Inactivation of unbound rat liver glucocorticoid receptor by N-alkylmaleimides at sub-zero temperatures.

A series of N-alkylmaleimides was shown to inactivate effectively the rat liver glucocorticoid receptor at neutral pH. A partial purification of the unbound cytosolic receptor by protamine sulfate precipitation and a careful stabilization of the essential thiol by dithiothreitol and sodium molybdate before the alkylation step appeared essential to obtain pseudo-first-order kinetics. Moreover, performing the experiment at -12 degrees C in buffer containing 40% glycerol as antifreeze agent resulted in increased receptor stabilization and a slowing-down of the inactivation process, which could then be more accurately studied. This process was demonstrated to be dose- and pH-dependent in the case of N-ethyl- and N-nonylmaleimides. Furthermore, comparison of the various N-alkylmaleimides revealed a striking increase of receptor inactivation with increasing chain length of the maleimide derivative. Full protection against inactivation was afforded by previous [3H]dexamethasone binding on the receptor. Long-chain N-alkylmaleimides inactivated by beta-mercaptoethanol were still able to inhibit the [3H]-dexamethasone binding noncovalently. Likewise N-nonylsuccinimide was shown to compete with [3H]dexamethasone for receptor binding. It is suggested that the chain length effect observed in the inactivation process is related to nonpolar interactions in the binding of maleimides to the receptor prior to the irreversible alkylation of sulfhydryl groups. These groups lie in a hydrophobic environment, probably in the steroid binding site itself.