The enzymatic deiodination of thyroxine (T4) is thiol dependent. Fasting (72 h) depresses hepatic T4 deiodination and lowers the hepatic content of nonprotein sulfhydryls (NP-SH) and reduced glutathione (GSH). It has been proposed that the fasting effect may be mediated through these alterations in hepatic sulfhydryls. To test the importance of tissue (hepatic) thiol content in the modification of T4 deiodination consequent to dietary manipulation, we examined the sequential deiodination of T4 to 3,5,3′-triiodothyronine (T3) (5′-deiodination) and 3,3′,5-triiodothyronine (reverse T3, rT3) (5-deiodination) in liver homogenates without added thiol from groups of rats fed Purina lab chow (P) (a protein-rich diet), glucose alone (G), or glucose plus cysteine (Gc) for 72 h or fasted (F) for the same period. The initial rate of each reaction was compared to the tissue concentrations of NP-SH and GSH.
Laurence A. Gavin, Francis A. McMahon, M. Moeller