Intracellular and extracellular sites of iodination in dispersed hog thyroid cells

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Abstract
Iodination and hormone synthesis was studied in isolated hog thyroid cells in suspension. Three iodination processes were characterized using pharmacological agents. The first, intracellular iodination, was dependent on active iodide transport, which was inhibited by NaClO4 or ouabain but not by catalase. It was linear for 6 h with an apparent Km for iodide of 1.5 .mu.M, was stimulated by thyrotropin or N6O2''-dibutyryl cAMP, yielded mostly iodinated thyroglobulin and was efficient for tetraiodothyronine synthesis. The second, extracellular iodination, which was sensitive to catalase, but not to NaClO4 or ouabain, plateaued after 2 h and the apparent Km was 16.5 .mu.M. It was insensitive to thyrotropin and dibutyryl cAMP; its major products were iodoprotein other than thyroglobulin and iodolipid and the yield of tetraiodothyronine was low. The third process, intracellular iodination from passively diffused iodide, was not sensitive to inhibitors. Other characteristics of passive intracellular iodination were intermediate between active intracellular iodination and extracellular iodination. The 3 processes are inhibited by similar concentrations of methimazole, and their apparent Km values, when corrected for the concentrating effect of iodide trapping, are all of the same order as the Km of purified thyroid peroxidases. Although their locations are different, the enzymic systems involved are apparently identical. Dispersed thyroid cells possess an extracellular site of iodination and an intracellular site of iodination with biochemical characteristics of physiological relevance.