Low N-ethylmaleimide concentrations activate ryanodine receptors by a reversible interaction, not an alkylation of critical thiols

Previous studies proposed that N-ethylmaleimide (NEM) alkylates 3 classes of thiols on skeletal muscle ryanodine receptors (RyRs) producing 3 phases of channel modification, as function of time and concentration. NEM (5 mM) decreased, increased, and then decreased the open probability (P(o)) of the channel by thiol alkylation, a reaction not reversed by reducing agents. We now show that low NEM concentrations (20-200 μM) elicit Ca²⁺ release from sarcoplasmic reticulum (SR) vesicles, but contrary to expectations, the effect was fully reversed by reducing agents or by washing SR vesicles. In bilayers, NEM (0.2 mM) increased P(o) of RyRs within seconds when added to the cis (not trans) side, and dithiothreitol (DTT; 1 mM) decreased P(o) in seconds. High (5 mM) NEM concentrations elicited SR Ca²⁺ release that was not reversed by DTT, as expected for an alkylation reaction. A non-sulfhydryl reagent structurally related to NEM, N-ethylsuccinimide (0.1-0.5 mM), also elicited SR Ca²⁺ release that was not reversed by DTT (1 mM). Other alkylating agents elicited SR Ca²⁺ release, which was fully (N-methylmaleimide) or partially (iodoacetic acid) reversed by DTT and inhibited by ruthenium red. Nitric oxide (NO) donors at concentrations that did not activate RyRs inhibited NEM-induced Ca²⁺ release, most likely by an interaction of NO with NEM rather than an inactivation of RyRs by NO. Thus, at low concentrations, NEM does not act as a selective thiol reagent and activates RyRs without alkylating critical thiols indicating that the multiple phases of ryanodine binding are unrelated to RyR activity or to NEM alkylation of RyRs.