Steroidal affinity labels of the estrogen receptor α.: 4.: Electrophilic 11β-aryl derivatives of estradiol

Ten electrophilic estradiol 11 beta-aryl derivatives were synthesized, with three different types of 11 beta-substituent: (i) p phi O(CH2)(2)X (compounds: 6, X = OSO2CH3; 7, X = I; 13, X = NHCOCH2-Cl; 15, X = N(CH3)COCH2Br; and 16, X = N(CH3)COCH2Cl); (ii) p phi O(CH2)(5)X (compounds: 17, X = I; 20, X = NHCOCH2Br; and 22, X = N(CH3)COCH2Br); and (iii) p phi C drop CCH2X (compounds: 27, X = NHCOCH2Cl; and 29, X = N(CH3)COCH2Cl). The range of their apparent affinity constants for binding the lamb uterine estrogen receptor alpha (ER alpha) was 3-40% that of estradiol. Six electrophiles, chloroacetamides 13, 16, 27, and 29, iodide 17, and bromoacetamide 20 (whose arm linking the electrophilic carbon to the 11 beta-phenyl group includes at least six bonds), were able to irreversibly inhibit the binding of [H-3]estradiol to ER (25-60% decrease in binding sites), with the following compound effectiveness order: 17 < 13 < 16 similar to 20 similar to 27 similar to 29. Mesylate 6, iodide 7 (whose linking arm includes only three bonds), and bromoacetamides 15 and 22 (which differ from 16 by the Cl to Br change and from 20 by the NH to NCH3 change, respectively) were much less effective (<10% decrease in binding sites, if any). The fact that the inactivation of estradiol-binding sites by the six electrophiles was totally prevented by estradiol indicated that they were ER affinity labeling agents. When ER was modified by methyl methanethiosulfonate, an SH-specific reagent, the different compounds led to very contrasting results in ER affinity labeling. With modified ER, iodide 17 and chloroacetamides 27 and 29 were practically inactive, chloroacetamides 13 and 16 and bromoacetamide 20 were still active but less effective than on the native ER, whereas tertiary bromoacetamides 15 and 22, found to be practically inactive on native ER, became the most effective electrophiles (similar to 45% and similar to 65% binding sites inactivated, respectively). The results indicate that in the steroid-filled hormone-binding pocket: (i) nucleophilic residues are localized on the beta-side but relatively remote from the steroid nucleus (distance from C-11 > "seven bonds"); (ii) relatively discrete changes in the electrophilic functionality, such as Cl to Br or NH to NCH3 of haloacetamido compounds, can markedly modify the positioning of the electrophilic center which could no longer react with the nucleophilic residues; and (iii) cysteine residues (probably homologues of human ER alpha cysteine 381 and/or cysteine 530) are, at least partly, the covalent attachment sites of the electrophiles. Moreover, modification of cysteine residues by methyl methanethiosulfonate changes the structure of the hormone-binding pocket, whose labeling by the various electrophiles is profoundly altered.