Design of anti-thyroid drugs: Binding studies and structure determination of the complex of lactoperoxidase with 2-mercaptoimidazole at 2.30 Å resolution

Abstract

Lactoperoxidase (LPO) belongs to mammalian heme peroxidase superfamily, which also includes myeloperoxidase (MPO), eosinophil peroxidase (EPO), and thyroid peroxidase (TPO). LPO catalyzes the oxidation of a number of substrates including thiocyanate while TPO catalyzes the biosynthesis of thyroid hormones. LPO is also been shown to catalyze the biosynthesis of thyroid hormones indicating similar functional and structural properties. The binding studies showed that 2-mercaptoimidazole (MZY) bound to LPO with a dissociation constant of 0.63 µM. The inhibition studies showed that the value of IC was 17 µM. The crystal structure of the complex of LPO with MZY showed that MZY bound to LPO in the substrate-binding site on the distal heme side. MZY was oriented in the substrate-binding site in such a way that the sulfur atom is at a distance of 2.58 Å from the heme iron. Previously, a similar compound, 3-amino-1,2,4-triazole (amitrole) was also shown to bind to LPO in the substrate-binding site on the distal heme side. The amino nitrogen atom of amitrole occupied the same position as that of sulfur atom in the present structure indicating a similar mode of binding. Recently, the structure of the complex of LPO with a potent antithyroid drug, 1-methylimidazole-2-thiol (methimazole, MMZ) was also determined. It showed that MMZ bound to LPO in the substrate-binding site on the distal heme side with 2 orientations. The position of methyl group was same in the 2 orientations while the positions of sulfur atom differed indicating a higher preference for a methyl group.

Regulation of Tyrosinase Enzyme Activity by Glutathione Peroxidase Mimics

“…21b,c,58 An X-ray study of the 7bound LPO complex has shown that 7 inhibits the LPOcatalyzed reactions by coordinating to the Fe-center of the enzyme via the C�S moiety (Fe•••S = 2.580 Å) (Figure S43). 58 Thus, it will be interesting to investigate whether the inhibitory activities of these selones/thiones (1−9) follow a similar trend in the case of the LPO-catalyzed oxidation reaction of 2,2′azino-bis-3-ethyl-benthiazoline-6-sulfonic acid (ABTS), as they have shown in the case of the inhibition of copper-containing tyrosinase enzyme. However, among all thione derivatives (4− 9), we found that the imidazole-based thiones 4 and 7 showed better inhibition efficiency than their benzimidazole and naphthimidazole counterparts, with the highest inhibitory potency observed in the case of commercially available antithyroid drug methimazole (4) with an IC 50 value of 7.4 μM (Table 4).…”

“…Imidazole-based thiones 4 and 7 and selone 1 are known to inhibit iron-containing metalloenzyme lactoperoxidase (LPO)-catalyzed oxidation reactions, which belong to the mammalian heme peroxidase superfamily. ,, An X-ray study of the 7 -bound LPO complex has shown that 7 inhibits the LPO-catalyzed reactions by coordinating to the Fe-center of the enzyme via the CS moiety (Fe···S = 2.580 Å) (Figure S43). Thus, it will be interesting to investigate whether the inhibitory activities of these selones/thiones ( 1 – 9 ) follow a similar trend in the case of the LPO-catalyzed oxidation reaction of 2,2′-azino-bis-3-ethyl-benthiazoline-6-sulfonic acid (ABTS), as they have shown in the case of the inhibition of copper-containing tyrosinase enzyme. However, among all thione derivatives ( 4 – 9 ), we found that the imidazole-based thiones 4 and 7 showed better inhibition efficiency than their benzimidazole and naphthimidazole counterparts, with the highest inhibitory potency observed in the case of commercially available antithyroid drug methimazole ( 4 ) with an IC 50 value of 7.4 μM (Table ).…”