Monoiodotyrosine is a precursor of thyroid hormone and results from iodization of tyrosine at the meta- position of the phenol ring. Two units can combine to form 3,3'-diiodothyronine. One unit can combine with diiodotyrosine to form triiodothyronine, as occurs in the colloid of the thyroid follicle. Monoiodotyrosine (MIT) crosses the lysosomal membrane of rat FRTL-5 thyroid cells by a carrier-mediated process. In egress studies, MIT lost from inside lysosomes was quantitatively recovered outside lysosomes as MIT, indicating that the compound was transported intact across the lysosomal membrane. In uptake studies, [125I]MIT entry required intact lysosomes and exhibited saturation kinetics. The existence of a lysosomal MIT carrier in thyroid cells may explain how this product of thyroglobulin catabolism is transported to the cytosol for iodine salvage and reutilization.
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