PYRIDOXAL-PHOSPHATE INDUCED ALTERATIONS IN GLUCOCORTICOID RECEPTOR CONFORMATION

The molecular properties of dexamethasone-re-ceptor complexes from rat thymocytes are markedly affected by pyridoxal phosphate as shown by both sucrose density gradient centrifugation and molecular exclusion gel chromatography. In the absence of added pyridoxal phosphate, unactivated cytoplasmic receptor complexes sediment as 7-8S and 2.9S species, whereas activated cytoplasmic receptor complexes sediment as 4-5S and 2.9S species. When unactivated cytoplasmic receptor complexes are exposed to pyridoxal phosphate either before or during sucrose density gradient centrifugation, the sedimentation profile is converted to one that is identical with that found for activated receptor; i.e., the 7-8S form is apparently converted to a 4-5S form. Dexamethasone receptors extracted from rat thymocyte nuclei with pyridoxal phosphate sediment as a single 3.5S peak. Findings consistent with the above are made by using gel filtration techniques. After treatment with pyridoxal phos- phate followed by reduction with sodium borohydride, all cytoplasmic and nuclear dexamethasone receptors are converted to a single 2.9S form. The 2.9S form is insensitive to the presence of 0.4 M KC1 during centrifugation. These observations suggest that there is an irreversible, covalent modification of dexamethasone-receptor complexes by pyridoxal phosphate and sodium borohydride. Presumably this modification occurs at a lysine residue(s) on the various forms of the receptor and causes either conformational changes in the receptors or the expression of a subunit that is common to and characteristic of all of the forms of the nuclear and cytoplasmic dexamethasone receptors found in rat thymocytes. The 2.9S forms of the dexamethasone receptors produced by treatment with pyridoxal phosphate and sodium borohydride appear to be similar in size with those produced by treatment of thymocyte cytosols with Ca2+. © 1979, American Chemical Society. All rights reserved.