Subcellular Alterations Causing Reduced Hepatic Thyroxine-5'-Monodeiodinase Activity in Fasted Rats*

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Abstract
Abnormalities in hepatic T4-5'-monodeiodinase activity were investigated in 72-h fasted rats. Fraction I (whole homogenate) and fraction II (2000 × g supernatant) had similar total T4-5'-monodeiodinase activities (per ml incubation mixture); activity in these fractions was reduced by 25% and 45% (P < 0.05 and P < 0.001) in fasted rats. Fraction III (2000–100,000 × g sediment) was inactive by itself. Dithiothreitol (DTT) caused variable stimulation of T4-5'-monodeiodinase activity in fraction I and consistent stimulation in fraction II. DTT completely restored activity to fraction III. In fractions I–III, the differences between DTT-stimulated total activities in the fed and fasted groups were significant, with the reactions from the fasted group having 21–42% less activity (P < 0.02 or less). The T4-5'-monodeiodinase specific activity (per mg protein) in the fasted group was reduced by 25–55% in fractions and II without DTT, and I/III with 5 mM DTT. In reconstitution experiments, fraction IV (100,000 × g supernatant, inactive by itself) from fasted rats was less potent than fraction IV from fed rats in restoring activity to fraction III, whether the latter originated from fed or fasted animals. Conversely, fraction III from fasted rats had significantly less activity than fraction III from fed rats when incubated in the presence of fraction IV from either fed or fasted rats. Diamide, which inactivates reduced glutathione (GSH), inhibited T45'-monodeiodinase in a linear, dose-dependent manner. The estimated diamide concentration required for complete inhibition of T4-5'-monodeiodinase activity in fraction II from the fasted animals was 23% less than that required for the fed animals (P < 0.05), consistent with previously reported GSH deficiency in liver of fasted rats. Injection of 400 ng T4 twice daily to fasted rats failed to restore T4-5'-monodeiodinase activity in fraction II from fasted rats to the levels found in fed rats; T4 treatment increased the serum free T4 index toward normal without raising the serum free T3 index above fasted levels. These results suggest that the decrease in GSH content in rat liver during fasting can account for only part of the loss of hepatic T4-5'-monodeiodinase activity and that the additional abnormality in the particulate fraction induced by fasting cannot be corrected by in vitro addition of reduced sulfhydryl groups.