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J L Leonard

Publications and source records attributed to J L Leonard.

82 records · Page 5Linked to original sources
PubMed1978

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.

Animals↗
PubMed1978

Thyroxine 5'-deiodinase activity of rat kidney: observations on activation by thiols and inhibition by propylthiouracil.

Enzymatic 5'-deiodination of T4 in various tissues is known to be stimulated by thiols and inhibited by propylthiouracil (PTU). Dithiothreitol (DTT) stimulation of rat kidney T4 5'-deiodinase followed saturation kinetics. Inhibition of the enzyme by PTU (10 (-5) M) was overcome in a concentration-dependent manner by DTT, methimazole (MMI), and thiourea. Pretreatment of catalytically active kidney microsomal preparations with PTS caused persistent inhibition of enzyme activity, assayed in the absence of PTU. Similarly, injection of PTU in rats in a dosage of 5 microgram/100 g BW or greater significantly impaired T4 5'-deiodination in subsequently isolated kidney microsomal preparations. Reagents that cleave disulfide bonds (DTT and KCN), as well as the reductant, Na2S2O4, and the thioureylene, MII, partially restored activity to PTU-inactivated enzyme in a time- and concentration-dependent way. These observations suggest that PTU inhibition of T4 5'-deiodinase results from an interaction of PTU with the enzym, possibly via a PTU-enzyme disulfide which can be prevented or reversed by thiols or thioureylene agents, and raise the possibility that protein sulfhydryl groups may undergo oxidation during the reductive 5'-deiodination of L-T4.

Animals↗