Hayase K, Yonekawa G, Yokogoshi H, Yoshida A
Department of Home Economics, Aichi University of Education, Japan.
J Nutr. 1991 Jul;121(7):970-8. doi: 10.1093/jn/121.7.970.
The purpose of the present study was to elucidate the mechanism by which thyroid hormone alters urea synthesis. The relative importance of urea cycle enzyme activities, substrate levels or the levels of urea cycle intermediates on urea production was investigated in a set of four experiments in which rats were fed a diet supplemented with 6-propyl-2-thiouracil (PTU, a thyroid inhibitor) and treated with triiodothyronine (T3). Compared with that of normal or control rats, the plasma level of T3 was significantly lower in rats fed a diet containing PTU only and elevated in rats given PTU + T3. Urinary excretion of urea and the liver content of ornithine in rats given PTU + T3 were significantly lower than in rats given PTU only. The liver level of ornithine was closely correlated to the excretion of urea in the present study. Fractional rates of protein synthesis in liver, kidney and small intestine were lower in the hypothyroid group. However, most free amino acid concentrations, except ornithine in liver and plasma and the activity of hepatic argininosuccinate synthetase (EC 6.3.4.5) of hypothyroid rats, were significantly reduced as compared with those of control or hyperthyroid rats. The results indicate that the increased hepatic ornithine content in the hypothyroid rats may be one of the regulatory factors causing changes in urea synthesis.
本研究的目的是阐明甲状腺激素改变尿素合成的机制。在一组四个实验中，研究了尿素循环酶活性、底物水平或尿素循环中间产物水平对尿素生成的相对重要性，实验中给大鼠喂食添加了6-丙基-2-硫氧嘧啶（PTU，一种甲状腺抑制剂）的饲料，并给予三碘甲状腺原氨酸（T3）进行处理。与正常或对照大鼠相比，仅喂食含PTU饲料的大鼠血浆T3水平显著降低，而给予PTU + T3的大鼠血浆T3水平升高。给予PTU + T3的大鼠尿素尿排泄量和肝脏鸟氨酸含量显著低于仅给予PTU的大鼠。在本研究中，肝脏鸟氨酸水平与尿素排泄密切相关。甲状腺功能减退组肝脏、肾脏和小肠的蛋白质合成分数率较低。然而，与对照或甲状腺功能亢进大鼠相比，除甲状腺功能减退大鼠肝脏和血浆中的鸟氨酸以及肝脏精氨琥珀酸合成酶（EC 6.3.4.5）活性外，大多数游离氨基酸浓度显著降低。结果表明，甲状腺功能减退大鼠肝脏鸟氨酸含量增加可能是导致尿素合成变化的调节因素之一。
