THE EFFECT OF A CA-2+-ACTIVATED PROTEASE ON THE GLUCOCORTICOID RECEPTOR OF LYMPHOID-TISSUES

The effect of 0.5-8 mM Ca2+ on the stability and DNA- or nuclear binding capability of the cytosol glucocorticoid receptors from different lymphoid tissues of chickens and rats was studied. Ca2+ treatment at 25.degree. C significantly increased the inactivation rate of the steroid-free receptor in chick thymus cytosol, but only slightly influenced it in the case of receptors from bursa of Fabricius or from rat thymus cytosol. Ca2+ ion strongly decreased the binding of cytosol hormone-receptor complex to isolated nuclei or DNA-cellulose. The decreased DNA-binding activity of the hormone-receptor complex could be seen in both cases if the Ca2+-treatment was carried out before or after the formation of hormone-receptor complex. This effect could be observed at much higher Ca2+ concentrations in the case of bursa and rat thymus cytosol than in the case of chick thymus cytosol. The effect of Ca2+ on heat-stability of the receptor was diminished by carbobenzoxy-D-Phe-Pro-Arg-aldehyde, an inhibitor of thrombin-like enzymes, while the effect on DNA-binding could be abolished with 3 different types of protease-inhibitors [butoxycarbonyl-D-Phe-Pro-Arg-aldehyde, trasylol, benzyloxycarbonylphenylalanine] as well as with SH-attacking agents [iodoacetate, 2,2''-dipyridyldisulfide]. The results are consistent with the hypothesis that the mechanism of Ca2+ action on the glucocorticoid receptors is probably the activation of some proteolytic enzymes, which are SH-proteases.