Metabolism of L-Thyroxine by Phagocytosing Human Leukocytes

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Abstract

Intact normal human leukocytes deiodinated L-thyroxine (T4) with the generation of inorganic iodide, chromatographically immobile origin material, and small quantities of L-triiodothyronine (T3). When phagocytosis was induced in the leukocytes through the addition of zymosan particles that had been opsonized by coating with plasma, T4-deiodination was greatly stimulated. In addition to the stimulation of T4-deiodination, the accumulation by the leukocytes of undegraded T4 was increased. Anoxia, which has previously been shown not to interfere with phagocytosis, did not prevent the increased cellular accumulation of T4 that phagocytosis induced, but virtually abolished T4-deiodination. On the other hand, calcium, which has previously been shown to be required for optimal phagocytosis, was required for the increase in both the cellular accumulation and deiodination of T4 that phagocytosis induced. Phospholipase-C, which has previously been shown to induce a metabolic burst that mimics that induced by phagocytosis, did not increase the cellular accumulation or deiodination of T4. On the other hand, colchicine, which has previously been shown to depress the metabolic burst that accompanies phagocytosis, did not prevent the increase in either the cellular accumulation or deiodination of T4 that phagocytosis induced. Thus, increased accumulation of T4 by the leukocytes during phagocytosis appears to be the primary factor responsible for the stimulation of deiodination that phagocytosis induces. The increased accumulation of T4 did not appear to be owing to engulfment of suspending medium surrounding the particles or to binding of T4 to the particles themselves. In addition to the enhanced cellular accumulation, other factors related to the metabolic burst that accompanies phagocytosis might also be involved in the stimulation of T4-deiodination. In leukocytes from two patients with chronic granulomatous disease, a disorder in which phagocytosis appears to occur normally but in which the metabolic burst and attendant increase in hydrogen peroxide generation do not occur, stimulation of T4-deiodination was either greatly diminished or totally lacking. In myeloperoxidase-deficient leukocytes, on the other hand, stimulation of T4-deiodination was at least as great as that in normal cells. Thus, we conclude that the primary factor responsible for the increased deiodination of T4 that phagocytosis induces is the enhanced cellular uptake of hormone. The increased generation of hydrogen peroxide that accompanies phagocytosis may be necessary for the enhanced deiodination of the accumulated T4, but the latter reaction does not require the mediation of myeloperoxidase.

Authors

Kenneth A. Woeber, Sidney H. Ingbar