Detection of a heat- and acid-stable 'progesterone'-binding protein in the rat lung

Using a modified charcoal method, we could detect a steroid-binding component in rat lung cytosol which specifically binds R5020, progesterone, and some of its natural derivatives. The concentration of binding sites is high (30-40 pmol/mg protein), the affinity is moderate, the Kd of the R5020 complex being approximately 10(-7) M. Proteolytic enzymes and sulfhydryl reagents destroyed the binding sites indicating the protein nature and the requirement for disulfide bonds. The protein sedimented in the 2 S range thus had an Mr of 10,000-15,000. Further characteristics are the extreme heat (30 min at 100 degrees C) and acid (pH 1) stability. These properties and the fact that it was not detected in serum, distinguish this binding protein from receptors and specific serum steroid binders.