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Spontaneous Deiodination of I¹³¹-Labeled Thyroid Extracts on Filter Paper

Department of Pharmacology, The University of Texas Southwestern Medical School Dallas, Texas

This work was supported by Grant No. A-3612 from the USPHS.

Abstract

When thyroid extracts, prepared from the thyroids of rats previously injected with I131, were applied to filter paper for chromatography, a spontaneous deiodination occurred, similar to that described in the preceding paper for individual iodinated amino acids. The effect was especially marked with dilute extracts, which displayed as much as 10-fold increases in I¹³¹-iodide after 25 min of drying on filter paper. This increase was not affected by previous heating of the thyroid extract at 100 C for 5 min, suggesting that thyroid deiodinases were not involved.

Even with minimal drying values, paper chromatography of thyroid extracts generally yielded somewhat higher results for radioiodide than those observed with paper electrophoresis. The latter procedure, therefore, is probably more reliable for accurate determination of I¹³¹-iodide in thyroid tissue.

Thyroid extracts that had been digested with pancreatin did not display appreciable spontaneous deiodination on filter paper. However, a large release of radioiodide (up to 10-fold) was observed during the digestion procedure itself, suggesting that marked deiodination occurred during proteolysis of thyroglobulin. Analysis of pancreatin digests of thyroid, therefore, may give misleading results for the distribution of I¹³¹ compounds within the gland.

Received December 18, 1962.