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.