<sc>l</sc>-Triiodothyronine and <sc>l</sc>-Reverse-Triiodothyronine Generation in the Human Polymorphonuclear Leukocyte

Kenneth A. Woeber; Betty A. Maddux

doi:10.1172/JCI109163

l-Triiodothyronine and l-Reverse-Triiodothyronine Generation in the Human Polymorphonuclear Leukocyte

Published September 1, 1978 - More info

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Abstract

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 K_m was 5 μM and the V_max 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.