title: Biochemical characterization of the Arctic char (Salvelinus alpinus) ovarian progestin membrane receptor.
abstract: Membrane progestin receptors are involved in oocyte maturation in teleosts. However, the maturation-inducing steroid (MIS) does not appear to be conserved among species and several progestins may fulfill this function. So far, complete biochemical characterization has only been performed on a few species. In the present study we have characterized the membrane progestin receptor in Arctic char (Salvelinus alpinus) and show that the 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) receptor also binds several xenobiotics, thus rendering oocyte maturation sensitive to environmental pollutants. We identified a single class of high affinity (Kd, 13.8 +/- 1.1 nM), low capacity (Bmax, 1.6 +/- 0.6 pmol/g ovary) binding sites by saturation and Scatchard analyses. Receptor binding displayed rapid association and dissociation kinetics typical of steroid membrane receptors, with t1/2 s of less than 1 minute. The 17,20beta-P binding also displayed tissue specificity with high, saturable, and specific 17,20beta-P binding detected in ovaries, heart and gills while no specific binding was observed in muscle, brain or liver. Changes in 17,20beta-P binding during oocyte maturation were consistent with its identity as the oocyte MIS membrane receptor. Incubation of fully-grown ovarian follicles with gonadotropin induced oocyte maturation, which was accompanied by a five-fold increase in 17,20beta-P receptor binding. In addition, competition studies with a variety of steroids revealed that receptor binding is highly specific for 17,20beta-P, the likely maturation-inducing steroid (MIS) in Arctic char. The relative-binding affinities of all the other progestogens and steroids tested were less than 5% of that of 17,20beta-P for the receptor. Several ortho, para derivatives of DDT also showed weak binding affinity for the 17,20beta-P receptor supporting the hypothesis that xenobiotics may bind steroid receptors on the oocyte's surface and might thereby interfere with oocyte growth and maturation.
authors.affiliation: The University of Texas, Austin Marine Science Institute, Port Aransas, Texas, USA. hberg@utmsi.utexas.edu
authors.forename: A Håkan
authors.initials: AH
authors.lastname: Berg
authors.forename: Peter
authors.initials: P
authors.lastname: Thomas
authors.forename: Per-Erik
authors.initials: PE
authors.lastname: Olsson
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D006063:Chorionic Gonadotropin / Q000494:pharmacology; D003634:DDT / Q000378:metabolism; D005260:Female; D005485:Flutamide / Q000378:metabolism; D005880:Gills / Q000378:metabolism; D006908:Hydroxyprogesterones / Q000378:metabolism; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism; D009865:Oocytes / Q000502:physiology; D010053:Ovary / Q000378:metabolism; D010636:Phenols / Q000378:metabolism; D011980:Receptors, Progesterone / Q000187:drug effects / Q000378:metabolism*; D014337:Trout / Q000502:physiology*
chemical_list: D006063:Chorionic Gonadotropin; D006908:Hydroxyprogesterones; D010636:Phenols; D011980:Receptors, Progesterone; C007433:17,20-dihydroxy-4-pregnen-3-one; D005485:Flutamide; C025256:nonylphenol; D003634:DDT; C016340:o,p'-DDT