Subcellular Distribution of Thyroxine 5′-Deiodinase in the Rat Kidney: A Plasma Membrane Location*

The enzymatic activity in rat kidney homogenates mediating the conversion of T4 to T2 has been previously shown to be particulate. The subcellular organelle responsible for T4 5'-deiodination, however, has not been identified. Enzymatic activity was assessed with outer ring 125I-labeled L-T4 as the substrate and the iodothyronines were separated by descending paper chromatography. Comparison of the distribution of T4 5'-deiodinase activity with that of established enzyme markers of subcellular organelles demonstrated: 1) T4 5'-deiodinase activity paralleled that of plasma membrane enzyme markers in subcellular fractions prepared by differential centrifugation, with 80% of the T4 5'-deiodinase and (Na+/K+)-ATPase equally distributed in the crude nuclear and microsomal fractions; 2) the smooth microsomal fraction, known to be contaminated with plasma membrane in kidney tissue preparations, contained the bulk of the T4 5'-deiodinase activity and plasma membrane marker enzymes found in the microsomal fraction; and 3) purified plasma membrane showed enrichment of T4 5'-deiodinase comparable to that of the known plasma membrane enzyme (Na+/K+)-ATPase. These data suggest that T4 5'-deiodinase in kidney tissue is an enzyme of the plasmalemma.