Possible sites of inactivation by N-Substituted maleimides are considered in the context of the results. This kinetic approach should have application in searching for and characterizing receptor-specific as well as other classes of taste cell inhibitors.