The concentration of rat tissue residual radioactivity was measured after extensive extraction with 95% ethanol 4 days following a combined injection of phenolic (β)- and nonphenolic (α)-ring iodine-labeled L-thyroxine (131I-T4-β3 and 125I-T4-α) or nonphenolic-ring 14CT4-α. The concentration of nonextractable radioactivity derived from the nonphenolic-ring was comparable to that from the phenolic-ring in the plasma, liver and heart but was somewhat greater than that of the phenolic-ring label in the kidney and muscle. The presence of constituents of both rings of L-thyroxine in the nonextractable radioactivity suggested that the intact iodothyronine might be present in the nonextractable material. Pronase hydrolysis of the liver and plasma nonextractable m 125I (NE126I) was performed 3 days following the injection of 126I triiodothyronine (126I-T3-β) and 7 days after 125I-T4-β. Paper chromatographic analysis of the NE125I pronase digests showed 3 components: 125I-iodothyronine, 126I-iodide and 126I-monoiodotyrosine. The 126I-iodothyronines recovered in the NE125I pronase digests greatly exceeded any possible contamination from residual extractable 125I-iodothyronine. Thus, following the distribution and metabolism of the thyroid hormones, a small fraction of hormone remains in the tissues and circulating proteins in a nonextractable form which presumably represents covalent linkage of hormone molecules with tissue structural and soluble proteins.
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