Thyroid hormone deiiodinases

Assay systems involving iodine-125 labelled iodothyronine substrates have been established which can detect deiodination with high sensitivity. A HPLC separation has also been established which complements the radiochemical assays. Using these assays, 5'-deiodinase enzyme activities have been localized to the plasma membranes, to the endoplasmic reticulum and to the lysosomes prepared from liver and kidney homogenates. A further activity in the cytosol of these tissues was also found, but evidence suggested, in part, that its presence there may be due to an artefact of the homogenisation procedures. Purification studies were performed on the cytosol 5'-deiodinase (because of its solubility) and on the combined microsomal 5'-deiodinase activity. Substantial purification (300 fold) of the 5'-deiodinase from cytosol was achieved using thyroxine as the ligand in affinity chromatography. Gel electrophoresis of this purified product suggested the presence of only a small number of proteins which combine in a regular fashion to produce aggregates of related structures. The combined plasma membrane and endoplasmic reticulum microsomal activity was significantly purified (3-10 fold) using iopanoic acid (which is a 5'-deiodinase inhibitor) as the ligand in affinity chromatography. The degree of purification compares favourably to that achieved by other workers. A model for deiodination of T4 is suggested in which the production of both T3 and rT 3 occurs predominantly at the plasma membrane. The rT 3 produced is then subject to rapid catabolism by deiodination in the lysosome. The rate of rT 3 deiodination by lysosomes is controlled by the inhibition or stimulation of lysosomal autophagy or changes in the lysosomal membrane or both by drugs, hormones or aminoacids. This leads to alterations in the intracellular levels of rT 3 which in turn feeds-back negatively on the plasma membrane enzyme (and also perhaps the endoplasmic reticulum deiodinase) to inhibit the production of T3. This then leads to a rise in plasma rT 3 followed soon after by a fall in plasma 13. It is further postulated that the final common pathway for dietary changes of rT 3 /T 3 levels is via the intracellular levels of aminoacids in the liver, especially those (e.g. Asn, Gln, Leu) which are gluconeogenic or anabolic. Finally drugs which are known to be lysomotrophic and markedly affect the stability of lysosomes are predicted to have effects on the ratio of rT3 to T3 found in plasma.