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

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Abstract
The enzymatic activity in rat kidney homogenates mediating the conversion of T4 [thyroxine] to T3 [triiodothyronine] is particulate but the subcellular organelle responsible for T4 5''-deiodination was not 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: 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; 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 purified plasma membrane showed enrichment of T4 5''-deiodinase comparable to that of the known plasma membrane enzyme (Na+/K+)-ATPase. T4 5''-deiodinase in kidney tissue is apparently an enzyme of the plasmalemma.

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