Taurog A, Dorris M L, Guziec F S
Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235.
Endocrinology. 1989 Jan;124(1):30-9. doi: 10.1210/endo-124-1-30.
We previously described an in vitro incubation system for studying the mechanism of inhibition of thyroid peroxidase (TPO)-catalyzed iodination by the antithyroid drug 1-methyl-2-mercaptoimidazole (MMI). Inhibition of iodination in this system may be reversible or irreversible, depending on the relative concentrations of iodide and MMI and on the TPO concentration. Metabolism of the drug occurs under both conditions, and in the present investigation we used 35S- and 14C-labeled MMI together with reverse phase HPLC to examine the metabolic products associated with reversible and irreversible inhibition of iodination by MMI. Under conditions of reversible inhibition, MMI was rapidly metabolized and disappeared completely from the incubation mixture. With [35S]MMI, the earliest detectable 35S-labeled product was MMI disulfide, which reached a peak after a few minutes and then declined to undetectable levels. Coincident with the decrease in disulfide was the appearance of two 35S peaks, the major one corresponding to sulfate/sulfite, and the other to a component eluting at 7.5 min. Similar results were obtained for the disulfide and for the 7.5 min metabolite with [14C]MMI. The major 14C-labeled metabolite containing no S appeared to be 1-methylimidazole. Under conditions of irreversible inhibition, MMI disulfide was also the earliest detectable 35S-labeled metabolite. However, MMI decreased more slowly, and after reaching a nadir at about 6 min returned gradually to a level about halfway between the initial and the minimum value. The reformation of MMI appeared to involve the nonenzymatic disproportionation of MMI disulfide. Formation of the 7.5 min peak was also observed, but there was no formation of sulfate/sulfite. The difference in metabolic pattern between the reversible and irreversible conditions is primarily related to the rapid inactivation of TPO that occurs under irreversible conditions. The metabolism of [35S]MMI in thyroids of rats injected with the labeled drug resembles more closely conditions of reversible inhibition, since sulfate/sulfite is the only 35S-labeled metabolite. Neither [35S]MMI disulfide nor the 7.5 min component was detected in rat thyroids in vivo. However, it was demonstrated that these components do not survive homogenization with thyroid tissue, and failure to detect them in vivo does not exclude them as likely intermediates in intrathyroidal MMI metabolism. Based on the observations reported in this study, we present a revised scheme for the mechanism of inhibition of TPO-catalyzed iodination by MMI.
我们之前描述了一种体外孵育系统，用于研究抗甲状腺药物1-甲基-2-巯基咪唑（MMI）抑制甲状腺过氧化物酶（TPO）催化碘化反应的机制。在该系统中，碘化反应的抑制可能是可逆的或不可逆的，这取决于碘化物和MMI的相对浓度以及TPO的浓度。在这两种情况下药物都会发生代谢，在本研究中，我们使用35S和14C标记的MMI以及反相高效液相色谱来检测与MMI对碘化反应的可逆和不可逆抑制相关的代谢产物。在可逆抑制条件下，MMI迅速代谢并从孵育混合物中完全消失。使用[35S]MMI时，最早可检测到的35S标记产物是MMI二硫化物，它在几分钟后达到峰值，然后下降到检测不到的水平。与二硫化物减少同时出现的是两个35S峰，主要的一个对应于硫酸盐/亚硫酸盐，另一个对应于在7.5分钟洗脱的一种成分。用[14C]MMI对二硫化物和7.5分钟代谢物也得到了类似的结果。不含硫的主要14C标记代谢物似乎是1-甲基咪唑。在不可逆抑制条件下，MMI二硫化物也是最早可检测到的35S标记代谢物。然而，MMI下降得更慢，在大约6分钟达到最低点后逐渐回升到初始值和最小值之间大约一半的水平。MMI的重新形成似乎涉及MMI二硫化物的非酶歧化反应。也观察到了7.5分钟峰的形成，但没有形成硫酸盐/亚硫酸盐。可逆和不可逆条件下代谢模式的差异主要与不可逆条件下发生的TPO快速失活有关。给大鼠注射标记药物后，甲状腺中[35S]MMI的代谢更类似于可逆抑制条件，因为硫酸盐/亚硫酸盐是唯一的35S标记代谢物。在大鼠甲状腺体内未检测到[35S]MMI二硫化物和7.5分钟成分。然而，已证明这些成分在与甲状腺组织匀浆后无法存活，在体内未检测到它们并不排除它们作为甲状腺内MMI代谢中可能的中间体。基于本研究报告的观察结果，我们提出了MMI抑制TPO催化碘化反应机制的修订方案。
