Biomimetic Studies on Anti-Thyroid Drugs and Thyroid Hormone Synthesis

Article abstract of DOI:10.1021/ja039860g

The selenium analogues of anti-thyroid drugs exhibit their anti-thyroid action by a mechanism different from that of MMI. The selenium analogue of MMI and related selenium compounds exhibit high GPx activity, providing a novel method for the reversible in

Full text of DOI:10.1021/ja039860g

Published on Web 02/14/2004
Biomimetic Studies on Anti-Thyroid Drugs and Thyroid Hormone Synthesis
Gouriprasanna Roy, Munirathinam Nethaji, and G. Mugesh*
Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received November 29, 2003; E-mail: mugesh@ipc.iisc.ernet.in
Thyroxine (T4) is the main secretory product of the thyroid gland,
and the deiodination of this prohormone to the biologically active
hormone, 3,5,3′-triiodothyronine (T3), is the first step in thyroid
hormone action. It is well known that type I iodothyronine
deiodinase (ID-1), a selenocysteine-containing enzyme, is respon-
sible for most of this conversion.1 The activation of thyroid
stimulating hormone (TSH) receptor by autoantibodies leads to an
overproduction of thyroid hormones, which can be controlled by
specific inhibitors such as 6-n-propyl-2-thiouracil (1, PTU) and
methimazole (3, MMI) that either block the thyroid hormone
biosynthesis or reduce the conversion of T4 to T3.1
Figure 1. Molecular structure of 5.
Table 1. Inhibition of LPO Activity by MMI, and 5-7
no.
compound
IC50 (µM)a
1
2
3
4
MMI
6.6
16.7
15.9
20.1
5
6
7
a Concentration of the compound causing 50% inhibition.
Although these are the most commonly employed drugs for
hyperthyroidism, the mechanism of their action is still not clear. It
has been proposed that these drugs form stable electron donor-
acceptor complexes with I2 and divert oxidized iodides away from
thyroglobulin, which effectively reduces the biosynthesis of thyroid
hormones.2 Another mechanism suggests that these drugs may
coordinate to the thyroid peroxidase (TPO), a heme enzyme, which
catalyzes the oxidation of iodides and the coupling of iodothyrosine
residues of thyroglobulin.3 It has also been reported that PTU can
block the conversion of T4 to T3 by reacting with the selenenyl
iodide intermediate (E-SeI) of ID-1 to form a selenenyl sulfide as
a dead end product.4
Figure 2. Inhibition curves for the LPO-catalyzed oxidation of ABTS.
Recently, the selenium analogues 2 (PSeU) and 4 (MSeI)
attracted considerable attention because these are expected to be
more nucleophilic than their sulfur analogues and the formation of
an -Se-Se- bond with ID-1 may occur more readily than the
formation of an -Se-S- bond.5 However, the selenium com-
pounds were found to be less potent than the sulfur analogues, and
the reason for the relatively low potency is still not clear.5 In view
of this, we have undertaken a biomimetic study on MSeI to probe
the mechanism by which the selenium derivatives exert their
inhibitory action. In contrast to MMI, the selenium analogue exists
in a diselenide form (5, MSeIox, Figure 1).6
may inhibit the LPO by a different mechanism. To verify this
hypothesis, we have carried out the inhibition experiments with
ebselen 6 and selenazole 7,6 which do not have any thione or
selenone moiety that has been proposed to be important for TPO
or LPO inhibition. Interestingly, the activities of 6 and 7 were found
to be comparable to that of 5, indicating that thione or selenone
moieties are not required for the inhibition of LPO. To the best of
our knowledge, compounds 5-7 represent the first examples of
selenium compounds that inhibit the LPO activity.
In this study, we employed the Fe-containing lactoperoxidase
(LPO) as a model for TPO, because both of the enzymes have been
shown to have similar properties.7 The IC50 values for the inhibition
of LPO-catalyzed oxidation 2,2′-azino-bis-3-ethylbenzthiazoline
sulfonic acid (ABTS) by the test compounds are summarized in
Table 1, and the inhibition curves for MMI and 5 are shown in
Figure 2.
As expected, MMI exhibited a strong inhibition with an IC50
value of 6.6 µM. The selenium analogue (5) also showed a strong
inhibition, although its activity was found to be almost 2 times
lower than that of MMI. This suggests that the selenium analogue
Recent studies on thyroid hormone metabolism suggest that
glutathione peroxidase (GPx),8 another selenoenzyme present in
the thyroid gland, degrades intracellular H2O2 and thereby inhibits
the iodination reactions.9 These observations indicate the possibility
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10.1021/ja039860g CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Reaction of 8 with MMI and MSeI
Table 2. Initial Rates (v0) for the Reduction of H2O2 in the
Presence of Selenium Catalystsa
no.
compound
GPxb
MSeIox (5)
Ebselen (6)
7
GPx + MSeIox
ebselen + MSeIox
7 + MSeIox
v0 (µM min-1
)
1
2
3
4
5
6
7
30.55 ( 0.87
81.05 ( 0.40
78.46 ( 0.63
132.70 ( 2.84
106.69 ( 3.97
150.56 ( 3.53
195.52 ( 2.03
readily cleaved by DTT to produce 11. This suggests that, although
MMI and MSeI can form the corresponding selenenyl sulfides and
diselenides with stable selenenyl iodides, the resulting compounds
are not stable under reducing conditions. This is in agreement with
the recent observations that MSeI exhibits very weak inhibition
on ID-1 even at higher concentrations.5c
a Conditions: GSH, 1 mM; EDTA, 1 mM; GSSG reductase, 0.6 unit/
mL; NADPH, 0.2 mM; H2O2, 1 mM; selenium catalysts, 0.025 mM; in 0.1
M potassium phosphate buffer, pH 7.3. b GPx: 5 nM.
In summary, we have shown that the selenium analogues of anti-
thyroid drugs exhibit their anti-thyroid activity by a mechanism
different from that of MMI. The inhibition of TPO by selenium
compounds is mainly due to their ability to act as GPx mimics,
and the insensitivity of ID-1 toward MSeI is due to the inability of
the drug to form a stable -Se-Se- bond. This study suggests that
suitably designed selenium compounds may solve the problems
associated with PTU and MMI, which are irreversible inhibitors
of TPO. Alternatively, the anti-thyroid drugs, together with GPx,
may constitute a defense system against reactive oxygen species
in the thyroid gland.
that some of the anti-thyroid drugs may inhibit the thyroid hormone
biosynthesis by reducing H2O2 (GPx-like activity), because the
oxidation of iron center in TPO by H2O2 is the first step in thyroid
hormone synthesis. Therefore, we focused our attention on the GPx
activity of MSeIox and some related compounds. Interestingly,
MSeIox exhibited high GPx activity, providing a novel mechanism
for its inhibitory action. The activity of MSeIox was found to be
comparable to that of ebselen (6),10 a well-known GPx mimic (Table
2). On the other hand, the sulfur analogue, MMI, did not show
any noticeable activity under identical experimental conditions.
The high GPx activity of 5-7 suggests that the selenium
analogues of the anti-thyroid drugs and other GPx mimics inhibit
the LPO activity by reducing H2O2 and these compounds may also
act as antioxidants and protect cells from oxidative damage. It has
been suggested that an increase in the H2O2 concentration in the
thyroid could damage the gland and be an important factor in the
pathophysiology of myxedematous cretinism.11
Acknowledgment. This study was supported by the Department
of Science and Technology (DST), New Delhi, India.
Supporting Information Available: Experimental procedures
(PDF) and X-ray data for 5 and 7 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
We also studied the effect of 3 and 5 on the GPx activity of the
natural enzyme and some GPx mimics because the electrophilic
reactivity of selenium in the selenenic acid (E-SeOH) intermediate
in the GPx cycle is similar to that of the E-SeI intermediate in the
deiodinase cycle. Therefore, the anti-thyroid drugs may interfere
with GPx by reacting with the E-SeOH to form stable -Se-S-
or -Se-Se- bonds. As it can be seen from Table 2, the activities
of GPx, ebselen (6), and compound 7 in the presence of MSeIox
are almost equal to the sum of the initial reduction rates observed
in the individual cases. The sulfur compound, MMI, also did not
inhibit the GPx or the model compounds, suggesting that the anti-
thyroid drugs do not interfere with other selenoenzymes such as
GPx in the thyroid gland.
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(6) Crystal data for 5 (C8H10N4Se2): Mr ) 320.12, monoclinic, space group
C2/c, a ) 12.618(7), b ) 7.536(4), c ) 11.418(6) Å, â ) 97.446(9)°, V
) 1076.5(10) Å3, Z ) 4, Fcalcd ) 1.975 Mg/m3, Mo KR radiation (λ )
0.71073 Å), T ) 293(2) K; R1 ) 0.0349, wR2 ) 0.0995 (I > 2σ(I)); R1
) 0.0387, wR2 ) 0.1021 (all data). Crystal data for 7 (C16H20N2O3Se):
Mr ) 367.30, triclinic, space group P, a ) 9.486(6), b ) 9.965(6), c )
10.953(6) Å, R ) 91.403(9); â ) 111.942(8); γ ) 118.051(8)°, V ) 821.8-
(8) Å3, Z ) 2, Fcalcd ) 1.484 Mg/m3, Mo KR radiation (λ ) 0.71073 Å),
T ) 293(2) K; R1 ) 0.0264, wR2 ) 0.0674 (I > 2σ(I)); R1 ) 0.0297,
wR2 ) 0.0693 (all data).
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