Composition of nonextractable radioactivity formed after injection of labeled L-thyroxine and 3, 5, 3’-triiodo-L-thyronine in rats

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 ¹⁴CT_4-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.