Solubilization and partial characterization of rat liver iodothyronine deiodinases

Rat liver cells contain iodothyronine deiodinating enzymes (iodothyronine-5 and 5'-deiodinases), which are associated with the endoplasmic reticulum. In the present study, the iodothyronine deiodinases have been solubilized from the microsomal fraction of rat liver with 1.0% cholate and 0.25% of the polyoxyethylene ether W-1. Cholate can be effectively removed from the cholate extract with a mixture of the polystyrene beads XAD-2 and XAD-7. However, after some time, aggregation of proteins occurred. Cholate solubilized iodothyronine-5'-deiodinase has an apparent molecular weight of 65,000 and a Stokes radius of 36-37 A. The sedimentation coefficient is 4.3 S in 0.4-0.6% cholate, 7.6 S in 0.05% W-1 ether and 12.8 S in the absence of detergent. The enzyme solubilized with W-1 ether has an apparent molecular weight of approx. 200,000 and a Stokes radius of 52-56 A in 0.025% W-1 ether. In the latter extract, the sedimentation coefficient of the deiodinase is 4.3-5.2 S under different conditions. On DEAE-Sepharose chromatography, 70% of the bound deiodinases eluted with 0.1 M NaCl. The purification of this fraction was only 2-fold. Covalent chromatography, using activated thiol-Sepharose, resulted in approximately 3-fold purification of the deiodinases solubilized with W-1 ether, whereas in case of the cholate extract, no purification at all was obtained. Glutathione-Sepharose affinity chromatography resulted in no enrichment of the deiodinases.