Extrathyroidal monodeiodination of l-thyroxine (T4) is the principal source of l-triiodothyronine (T3) and l-reverse-triiodothyronine (rT3) production. To define some of the cellular factors involved, we examined T3 and rT3 generation from added nonradioactive T4 in human polymorphonuclear leukocytes, using radioimmunoassays to quantify the T3 and rT3 generated. Under optimum incubation conditions which included a pH of 6.5 in sucrose-acetate buffer, the presence of dithiothreitol as a sulfhydryl-group protector, and incubation in an hypoxic atmosphere, significant net generation of T3 and rT3 was observed. Of the several subcellular fractions studied, the particulate fraction obtained by centrifugation at 27,000 g was found to possess the highest T3- and rT3-generating activities per unit quantity of protein. With respect to T3 generation from substrate T4, the Km was 5 μM and the Vmax was 7.2 pmol/min per mg protein. Propylthiouracil, methimazole, and prior induction of phagocytosis inhibited both T3 and rT3 generation, but T3 generation was inhibited to a greater extent. rT3, in a concentration equimolar to that of substrate T4, did not alter T3 generation, but inhibited T3 generation when the molar ratio of rT3 to T4 approached 10:1. Under the incubation conditions employed, particulate fractions of leukocytes obtained from five cord blood samples displayed an essentially normal relationship between T3- and rT3-generating activities, despite the distinctly divergent serum T3 and rT3 concentrations in these samples. From our findings, we draw the following conclusions: (a) the human polymorphonuclear leukocyte possesses the ability to generate T3 and rT3 from substrate T4; (b) the T3- and rT3-generating activities are associated principally with the 27,000 g particulate fraction and display enzymic characteristics with a sulfhydryl-group requirement; (c) T3-generating activity appears to be more susceptible to inhibitory influences than rT3-generating activity; and (d) in cord blood leukocytes, the putative enzymes catalyzing T3 and rT3 generation appear to be functionally intact under the experimental conditions employed.
Kenneth A. Woeber, Betty A. Maddux