The involvement of receptro sulphydryl groups in the binding of steroids to the cytoplasmic glucocorticoid receptor from rat thymus.

The glucocorticoid receptor protein present in the high-speed supernant fraction of rat thymus tissue is extremely unstable, having a half-life of about 2 h at 4 degrees C. It was found that the decline in steroid-binding capacity could be slowed, though not arrested completely, by the addition of sulphydryl-protecting agents such as 2-mercaptoethanol or dithiothreitol, and by EDTA. The inactivation was also partly reversed by these agents. 0.5 mM N-ethylmaleimideor p-chloromercuriphenylsulphonic acid inactivated the recptor at 4 degrees C, but the presence of bound steroid protected the receptor against this inactivation. Bound steroid did not protect the receptor against the action of higher concentrations of these reagents. Treatment of intact thymus cells with 2,4-dinitrophenol resulted in a reduction in the steroid-binding capacity of the supernatant fraction derived from these cells. This effect of 2,4-dinitrophenol could not be reversed by the presence of dithiothreitol in the extraction buffer. It is concluded that the inactivation of the receptor in vitro is at least partly due to the oxidation of one or more sulphydryl groups necessary for steroid binding; the process of oxidation does not account for the reduction in steroid binding observed in intact thymus cells under conditions of energy deprivation.