Estrogen photoaffinity labels. 1. Chemical and radiochemical synthesis of hexestrol diazoketone and azide derivatives; photochemical studies in solution

Two photosensitive estrogen derivatives, hexestrol diazoketopropyl ether (5) and hexestrol azide (8a), have been synthesized in radiolabeled form, and their photochemical behavior in solution has been studied. The radiolabeled compounds were prepared in good yields according to improved synthetic procedures; they are stable and were obtained with specific activities in the range of 50-100 Ci per mmol and radiochemical purities in excess of 95%. A simpler model system, phenyl diazoketoprophyl ether, was used to study the photochemical behavior of the diazoketopropyl ether group. Direct irradiation of this compound at 254 nm in methanol led to 33% insertion product (methoxyketone) and 67% Wolff rearrangement product (ester). Irradiation of [3H]hexestrol diazoketopropyl ether (5) in methanol gives mainly nonpolar photoproducts (presumed to be the methoxy ketone and ester); however, irradiation in aqueous medium leads to large amounts of free hexestrol (52%). Photolysis of hexestrol azide (8a) in either methanol or water gives the corresponding amine in low yield as the only identifiable photoproduct.

Estrogen photoaffinity labels. 2. Reversible binding and covalent attachment of photosensitive hexestrol derivatives to the uterine estrogen receptor

The ability of two radiolabeled, photoreactive estrogen analogues, [3H]hexestrol diazoketopropyl ether ([3H]Hex-DKP) and [3H]hexestrol azide ([3H]Hex-N3), to covalently label the uterine estrogen receptor is studied. Lamb uterine estrogen receptor preparations that have been partially purified (ammonium sulfate precipitation, Sephadex G-200 chromatography) and disaggregated by limited trypsinization can be electrophoresed on polyacrylamide gels under conditions where binding activity is preserved. This electrophoretic procedure was used to fractionate the proteins labeled by the two photoreactive estrogen analogues. Prior to photolysis, peaks of radioactivity indicating estrogen specific binding of [3H]-Hex-N3 and [3H]Hex-DKP are evident on the gels, although dissociation of the latter compound is extensive. When preparations of uterine estrogen receptor that contain the photoreactive derivatives are irradiated and then electrophoresed, reversibly labeled proteins can be distinguished from irreversibly labeled ones (covalently bonded), by extraction of the individual gel slices with organic solvents. While no irreversible binding to receptor appears to result from irradiation with [3H]Hex-DKP, irradiation with [3H]Hex-N3 does covalently label the estrogen receptor. The receptor covalently labeled with [3H]Hex-N3 has the same electrophoretic mobility as the unlabeled receptor; the covalent labeling process is estrogen-site specific, and the efficiency of labeling (15-20%) is consistent with the inactivation efficiency of Hex-N3, previously measured by an indirect assay. This is the first example of the labeling of a steroid hormone receptor by photoaffinity labeling.

Ultraviolet photosensitivity of the estrogen binding protein from rat uterus. Wavelength and ligand dependence. Photocovalent attachment of estrogens to proteins

Ultraviolet irradiation of the estrogen binding protein in rat uterine cytosol results in a progressive photoinactivation which is rapid at 254 nm and slower at greater than 315 nm. Both unfilled and estradiol-filled sites are inactivated at approimately the same rates at 254 nm (t 1/2 equals 8 min and 11 min, respectively), but at 315 nm, empty sites are consumed much more rapidly (t 1/2 equals 3.4 hr) than filled ones (t 1/2 equals 24 hr). The protective effect of the estrogen ligand at this wavelength appears to depend on its binding to the estrogen-specific binding site, as inactivation rate studies at different concentrations of estrone, estradiol, and estriol show a good correlation between the extent of protection and the fractional saturation of the high affinity estrogen binding sites. Scatchard analysis indicates that inactivation is the result of a loss of binding sites and not a decrease in their affinity, and sedimentation analysis shows that increased heterogeneity and aggregation of the estrogen binding species accompanies the photoinactivation process. Photoinactivation appears to be the result of direct irradiative damage of the animo acid residues, as the inactivation rate is the same under air and nitrogen atmospheres, and is unaffected by nucleophiles, reductants, and radical scavengers. When photoinactivation is measureed by irradiation of cytosol containing [3-H]estradiol, a concurrent photocovalent attachment process is noted; the steroid becomes linked to protein in a solvent-extractable manner (boiling ethanol inextractable). This attachment, however, does not appear to be related to the steroid binding at the estrogen binding site. Its rate is affected by reductants and scavengers. A similar photocovalent attachment reaction occurs when bovine serum albumin or ovalbumin is irradiated in the presence of [3-H]estradiol or [3-H]diethylstilbestrol. The detailed reactions involved in this photocovalent attachment process have not been defined at present.

Iodohexestrols. I. Synthesis and photoreactivity of iodinated hexestrol derivatives

The five possible ortho-iodinated hexestrols, containing from 1-4 iodines per molecule, have been synthesized as potential iodine-containing estrogenic ligands. After chromatographic separation and purification, each derivative has been fully characterized by spectroscopic methods. [3-H]-3-Iodohexestrol, [131-I]- and [125-I]-3-iodohexestrol, and [125-IA1-3,5-diiodohexestrol have also been prepared. Photolysis of 3,5-diiodohexestrol in methanol results in rapid deiodination to 3-iodohexestrol; further reduction to hexestrol is slower. Photolysis of 3-iodohexestrol in benzene gives 3-phenylhexestrol.

Iodohexestrols. II. Characterization of the binding and estrogenic activity of iodinated hexestrol derivatives, in vitro and in vivo

The affinity of ortho-iodinated hexestrols for the estrogen binding protein from rat uterus, determined by competitive binding assay, decreases with progressive iodine substitution; 3-iodohexestrol (I-Hex) has a binding affinity 42% that of estradiol. Analysis of [3-H]-I-Hex binding in rat uterine cytosol by sucrose density gradient centrifugation shows both an estrogen-specific binding component (8 S) and a more abundant component (4 S) that is not estrogen specific. Scatchard analysis indicates that this latter binding is of high affinity (Kd equals to 3.7-8.3 times 10- minus-9 M) but is not uterine specific. Polyacrylamide gel electrophoresis shows that most of the [3-H]-I-Hex binding activity in serum and uterine cytosol is distinct from and anodic to the principal protein component (albumin), and that is comigrates with [14-C]thyroxine binding activity. In in vitro incubation of rat uteri, I-Hex can block the specific uptake of [3-H]estradiol into the nuclear fraction; it itself causes a translocation of estrogen-specific binding capacity (as measured by exchange) from cytoplasm to nuclei, and can induce the synthesis of an estrogen-specific uterine protein, all under conditions where it is not metabolically deiodinated to hexestrol. The uterotrophic activities of the iodohexestrols are in most cases comparable to that expected on the basis of their competitive binding affinities. However, selective, estrogen-specific uptake of [3-H]-I-Hex into rat uterus, either in vitro or in vivo, cannot be demonstrated.