Hydrolysis of Thyroglobulin in the Thyroid

The thyroid gland possesses a proteolytic enzyme this was first demonstrated by De Robertis (1941) in follicular colloid extracted from single follicles of rats thyroids the proteolytic activity of the enzyme was increased after the administration of thyrotropin and was decreased after administration of iodide. The activity of the enzyme was shown (De Robertis and Nowinski, 1946) to vary in different pathological conditions in human beings in cases of severe toxic goiter it was increased to twice the normal activity in simple colloid goiter it was appreciably lowered. 

Leblond, Franklin, and Quastel (1950) have shown that rat thyroids contain small amounts of free iodinated amino acids which can be extracted with butanol from the thyroid without previous hydrolysis. These include monoiodotyrosine, diiodotyrosine, thyroxine, and triiodothyronine, and they are presumably formed by proteolysis of thyroglobulin. Their fate has been the subject of much experimental work in recent years. 

It was further found by these authors that the dehalogenase was inactive towards the iodinated tyrosines when they were bound in thyroglobulin only free amino acids were attacked (personal communication). The metabolism of the iodinated tyrosines can therefore be regarded as taking place entirely within the thyroid after proteolysis from thyroglobulin they are completely dehalogenated and the iodide formed can be re-utilized for the cycle of thyroid hormone synthesis. 

The fate of thyroxine and triiodothyronine after their proteolytic release from thyroglobulin wiU now be considered. 

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