IODOAMINO ACID COMPOSITION OF POORLY IODINATED HUMAN THYROGLOBULIN FRACTIONATED BY ISOPYCNIC CENTRIFUGATION

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Abstract
Distribution of iodotyrosines and iodothyronines was studied in poorly iodinated 19 S thyroglobulins [TG] isolated by sucrose gradient centrifugation from 5 samples of human thyroid. The 19 S thyroglobulins were fractionated by isopycnic centrifugation in 34.5% rubidium chloride gradient. The distribution of iodoamino acids depends on the total I content of each fraction. The variations in MIT [monoiodotyrosine] and DIT [diiodotyrosine] residues with increasing levels of iodination are not identical, the elevation of DIT residues being higher than than of MIT. The number of T4 [thyroxine] residues increases quite rapidly after 5 atoms of iodine while T3 [triiodothyronine] increases slightly and slowly. Even after fractionation of poorly iodinated TG, the synthesis of thyroid hormones is observed at a level of I content as low as 1.4 and 1.8 atoms per molecule of TG. The TG fractions obtained by isopycnic centrifugation are still heterogeneous, although less than the initial non-fractionated TG. To explain the data, it must be admitted that a large proportion of TG molecules are not iodinated.

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