Glaucous gulls (Larus hyperboreus) from Svalbard, Norway (marine), and herring gulls (Larus argentatus) from the Laurentian Great Lakes (freshwater) of North America are differentially exposed to persistent and bioaccumulative anthropogenic contaminants, such as polychlorinated biphenyls (PCBs) and polybrominated diphenyl ether (PBDE) flame retardants and metabolic products. Such compounds can potentially perturb hormone transport via binding interactions with proteins such as transthyretin (TTR, prealbumin). In this present study, we isolated, cloned and sequenced TTR cDNA from the brain and liver of two species (herring and glaucous gull), which, to our knowledge, is the first report describing the TTR nucleic acid and amino acid sequences from any gull species. Identical TTR nucleotide and amino acid sequences were obtained from both gull species (liver and brain). Recombinant TTR (rTTR) was expressed and purified, and determined as a monomer of 18 kDa and homodimer of 36 kDa that putatively is comprised of the two protein monomers. Concentration dependent, competitive TTR-binding curves with each of the natural TTR ligands 3,5,3'-triiodothyronine (T(3)) and thyroxine (T(4)) were generated as well as by treatment with a range of concentrations (10(-3)-10(5)nM) of 2,2',3,4',5,5',6-heptaCB (CB187), 2,2',4,4'-tetrabromoDE (BDE47), and hydroxyl- (OH) and methoxyl (MeO)-containing analogs (i.e., 4-OH-CB187, 6-OH-BDE47, 4'-OH-BDE49, 4-MeO-CB187, and 6-MeO-BDE47). Relative to the nonsubstituted BDE47 and CB187 and their MeO-substituted analogs, the OH-substituted analogs all had lower K(i) and K(d) values, indicating greater affinity and more potent competitive binding to both T(3) and T(4). The OH-substitution position and/or the diphenyl ether substitution of the four bromine atoms resulted in more potent, greater affinity, and greater relative potency for 4'-OH-BDE49 relative to 6-OH-BDE47. CB187 was more comparable in binding potency and affinity to 4-OH-CB187, then was 6-OH-BDE47 and 4'-OH-BDE49 relative to BDE47 where the binding potency and affinity was several orders of magnitude greater for 6-OH-BDE47 and 4'-OH-BDE49. This indicated that the combination of the more thyroid hormone-like brominated diphenyl ether backbone (relative to the chlorinated biphenyl backbone), and in combination of having an OH-group, results in a more effective competitive ligand on gull TTR relative to both T(3) and T(4). Known circulating levels of 4-OH-CB187, 6-OH-BDE47, and 4'-OH-BDE49 in the plasma of free-ranging Svalbard glaucous gulls were comparable to the concentration of in vitro competitive potency of T(3) and T(4) with gull TTR. These results suggest that environmentally relevant and selected OH-containing PCB, and to a lesser extent PBDE congeners have the potential to be physiologically effective in these gull species via perturbation of T(4) and T(3) transport.

Environmentally relevant concentrations of selected polychlorinated biphenyl (PCB) and polybrominated diphenyl ether (PBDE) flame retardant congeners and their hydroxylated (OH) and methoxylated (MeO) analogues that can perturb thyroid hormone-dependent processes were comparatively examined with respect to competitive binding with thyroxine (T(4)) and 3,5,3'-triiodothyronine (T(3)) thyroid hormones (THs) on recombinant human and gull albumin and transthyretin transport proteins. The liver tissue was from glaucous gulls (Larus hyperboreus) from Norway and herring gulls (Larus argentatus) from the Great Lakes of North America. We isolated, cloned, sequenced, purified, and expressed the cDNA (cDNA) of albumin from liver of herring and glaucous gull. Albumin amino acid sequences were identical for both gull species. Concentration-dependent, competitive binding curves were generated for T(4) and T(3) binding alone and for selected substrates using gull and human recombinant albumin (recALB). Human recALB had high preference for T(4) relative to T(3), whereas it was reversed for gull recALB. Binding assays with recALB and recTTR gull proteins showed that relative to 2,2',4,4'-tetrabromoDE (BDE-47) and 2,2',3,4',5,5',6-heptaCB (CB-187) and the MeO-substituted (4-MeO-CB187 and 6-MeO-BDE47) analogues, 4-OH-CB187, 6-OH-BDE47, and 4'-OH-BDE49 had the greatest binding affinity and potency, and that competitive binding was greater for T(3) relative to T(4). These results indicate that xenobiotic ligand binding to human ALB or TTR cannot be used as a surrogate for gull binding interactions. The combination of TH-like brominated diphenyl ether backbone (relative to the chlorinated biphenyl backbone), and the presence of OH-group produced a more effective competitive ligand on human and gull recALB and recTTR relative to both T(3) and T(4). This suggests the possibility that OH-substituted organohalogen contaminants may be an exposure concern to the thyroid system in free-ranging gulls as well as for humans.

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