2-Iodoestradiol binds with high affinity to human sex hormone binding globulin (SHBG).
Human sex hormone binding globulin (SHBG) binds a set of steroids that differ slightly from each other in structure. Dihydrotestosterone and testosterone are bound with high affinity by SHBG whereas estradiol is bound with a lower affinity. In this work we have studied the binding to human SHBG of the derivatives obtained by substituting iodine in the aromatic A-ring of estradiol. Three A-ring iodinated estradiol derivatives, 2-iodoestradiol, 4-iodoestradiol and 2,4-di-iodoestradiol, were obtained by treating 17 beta-estradiol with NaI and Chloramine T and separating the reaction products by HPLC. Their structures were confirmed by mass spectrometry and 1H-NMR. The corresponding radioactive compounds were obtained with use of Na[125I] in the same synthesizing procedure. Incubation of whole serum, serum albumin and purified SHBG with each of the three [125I]iodoestradiols followed by agarose gel electrophoresis showed only 2-iodoestradiol to have a strong binding to SHBG. This steroid was also bound to albumin, but with a lower affinity. Besides SHBG and albumin, there were no other binders of 2-iodoestradiol in human serum. The affinity constant for the binding of 2-iodoestradiol to purified human SHBG at 37 degrees C and physiological pH was determined by a dextran-coated charcoal method to be 2.4 x 10(9) M-1 (i.e. exceeding that of dihydrotestosterone). It was found that 0.9 mol of 2-iodoestradiol was bound per mol of SHBG dimer (93 kDa) at saturation, and that 2-iodoestradiol competed with dihydrotestosterone for the same binding site of SHBG. It was concluded that 2-iodoestradiol has a remarkably high affinity for human SHBG, and that its gamma-emitting 125I-analog is useful for binding studies of human SHBG.
Fernlund
P
P
Department of Clinical Chemistry, University of Lund, Malmö General Hospital, Sweden.
Gershagen
S
S
D001667:Binding, Competitive; D055598:Chemical Phenomena; D002621:Chemistry; D002851:Chromatography, High Pressure Liquid; D013196:Dihydrotestosterone / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004958:Estradiol / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D012709:Serum Albumin / Q000378:metabolism; D012738:Sex Hormone-Binding Globulin / Q000378:metabolism*
D012709:Serum Albumin; D012738:Sex Hormone-Binding Globulin; D013196:Dihydrotestosterone; C053381:2-iodoestradiol; D004958:Estradiol