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BIOCHEMICAL STUDIES ON THE IODINE ORGANIFICATION DEFECT OF PENDRED'S SYNDROME

Biochemical studies were performed on the thyroid of a typical case of Pendred's syndrome who underwent total thyroidectomy. The patient, a euthyroid 14 year old female, presented congenital nerve deafness, goitre and impairment of thyroidal iodine organification.

Thyroid peroxidase (TPO) activity was quantitatively normal. Using crude preparations, enzyme activity ranged from 176–366 nmoles of I^–-incorporated (inc.) to tyrosine/g of tissue. Normal values are 220–410 nmoles I^– inc./g. Digitonin pseudosolubilized TPO, assayed by the tyrosine-iodinase method, showed 44.9 nmoles I^– inc./mg of protein for the Pendred's tissue, and 44.5 nmoles I^– inc./mg for the control glands. Soluble TPO was utilized for enzyme-kinetic studies. The Km for H₂O₂, obtained in the triiodide assay with iodide as hydrogen donor, was in the same order of magnitude for both Pendred's and control thyroids, being 3 x 10^–4 M and 3.5 x 10^–4 M, respectively. In the guaiacol assay, the Pendred's Km value of 4 x 10^–4 M was also normal for H₂O₂. Pre-incubation of enzyme preparations with 5 x 10^–5 M haematin did not stimulate TPO activity.

Thyroidal NADPH-cytochrome c reductase and monoamine oxidase activities were normal. A low concentration, 37 mg/g, of a qualitatively normal thyroglobulin was found in Pendred's goitre. The iodine content of thyroglobulin was 0.19 %.

It is concluded that the four major components of the iodination process: 1) thyroid peroxidase, 2) iodide, 3) thyroglobulin and 4) H₂O₂ would be normally present in Pendred's glands. It is postulated that a failure of these major components to come together at the iodination site might be the explanation of the organification defect of Pendred's syndrome.

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				S P Pryor, A C Madeo, J C Reynolds, N J Sarlis, K S Arnos, W E Nance, Y Yang, C K Zalewski, C C Brewer, J A Butman, et al. SLC26A4/PDS genotype-phenotype correlation in hearing loss with enlargement of the vestibular aqueduct (EVA): evidence that Pendred syndrome and non-syndromic EVA are distinct clinical and genetic entities J. Med. Genet., February 1, 2005; 42(2): 159 - 165. [Full Text] [PDF]