Abstract
We have compared the relative potencies of various thiouracils (TUs) and related compounds in inhibition of 5′-monodeiodination of T4 to T3 by rat liver homogenate. T4 (2.5 (μM) was incubated with 0.13 geq liver homogenate in 0.1 M Tris buffer (pH 7.4) for 30 min in the presence or absence of various test agents, and the amount of T3 produced was quantitated by a specific RIA. Thiourea, thiobarbiturate (2-thiobarbituric acid), imidazoles and imidazolines (2-thiohydantoin, 2,4-dihydroxy-5- thiohydantoin, and 2-mercaptoimidazole), and thiopyrimidines (4-mercaptopyrimidine and 2,4-hydroxy-5-thiopyrimidine) had 3% or less of the activity of 2-TU. Among TU derivatives, 5-iodo-2-TU was the most active agent, with about a 30-fold greater inhibiting activity (on a molar basis) than 2-TU. 6-N-propyl TU (PTU) exhibited 2.5 times the inhibiting activity of TU, whereas 6-carbethoxy-2-TU had 1.4 times the activity, and others followed in descending order: 5-N-butyl-6-methyl-TU (1.3 times), 4-hydroxy-6-methyl-TU (0.4 t...
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