The Role of Sulfhydryl Groups on the Impaired Hepatic 3′,3,5-Triiodothyronine Generation from Thyroxine in the Hypothyroid, Starved, Fetal, and Neonatal Rodent

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Abstract

The role of nonprotein sulfhydryl groups (NPSH) in the decreased in vitro hepatic 3′,3,5-triiodothyronine (T3) generation from thyroxine (T4) in the starved, hypothyroid, fetal and 1- to 4-d-old neonatal rat and dwarf mouse was assessed. NPSH were measured in fresh 25% liver homogenates prepared in 0.1 M PO4/10 mM EDTA buffer. As compared with values in adult male rats, NPSH concentration was decreased in the 2-d-starved (1.1±0.04 (mean±SE) vs. 2.2±0.15 mmol/250 g wet liver weight, P < 0.001), fetal (1.0±0.04 vs. 3.2±0.08, P < 0.001), 1-d-old neonatal (1.1±0.03 vs. 2.1±0.04, P < 0.001), and hypothyroid (thyroidectomized 60 d) (1.4±0.06 vs. 2.2±0.15 P < 0.001) rat. NPSH were also decreased in the hypothyroid, hypopituitary dwarf mouse as compared with values in their normal litter mates (1.3±0.03 vs. 2.0±0.2, P < 0.01). Chronic administration of T3 (0.5 μg/100 g body wt per d) markedly increased hepatic T3 generation from T4 in the thyroidectomized rat and in the dwarf mouse to values similar to those observed in the normal rodent without affecting NPSH concentration. In contrast, T3 administration to the starved rat did not alter either hepatic T3 generation from T4 or NPSH. Reduced glutathione concentration was also markedly decreased in the starved rat (fed; 1.05±0.075 mmol/250 g wet tissue vs. starved 0.38±0.02, P < 0.001). Dithiothreitol (DTT), a thiol reducing agent, increased hepatic T3 generation from T4 in the normal adult male rat by 45±5% in six experiments. When compared to DTT-stimulated control homogenates, the addition of DTT completely restored hepatic T3 generation in starved rats, partially restored T3 generation in 1- and 4-d-old neonates, but had little or no effect in the fetal and hypothyroid rat and dwarf mouse. Liver homogenates stored for 6 mo at −20°C lost their capacity to generate T3 from T4. NPSH concentrations in the frozen homogenates decreased progressively with increasing storage and were absent by 6 mo. 5′-Deiodinase activity correlated with NPSH concentration in the stored homogenates (r = 0.95, P < 0.005). Addition of DTT partially restored hepatic T3 generation in the frozen homogenate. It is concluded that NPSH are important for the action of the liver 5′-deiodinase. The decreased hepatic T3 generation in the starved rat is associated with decreased NPSH but not with a decrease in the absolute quantity of 5′-deiodinase because provision of sulfhydryl groups restored hepatic T3 generation to normal. In contrast, the decreased hepatic T3 generation in the adult hypothyroid rodent and in the fetal rat is probably due to a decrease in the enzyme concentration per se. In the 1- and 4-d neonatal rat, the decrease in hepatic T3 generation is secondary to a decrease in NPSH and the deiodinating enzyme.

Authors

Arthur R. C. Harris, Shih-Lieh Fang, Lee Hinerfeld, Lewis E. Braverman, Apostolos G. Vagenakis