Effects of 3-nitro-L-tyrosine on thyroid function in the rat: an experimental model for the dehalogenase defect.

Abstract

The effects on thyroid function of an inhibitor of tyrosine dehalogenase, 3-nitro-L-tyrosine (MNT) have been investigated in rats. In preliminary studies, marked inhibition of iodotyrosine deiodination was demonstrated in rats drinking 8 mM MNT. A series of experiments was then performed in which rats received Remington low iodine diet and 8 mM MNT as drinking fluid. This regimen had the following effects, compared to the effects of a low iodine diet alone: (a) a decrease in serum protein-bound iodine, elevation of serum thyrotropin level, goiter, and growth inhibition all prevented or reversed by iodine supplements: (b) on initiation of MNT, a 2- to 3-fold increase in the rate of release of radioiodine from the thyroid and concomitant urinary excretion of large amounts of organic iodine: and (c) after 2 wk of MNT, a greatly increased rate of thyroidal uptake and release of (131)I, an increase in the ratio of monoiodotyrosine-(131)I to diiodotyrosine-(131)I in thyroid proteolysates and the appearance of labeled iodotyrosines in serum. Acute administration of MNT intraperitoneally to rats on either an iodine-deficient or iodine-sufficient diet did not inhibit thyroidal uptake of (131)I or alter the distribution of (131)I among thyroidal iodoamino acids. It is concluded that MNT is an effective inhibitor of iodotyrosine deiodination in vivo, without other important actions on thyroid function. Thus, MNT treatment affords a model for the human dehalogenase defect. By provoking iodotyrosine secretion and consequent urinary loss of iodine, MNT can exaggerate the effects of a low iodine intake, producing goitrous hypothyroidism despite a rapid rate of iodine turnover in the thyroid.