A Study of the Action of Purified Thyroid Protease on l31I-Labelled Thyroglobulin

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Abstract
The preparation of pig thyroglobulin labeled with 131I is described and the results of the action of purified thyroid protease on different samples are presented. The major iodine-containing compounds liberated were found to be mono- and di-iodotyrosine, thyroxine, and tri-iodothyronine, together with a "peptide" fraction; of these mono-iodotyrosine, di-iodotyrosine, and the "peptide" predominated. Specific activities of these components were determined and compared with the values obtained following alkaline hydrolysis of the labeled protein. A radioactive spot of intensity similar to that of thyroxine and of slightly lower RF value in the solvent system used was also observed. Its identity has not yet been established. Preliminary experiments with labeled synthetic iodopeptides as substrates indicate that the protease possesses peptidase activity at acid pH which may be preferentially concerned with the fission of bonds joining aromatic amino acid residues; the thyroid enzyme would thus behave, in this respect, similarly to pepsin.

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