Purification and characterization of a heat- and acid-stable progestin binding protein of rat lung

We report on the purification and characterization of a heat- and acid-stable progestin binding protein of rat lung cytosol, the binding behaviour and capacity of which have already been described by us. Using heat treatment, fractionated acetone precipitation, DEAE, HAP and Sephadex G-100 chromatography, the protein was pure when analyzed by SDS-gel electrophoresis. The molecular weight was found to be 12-13,000 by sucrose density and analytical ultracentrifugation and by SDS-gel electrophoresis whereas gel chromatography revealed an apparent molecular weight of 22,000 and a Stockes' radius of 21 A. Among several species tested, only mouse lung contained a similar binding protein, which was only slightly different with respect to binding specificity. The interaction of [3H]R-5020 occurred with the following constants k1 = 5.1 x 10(4) M-1 min-1, k-1 = 0.9-3.5 x 10(-2) min-1 giving an equilibrium dissociation constant of 1.8-6.7 x 10(-7) M compared to 3 x 10(-7) M obtained by Scatchard plot analysis.