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

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.

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