Cochaux P, Van Sande J, Swillens S, Dumont J E
Institute of Interdisciplinary Research, School of Medicine, Free University of Brussels, Belgique.
Eur J Biochem. 1987 Dec 30;170(1-2):435-42. doi: 10.1111/j.1432-1033.1987.tb13718.x.
The characteristics of the iodide-induced inhibition of cyclic AMP accumulation in dog thyroid slices have been previously described [Van Sande, J., Cochaux, P. and Dumont, J. E. (1985) Mol. Cell. Endocrinol. 40, 181-192]. In the present study we investigated the characteristics of the iodide-induced inhibition of adenylate cyclase activity in dog and horse thyroid. The inhibition of cyclic AMP accumulation by iodide in stimulated horse thyroid slices was similar to that observed in dog thyroid slices. The inhibition was observed in slices stimulated by thyroid-stimulating hormone, cholera toxin and forskolin. Increasing the concentration of the stimulators did not overcome the iodide-induced inhibition. Adenylate cyclase activity, assayed in crude homogenates or in plasma-membrane-containing particulates (100,000 x g pellets), was lower in homogenates or in particulates prepared from iodide-treated slices than from control slices. This inhibition was observed on the cyclase activity stimulated by forskolin, fluoride or guanosine 5'-[beta, gamma-imino]triphosphate, but also on the basal activity. It was relieved when the homogenate was prepared from slices incubated with iodide and methimazole. Similar results were obtained with dog thyroid. The inhibition persisted when the particulate fraction was washed three times during 1 h at 100,000 x g, in the presence of bovine serum albumin or increasing concentration of KCl. It was similar whatever the duration of the cyclase assay, in a large range of protein concentration. These results indicate that a stable modification of adenylate cyclase activity, closely related to the plasma membrane, was induced when slices were incubated with iodide. Iodide inhibition did not modify the affinity of adenylate cyclase for its substrate (MgATP), but induced a decrease of the maximal velocity of the enzyme. The percentage inhibition was slightly decreased when Mg2+ concentration increased, and markedly decreased when Mn2+ concentration increased. A detectable adenylate cyclase activity was demonstrated when intact slices were incubated in the presence of [alpha-32P]ATP, probably because of the presence of broken cells produced during the slicing. Iodide had no direct effect on this cyclase system, which confirms that iodide needs the integrity of the cell to induce the inhibition and suggests that the inhibition is not transmitted between cells.
碘化物对犬甲状腺切片中环磷酸腺苷（cAMP）积累的抑制特性已有前人描述[Van Sande, J., Cochaux, P. 和 Dumont, J. E. (1985) Mol. Cell. Endocrinol. 40, 181 - 192]。在本研究中，我们调查了碘化物对犬和马甲状腺中腺苷酸环化酶活性的抑制特性。碘化物对刺激后的马甲状腺切片中cAMP积累的抑制作用与犬甲状腺切片中观察到的相似。在促甲状腺激素、霍乱毒素和福斯高林刺激的切片中均观察到了这种抑制作用。增加刺激剂的浓度并不能克服碘化物诱导的抑制作用。在粗匀浆或含质膜颗粒（100,000×g沉淀）中测定的腺苷酸环化酶活性，在由碘化物处理过的切片制备的匀浆或颗粒中低于对照切片。这种抑制作用在福斯高林、氟化物或鸟苷5'-[β,γ-亚氨基]三磷酸刺激的环化酶活性中观察到，也在基础活性中观察到。当从用碘化物和甲巯咪唑孵育的切片制备匀浆时，抑制作用得以缓解。犬甲状腺也得到了类似的结果。当颗粒部分在100,000×g下在1小时内洗涤三次，在存在牛血清白蛋白或增加KCl浓度的情况下，抑制作用仍然存在。在很大范围的蛋白质浓度下，无论环化酶测定的持续时间如何，结果都是相似的。这些结果表明，当切片用碘化物孵育时，诱导了与质膜密切相关的腺苷酸环化酶活性的稳定改变。碘化物抑制并未改变腺苷酸环化酶对其底物（MgATP）的亲和力，但诱导了酶的最大速度降低。当Mg2+浓度增加时，抑制百分比略有降低，而当Mn2+浓度增加时，抑制百分比明显降低。当完整切片在[α-32P]ATP存在下孵育时，可检测到腺苷酸环化酶活性，这可能是由于切片过程中产生的破碎细胞的存在。碘化物对该环化酶系统没有直接影响，这证实了碘化物需要细胞的完整性来诱导抑制作用，并表明抑制作用不会在细胞间传递。
