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 K d of the R5020 complex being ∼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 M r of 10 000–15 000. Further characteristics are the extreme heat (30 min at 100°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.
