(Table 1) Concentration of PCB and other contaminants in blood plasma of polar bears (Ursus maritimus) from Svalbard

Persistent chemicals accumulate in the arctic environment due to their chemical reactivity and physicochemical properties and polychlorinated biphenyls (PCBs) are the most concentrated pollutant class in polar bears (Ursus maritimus). Metabolism of PCB and polybrominated biphenyl ether (PBDE) flame-retardants alter their toxicological properties and these metabolites are known to interfere with the binding of thyroid hormone (TH) to transthyretin (TTR) in rodents and humans. In polar bear plasma samples no binding of [125I]-T4 to TTR was observed after incubation and PAGE separation. Incubation of the plasma samples with [14C]-4-OH-CB107, a compound with a higher binding affinity to TTR than the endogenous ligand T4 resulted in competitive binding as proven by the appearance of a radio labeled TTR peak in the gel. Plasma incubation with T4 up to 1 mM, a concentration that is not physiologically relevant anymore did not result in any visible competition. These results give evidence that the binding sites on TTR for T4 in wild living polar bears are completely saturated. Such saturation of binding sites can explain observed lowered levels of THs and could lead to contaminant transport into the developing fetus.持久性化学品（Persistent chemicals）凭借其化学反应性与理化特性，可在北极环境中持续累积；多氯联苯（polychlorinated biphenyls，PCBs）是北极熊（Ursus maritimus）体内浓度最高的污染物类别。多氯联苯与多溴联苯醚（polybrominated biphenyl ether，PBDE）阻燃剂的代谢过程会改变其毒理学特性，且已有研究证实，此类代谢物会在啮齿类动物与人体中干扰甲状腺激素（thyroid hormone，TH）与运甲状腺素蛋白（transthyretin，TTR）的结合。对北极熊血浆样本进行孵育及聚丙烯酰胺凝胶电泳（polyacrylamide gel electrophoresis，PAGE）分离后，未观测到碘[125I]-甲状腺素T4与TTR的结合。将血浆样本与[14C]-4-羟基多氯联苯107（[14C]-4-OH-CB107，一种对TTR的结合亲和力高于内源性配体T4的化合物）共同孵育后，凝胶中出现放射性标记的TTR峰，从而证实发生了竞争性结合。而将血浆样本与浓度达1mM的T4（该浓度已超出生理相关范围）共同孵育时，未观测到任何可见的竞争性结合现象。上述研究结果表明，野生北极熊体内TTR上结合T4的位点已完全被占据。这种结合位点的饱和状态可解释已观测到的甲状腺激素水平降低现象，且可能导致污染物转运进入发育中的胎儿体内。