The fate of the "lost side chain" during thyroid hormonogenesis.

Thyroglobulin labeled with L-[U-‘*C]tyrosine was prepared, purified, and iodinated in order to investigate the fate of the 3-carbon fragment which remains when 2 iodotyrosine residues (18 carbon atoms) in thyroglobulin react with each other in the course of thyroid hormonogenesis to form 1 hormone residue (15 carbon atoms). Incubation of hog thyroid slices with L-[U-‘*C]tyrosine led to the formation of 150 pmol of labeled thyroglobulin/g of wet tissue. After homogenization, the mixture of this newly formed iodine-free thyroglobulin and of preformed unlabeled thyroglobulin present in the 105,000 x g supernatant was purified by gel filtration through Sephadex G-200. Iodination of this purified thyroglobulin with iodide, glucose-glucose oxidase, and thyroid peroxidase led to the formation of ‘*C-labeled iodotyrosine and hormone (triiodothyronine and thyroxine) residues. An acidic labeled compound was found in the enzymatic hydrolysate of the iodinated thyroglobulin, in addition to labeled tyrosine, iodotyrosines, iodothyronines, and various unidentified labeled compounds. The formation of the acidic compound and that of the hormone followed similar kinetics. The radioactivity of the acidic compound was always one-fifth of that of the hormone at any period of iodination up to 90 min. Therefore, this compound must be a 3-carbon acid which is formed at the same time and at the same rate as the hormone. It was shown to be pyruvic acid. It behaved chromatographically like pyruvic acid, formed a 2,4-dinitrophenylhydrazone which behaved chromatographically like that of pyruvic acid, and was reduced to lactic acid by NADH in the presence of lactate dehydrogenase. The labeled pyruvic acid liberated upon hydrolysis of iodinated thyroglobulin is probably formed from dehydroalanine residues in the native protein.