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Identification of Two Nuclear Androgen Receptors in Kelp Bass (Paralabrax clathratus) and Their Binding Affinities for Xenobiotics: Comparison with Atlantic Croaker (Micropogonias undulatus) Androgen Receptors¹

^a Department of Marine Science, The University of Texas Marine Science Institute, The University of Texas at Austin, Port Aransas, Texas 78373

Two distinct nuclear androgen receptors (ARs) were identified in brain and ovarian tissues of kelp bass, Paralabrax clathratus, termed kbAR1 and kbAR2, which correspond to the two nuclear ARs we have previously characterized in Atlantic croaker, Micropogonias undulatus, termed acAR1 and acAR2. Scatchard analysis of nuclear fractions of whole brain tissue demonstrated that kbAR1 had a single class of high-affinity binding sites for testosterone (T; K_d of 1.8 nM and B_max of 1.0 pmol/g tissue), whereas cytosolic fractions of kbAR2 ovarian tissue had a single class of high-affinity binding sites for dihydrotestosterone (DHT; K_d of 0.1 nM and B_max of 0.5 pmol/g tissue). Competition studies showed that both kbAR1 and kbAR2 were specific for androgens. However, kbAR1 bound only T with high affinity, whereas kbAR2 bound DHT, mibolerone, 17-methyl-testosterone, T, and 11-ketotestosterone with high affinity. In addition, we examined the binding affinities of dichlorodiphenyltrichloroethane and its derivatives, several hydroxylated polychlorinated biphenyl (PCB) congeners, PCB mixtures, and the fungicide vinclozolin and its two metabolites M1 and M2 for the two ARs in Atlantic croaker ovarian, testicular, and brain tissues and in kelp bass ovarian and brain tissues. Only 4,4'-PCB-3-OH and 2',5'-PCB-3-OH demonstrated greater than 50% displacement of [³H]testosterone from either acAR1 or kbAR1. In contrast, with the exception of vinclozolin, all of the xenobiotics examined demonstrated binding to acAR2 in testicular and ovarian tissues. The binding affinities were highest in the testicular tissue with M2, 2,2'5'-PCB-4-OH, and o,p'-DDD all binding with EC₅₀s less than 10 µM. The binding affinities of xenobiotics to kbAR2 in ovarian tissue were similar to their binding affinities for ovarian acAR2. The finding that AR1 and AR2 possess different binding affinities for natural androgens and synthetic steroids, as well as for xenobiotics, suggests that the activities of androgens and of certain xenobiotics will depend upon the type of AR present within the target tissue.

¹ This research was supported by PHS grant ESO 4214, Texas A&M University Sea Grant College Program grant RIMBT-3 and a NOAA grant to P.T. and the E.J. Lund Fellowship, University of Texas Marine Science Institute to T.S.

² Correspondence: Todd Sperry, The University of Washington, Dept. of Zoology, Box 315800, Seattle, WA 98195. FAX: 206 543 3041.

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