Oak Ridge Institute for Science and Education, Oak Ridge, TN, United States; Integrated Systems Toxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC, United States.
Toxicology. 2013 Oct 4;312:97-107. doi: 10.1016/j.tox.2013.08.006. Epub 2013 Aug 17.
Thyroperoxidase (TPO), the enzyme that catalyzes the synthesis of thyroid hormone, is a known target for thyroid-disrupting chemicals. In vivo toxicological evidence supporting TPO-inhibition as one molecular-initiating event that leads to thyroid disruption is derived largely from rat models; however, a significant fraction of research on the inhibition of TPO by xenobiotics has been conducted using porcine TPO. The current work tested the hypothesis that porcine and rat thyroid microsomes exposed to TPO-inhibiting chemicals would demonstrate different responses in a guaiacol oxidation assay. A primary objective of this work is to establish the degree of concordance between rat and porcine TPO inhibition data. Microsomes were isolated from both rat and pig thyroid glands, and the guaiacol oxidation assay was performed for a training set of 12 chemicals, including previously reported TPO inhibitors, thyroid-disrupting chemicals thought to perturb other targets, and several previously untested chemicals, to determine the relative TPO inhibition responses across species. Concentration-response curves were derived for methimazole (MMI), dibutylphthalate (DBP), diethylhexylphthalate (DEHP), diethylphthalate (DEP), 3,5-dimethylpyrazole-1-methanol (DPM), iopanoic acid (IOA), 2-mercaptobenzothiazole (MBT), sodium perchlorate (PERC), p-nonylphenol (PNP), 4-propoxyphenol (4POP), 6-propylthiouracil (PTU), and triclosan (TCS). MMI, PTU, MBT, DPM, 4POP, and at extremely high concentrations, PERC, inhibited TPO activity. Results demonstrated a strong qualitative concordance of response between the two species. All chemicals that inhibited TPO in porcine microsomes also inhibited TPO in rat microsomes. Hill model-derived IC50 values revealed approximate 1.5- to 50-fold differences in relative potency to MMI between species for positive chemicals. DPM, MBT, 4POP, and PTU exhibited greater relative potency to MMI using rat TPO versus porcine TPO, but rank order potency for inhibition was similar for the other test chemicals, with: PTU>MBT>DPM>4POP>PERC for rat TPO and MBT>PTU>DPM>4POP>PERC for porcine TPO. These data support the extrapolation of porcine TPO data to potential thyroid-disrupting activity in rodent models to evaluate TPO-inhibiting chemicals.
甲状腺过氧化物酶（TPO）是催化甲状腺激素合成的酶，是一种已知的甲状腺破坏化学物质的靶标。支持 TPO 抑制作为导致甲状腺破坏的一个分子起始事件的体内毒理学证据主要来自大鼠模型；然而，外源化学物对 TPO 抑制的研究很大一部分是使用猪 TPO 进行的。目前的工作检验了这样一个假设，即暴露于 TPO 抑制性化学物质的猪和大鼠甲状腺微粒体在愈创木酚氧化测定中会表现出不同的反应。这项工作的一个主要目的是确定大鼠和猪 TPO 抑制数据之间的一致性程度。从小鼠和猪甲状腺中分离出微粒体，并对 12 种化学物质进行愈创木酚氧化测定，包括先前报道的 TPO 抑制剂、被认为扰乱其他靶标的甲状腺破坏化学物质以及几种以前未测试的化学物质，以确定跨物种的相对 TPO 抑制反应。为甲巯咪唑（MMI）、邻苯二甲酸二丁酯（DBP）、邻苯二甲酸二（2-乙基己基）酯（DEHP）、邻苯二甲酸二乙酯（DEP）、3,5-二甲基吡唑-1-甲醇（DPM）、碘帕醇（IOA）、2-巯基苯并噻唑（MBT）、高氯酸钠（PERC）、壬基酚（PNP）、4-正丙氧基苯酚（4POP）、6-丙基硫氧嘧啶（PTU）和三氯生（TCS）推导了浓度-反应曲线。MMI、PTU、MBT、DPM、4POP 和在极高浓度下，PERC 抑制 TPO 活性。结果表明，两种物种的反应具有很强的定性一致性。在猪微粒体中抑制 TPO 的所有化学物质也抑制了大鼠微粒体中的 TPO。用 Hill 模型推导的 IC50 值表明，对于阳性化学物质，物种间对 MMI 的相对效力约为 1.5 至 50 倍。DPM、MBT、4POP 和 PTU 对大鼠 TPO 的相对效力大于猪 TPO，但对其他测试化学物质的抑制作用强度相似，PTU>MBT>DPM>4POP>PERC 用于大鼠 TPO 和 MBT>PTU>DPM>4POP>PERC 用于猪 TPO。这些数据支持将猪 TPO 数据外推到评估 TPO 抑制性化学物质的啮齿动物模型中的潜在甲状腺破坏活性。
