Inactivation of glucocorticoid-binding capacity by protein phosphatases in the presence of molybdate and complete reactivation of dithiothreitol

Abstract

Glucocorticoid receptor in rat liver cytosol is inactivated (rendered unable to bind steroid) by incubation with calf intestine alkaline phosphatase or highly purified rabbit muscle phosphoprotein phosphatase (phosphorylase phosphate, protein phosphatase C). The receptor is inactivated by both enzymes even when 10 mM sodium molybdate is present. Receptors that are inactivated by phosphatases in the presence of molybdate can be reactivated to the steroid-binding state by addition of dithiothreitol, but receptors that are inactivated in the absence of molybdate cannot be reactivated. These observations suggest that dephosphorylation leads to oxidation of a moiety (-SH) on the receptor that is required for steroid binding. Molybdate apparently preserves the receptor in a form such that reduction returns the receptor to the steroid binding state. We would propose that molybdate may act by complexing with sulfur groups on the receptor.