Subcellular distribution of iodothyronine 5'-deiodinase in cerebral cortex from hypothyroid rats

We have examined iodothyronine deiodination in subcellular fractions of cerebral cortex obtained from hypothyroid rats. Enzymatic activities were measured at 37 degrees C in the presence of 20 mM dithiothreitol with 125I-labeled T4 and 125I-labeled rT3 as substrates for 5'-deiodination and 131I-labeled T3 as the substrate for the 5-deiodinase. Reaction products were separated by descending paper and/or ion-exchange chromatography. Cerebral cortex subcellular fractions were also characterized by marker enzyme analysis and electron microscopy. Under optimal reaction conditions more than 80% of the 5'-deiodinase was recovered after fractionation. Both 5'-deiodinase and (Na+ + K+)-ATPase showed similar subcellular distributions and were enriched approx. 3-fold in the easily sedimenting membrane fraction and nerve terminal plasma membranes. Crude microsomal membranes (6 X 10(6) g X min pellet) also showed 2-fold enrichment of these enzymes. Nuclei and isolated mitochondria were devoid of deiodinating activity. T4 and T3 5-deiodinating activity was absent in the easily sedimenting membranes and present but not enriched in particulate fractions containing microsomal membranes. These data suggest that iodothyronine 5'-deiodinase is associated with plasma membrane fractions in the cerebral cortex.