Inhibition of peroxidase-catalyzed iodination by thioamides: experimental and theoretical study of the antithyroid activity of thioamides

Abstract

The reaction of di-iodine with N-methyl-2-mercapto-benzothiazole (NMBZT) in the presence of ferric tetra-phenyl-porphyrin chloride (FeTPPCl) in 6 ∶ 3 ∶ 1 (I₂ ∶ thioamide ∶ FeTPPCl) molar ratio yields a mixture of products consisting of the iodonium {[(NMBZT)₂I]⁺·[(I₇)⁻]}_n salt, 1, and the FeTPPCl complex, 2. The compounds were characterized by X-ray diffraction analysis. The crystal structure of 1 was identical to the one already published, while of compound 2 at 294(1) K re-evaluates previous work on it. The inhibition activity of the thioamide ligands (N-methyl-2-mercapto-benzothiazole (NMBZT), 2-mercapto-benzothiazole (MBZT), 5-chloro-2-mercapto benzothiazole (CMBZT), 2-mercaptothiazolidine (MTZT), 2-mercaptopyridine (PySH), thiourea (TU), 1,3-bis(3-pyridyl-methyl)-2-thiourea (PyTU), 2-mercapto-3,4,5,6-tetrahydropyrimidine (THP), 2-mercaptopyrimidine (PmSH), 6-propyl-2-thiouracil (PTU), 6-methyl-2-thiouracil (MTU), di-thiouracil-2,4 (DTUC), 2-mercapto-4-methyl-pyrimidine hydro chloride (MPmCl), methimazole (MMI), 2-mercapto-benzimidazole (BZIM), 5-nitro-2-mercapto-benzimidazole (NiMBZIM), 5-methyl-2-mercapto-benzimidazole (MBZIM), 2-hydroxy-pyrimidine (PmOH), 2-hydroxypyridine (PyOH)) against the catalytic oxidation of iodides by H₂O₂ in the presence of FeTPPCl (a model of the active site of Thyroid Peroxidase (TPO)) was measured as a result of the yield of I₃⁻ resulting from the oxidation of I⁻ and was also evaluated theoretically using electronic structure calculation methods (DFT). The compounds exhibiting high and intermediate degree of inhibition (between 27% and 17%) are MBZT, MMI, PySH, NMBZT, PmSH, MBZIM, NiBZIM and MTZT. The kinetic study of inhibition of lactoperoxidase (LPO) (a model of TPO) is based on the assessment of LPO activity in the presence of its inhibitors—the thioamide compounds. LPO activity can be assayed by measuring guaiacol (gua) peroxidation to tetraguaiacol in the presence of H₂O₂. The results illustrated that the lowest IC₅₀ values (<10 μM) required for the LPO inhibition were exhibited by DTUC, NiMBZIM, MBZT, PySH thioamides. Low concentrations between 10 and 30 μM are obtained in the case of CMBZT, BZIM, MPmCl, PmSH, MBZIM, and MMI. DFT calculations of the geometric and energetic profiles threw light on the mechanism of the inhibition process by the thioamide compounds scrutinizing all crucial reaction steps and are in agreement with the experimental results.