title: A granule-associated L-thyroxine deiodinating system in the human leukocyte.
abstract: Recent work has shown that leukocytes deiodinate L-thyroxine (T4) and that deiodination is enhanced when phagocytosis is induced. To define the subcellular locus and characteristics of this phenomenon, the deiodinative activities of different cell fractions obtained from human leukocytes (greater than 90% polymorphonuclear) were studied. Of the several fractions obtained, the granule fraction was found to possess the bulk of T4-deiodinative activity; as little as 9 mug/ml of granule protein effected significant deiodination of added labeled T4. Prior exposure of intact leukocytes to opsonized zymosan particles to induce phagocytosis resulted in an increase in the T4-deiodinative activity of the subsequently isolated granule fraction alone, strengthening the validity of the finding that T4-deiodinative activity in the human leukocyte resides in the granule fraction. The characteristics of the granule-associated deiodinating system suggest that it is enzymic in nature with a Km of the order of 10(-6)M.
mesh_terms: D003594:Cytoplasmic Granules / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007455:Iodine / Q000378:metabolism; D007962:Leukocytes / Q000378:metabolism / Q000648:ultrastructure*; D010587:Phagocytosis; D013974:Thyroxine / Q000378:metabolism*
chemical_list: D007455:Iodine; D013974:Thyroxine