High and low affinity progesterone (P)-binding proteins similar to those described in the chick oviduct were characterized for the first time in liver cytosol in an oviparous turtle, Chrysemys picta. Two forms of high affinity P receptor were separated by diethyl aminoethyl (DEAE)-Sepharose anion exchange chromatography; the A form eluted in low salt buffer, and the B form at a KC1 concentration of 0.20 M in a linear gradient. After photoaffinity labeling of crude cytosol with [3H] R5020, two overlapping peaks of radioactivity were detected with sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After separation with DEAE-Sepharose and photoaffinity labeling, the A form migrated as a single peak with a mol wt of 90,000, and the B form as a single peak with a mol wt of 123,000. The affinity (Kd, 2.9 × 10-9 M), capacity (1 pmol/mg protein), and binding specificity were characteristic of a P receptor. P competed most effectively for [3H]P binding. R5020 and deoxycorticosterone were good competitors; 5 -Î±pregnenolone, 17Î±hydroxyprogesterone, and testosterone were quite effective; cortisol, corticosterone, aldosterone, and 5 -Î±dihydrotestosterone were poor competitors. A lower affinity P-binding component (Kd, 1 × 10-7 M; maximum binding, 15 pmol/mg protein) coeluted from DEAE-Sepharose columns with the A and B receptor forms. This is the first description of a hepatic P receptor in any vertebrate.
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