16alpha,17alpha-Cycloalkane Derivatives of Progesterone Intensively Bind to a Rat Serum Protein

A. N. Smirnov^1*, E. V. Pokrovskaya¹, I. S. Levina², L. E. Kulikova², A. V. Kamernitzky², and V. P. Shevchenko³

¹Laboratory of Endocrinology, School of Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-4309; E-mail: smirnov_an@mail.ru

²Group of Chemistry of Steroids and Oxylipines, Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninski pr. 47, Moscow, 117913 Russia

³Laboratory of Isotope-Modified and Physiologically Active Compounds, Institute of Molecular Genetics, Russian Academy of Sciences, pl. Akademika Kurchatova 2, Moscow, 123182 Russia

^* To whom correspondence should be addressed.

Received November 3, 2000; Revision received December 21, 2000

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The interaction of 6alpha-methyl-[1,2-³H]16alpha,17alpha-cyclohexanoprogesterone with rat serum proteins has been studied. Specific binding of this ligand characterized by K_d = 0.36 ± 0.10 µM and concentration of binding sites (B_max) of about 1 µM (27.8 ± 12.5 pmol/mg total protein) was found. According to competitive analysis, the affinity of the studied progestins to a protein that differs from transcortin was to some extent correlated with their hydrophobicity. The dissociation kinetics of ³H-ligand-protein complexes were biphasic, the binding sites forming stable and labile complexes with ³H-ligand being eluted in the same region during ion-exchange chromatography. In overall properties, the serum protein differs from the progesterone receptor and the pregna-D´-pentarane-specific protein from rat uterus. It is suggested that the revealed protein may provide high progestagenic activity of 6alpha-methyl-16alpha,17alpha-cyclohexanoprogesterone by prolonging its retention in the bloodstream.

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KEY WORDS: progestins, steroids, progesterone analogs, transport proteins, binding kinetics

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