16 alpha,17 alpha-cycloalkane derivatives of progesterone intensively bindto a rat serum protein

The interaction of 6 alpha -methyl-[1,2-H-3]16 alpha ,17 alpha -cyclohexano progesterone with rat serum proteins has been studied. Specific binding of this ligand characterized by K-d = 0.36 +/- 0.10 muM and concentration of b inding sites (B-max) of about 1 muM (27.8 +/- 12.5 pmol/mg total protein) w as found. According to competitive analysis, the affinity of the studied pr ogestins to a protein that differs from transcortin was to some extent corr elated with their hydrophobicity. The dissociation kinetics of H-3-ligand-p rotein complexes were biphasic, the binding sites forming stable and labile complexes with H-3-ligand being eluted in the same region during ion-excha nge chromatography. In overall properties, the serum protein differs from t he progesterone receptor and the pregna-D'-pentarane-specific protein from rat uterus. it is suggested that the revealed protein may provide high prog estagenic activity of 6 alpha -methyl-16 alpha ,17 alpha -cyclohexanoproges terone by prolonging its retention in the bloodstream.