Inhibition of Hepatic Outer Ring Monodeiodination of Thyroxine and 3,3′,5′-Triiodothyronine by Sodium Salicylate*

Abstract

We have studied, in vitro and in vivo, the effect of sodium salicylate on outer ring monodeiodination of T₄ to T₃ and of rT₃ to 3,3′-diiodothyronine by rat liver. For T₄ to T₃ conversion studies, 2.5 μm T₄ were incubated with 0.22 geq rat liver in 0.15 m phosphate buffer (pH 7.4) for 15 min in the presence of 0–2 mm sodium salicylate, and the amount of T₃ produced was quantitated by a specific RIA. Salicylate caused a dose-dependent reduction in T₃ generation, with 1.1 mm salicylate causing a 50% reduction. The inhibition of T₃ production persisted even in the presence of dithiothreitol (2 mm). Phenacetin (up to 1.9 mm) and acetaminophen (up to 6 mm) caused no detectable effect on T₃ production. However, 8-anilino-1-naphthalene sulfonic acid, which, like salicylate, inhibits the binding of iodothyronines to T₄-binding globulin, reduced T₄ to T₃ conversion in a dose-dependent manner; a 50% reduction was caused by about 1.7 mm 8-anilino-1-naphthalene sulfonic acid. A Lineweaver-Burk plot of the data obtained using 0.26–7.7 μm T₄ with and without 0.56, 1.4, or 2.8 mm salicylate suggested that salicylate is a competitive inhibitor of T₄ to T₃ conversion (Ki = ∼0.48 mm). Similar studies with rT₃ (0.04–1.5 mm) with and without salicylate (2.8 mm) suggested that salicylate is also a competitive inhibitor of outer ring monodeiodination of rT₃ (Ki = ∼4.2 mm). For in vivo studies, groups of Sprague-Dawley rats were injected ip daily with T₄ (1 μg/100g·day) with and without sodium salicylate (24 or 48 mg/day) for 4 days, and serum was examined for T₄) T₃, and salicylate concentrations, while liver homogenates were examined for nonprotein sulfhydryl groups (NP-SH) and outer ring monodeiodination of T₄ and rT₃. Administration of the two doses (24 and 48 mg/day) of salicylate resulted in mean serum salicylate concentrations of 11 and 18 mg/dl, respectively, and there was (respectively) a 58% and 70% reduction in serum T₄, a 30% and 62% reduction in serum T₃, a 30% and 54% reduction in T₃ production from T₄, and a 28% and 49% reduction in 3,3′-diiodothyronine production from rT₃. Hepatic concentration of NP-SH was unaffected by salicylate treatment. The various data suggest that 1) salicylates competitively inhibit outer ring monodeiodination of T₄ and rT₃, 2) salicylate administration does not effect hepatic NP-SH, and 3) the binding site of T₄-binding globulin and the active site of the hepatic 5′-iodothyronine monodeiodinase may be similar. (Endocrinology106: 1728, 1980)