[Multicomponent system of estrogen-binding proteins of the liver. Isolation and comparative properties of 4-5S and other rat liver high molecular weight proteins, which specifically bind estradiol]

Forms I and II of the estrogen receptors of female rat liver were isolated by desalting of the cytosol fraction desalting with 40% saturated (NH4)2SO4 with a subsequent gel-filtration through Ultragel AcA 44. In the absence of 0,3 M NaCl form I produces heavy aggregates and is eluted from Sepharose 6B in excluse volume. The molecular weight of from II under the same conditions as measured during gel-filtration and gradient centrifugation is 80 000. In the presence of 0,3 M NaCl the molecular weights of forms I and II are 63 000 and 77 000, respectively. The pH optima for the estradiol-binding activity of forms I and II are 6,8--8,5 and 7,4--9,1, respectively. Form II is more stable to heating and storage at 4 degrees. The binding activity of both forms of the receptors is eliminated by p-chloromercurybenzoate. The equilibrium constant for form II association with estradiol exceeds that of form I 2,5-fold and is equal to 2,4 x 10(10) M-1. The presence of 0,3 M NaCl does not affect this parameter in both cases. The association of forms I and II with estradiol and dissociation of the complexes are of a two-step type. The k+1 values for forms I and II are identical, whereas the k-1 value for form I is about two times that for form II. In terms of their competitive activity the hormone compounds tested can be arranged in the following order: estradiol less than or greater than hexestrol much greater than estrone congruent to estriol congruent to 17-deoxyestradiol much greater than 5 alpha-dihydroxytestosterone congruent to testosterone. Progesterone and corticosterone do not reveal competitive properties. The functional similarity of forms I and II of estrogen receptors from rat liver suggests that these proteins are isoreceptors.