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Iodination and oxidation of thyroglobulin catalyzed by thyroid peroxidase.

1984; Elsevier BV; Volume: 259; Issue: 1 Linguagem: Inglês

10.1016/s0021-9258(17)43667-2

1083-351X

Masao Nakamura, Isao Yamazaki, Hidehiko Nakagawa, Sachiya Ohtaki, Nobuo Ui,

Electrochemical sensors and biosensors

The kinetics of iodination and oxidation of hog thyroglobulin were studied with purified hog thyroid peroxidase and the results were compared with the reactions of free tyrosine.From Lineweaver-Burk plots and on the basis of a value of 0.83 for AemM at 289 nml iodine atom incorporated, the rate constant for transfer of an assumed enzyme-bound iodinium cation to thyroglobulin was estimated to be 6.7 X lo7 and 2.3 X lo7 I -' s" in native (iodine content = 1.0%) and more iodinated (iodine content = 1.2%) thyroglobulins, respectively.This iodine-transferring reaction was stimulated by iodothyronines, similarly as observed in the reaction with free tyrosine.The iodination of thyroglobulin was inhibited by GSH, the inhibition being competitive with thyroglobulin.Thyroglobulin was oxidized in the presence of a thyroid peroxidase system without giving any appreciable change in absorbance around 300 nm.From stopped flow data, the oxidation was concluded to occur by way of two-electron transfer and the rate constant for the reaction of thyroid peroxidase Compound I with thyroglobulin was estimated to be 1.0 X lo7 M" s-'.The stopped flow kinetic pattern was similar to that observed on the reaction with free tyrosine and monoiodotyrosine.About 6 mol of hydrogen peroxide were consumed per mol of thyroglobulin.Thyroid peroxidase catalyzed thyroglobulin-mediated oxidation of GSH, but lactoperoxidase did not.Thyroid peroxidase plays catalytic roles in two steps of the biosynthesis of thyroxine: (a) the iodination of tyrosine residues of thyroglobulin and (b) the coupling of diiodotyrosine to produce thyroxine (1-3).Since the iodination itself takes place in two steps from tyrosine to diiodotyrosine via monoiodotyrosine, the iodine distribution in thyroglobulin has been analyzed at different stages of its enzymatic iodination (4-11).The iodine distribution has been analyzed also for various thyroglobulin preparations iodinated chemically (12-16) and in uiuo (14-20).These results show that thyroxine is formed at early stages of iodination of thyroglobulin, suggesting that the native structure of thyroglobulin plays an important role in "preferential formation" of thyroxine (8).This idea has been justified by the findings that a specific peptide structure of thyroglobulin is involved in the biosynthesis of thyroxine (21-26).