Chemical modification of the avian progesterone receptor by pyridoxal 5'-phosphate.

Pyridoxal BP-phosphate inhibits the binding of the avian progesterone receptor to ATP-Sepharose, probably through a Schiff base interaction. It also blocks other interactions characteristic of the activated or transformed receptor, such as its binding to nuclei, DNA-cellulose, and phosphocellulose. In an attempt to explain these inhibitory effects, we have characterized the progesterone receptor after treatment with pyridoxal 5’-phosphate. The cytosol receptor which was fractionated by ammonium sulfate precipitation was treated with pyridoxal 5’-phosphate and then with the reducing agent, sodium borohydride. This treatment causes an irreversible inhibition of the above receptor interactions but it has no effect on the steroid-binding properties of the receptor. The treatment does not change the sedimentation of receptor on high salt sucrose gradients; however, in low salt sucrose gradients the receptor does not aggregate but remains in a 3.5 to 4 S form. The A and B forms of the treated receptor can be resolved by chromatography on DEAE-cellulose or hydroxylapatite, but the elution patterns are shifted from those of nontreated receptor. Also, electrofocusing of the receptor shows that it shifts to a more acidic isoelectric point. In addition to these charge alterations, the modified receptor is much more stable and can withstand incubation at 37°C for up to 2 h. These results indicate a direct action of pyridoxal 5’-phosphate to produce chemical modifications in the receptor molecule.