AN ANALYSIS OF THE STRUCTURE AND ANTIGENICITY OF DIFFERENT FORMS OF HUMAN THYROID PEROXIDASE

Thyroid peroxidase (TPO) is an important enzyme in the production of thyroid hormone and one of the major autoantigens in autoimmune thyroid disease. The gene for human thyroid peroxidase encodes a single 933 amino acid polypeptide chain. However, several reports have suggested that it exists in both high- and low-molecular-weight forms and the exact structure of the native enzyme is nor: known. We examined the structure of TPO using two monoclonal antibodies against different portions of TPO, a polyclonal mouse antiserum raised against a 300 amino acid fragment of TPO and autoantibodies directed against TIPO obtained from patients with autoimmune thyroid disease. Western blots performed under nonreducing conditions identified three bands of approximately 220-230 kDa and two bands of 105 and 110 kDa that appeared to be immunologic TPO. After reduction, the TPO activity migrated as a smear of bands from 105 to 110 kDa, suggesting that the higher molecular weight form of the enzyme is a disulfide-linked dimer. Patients with autoimmune thyroid disease showed higher rates of recognition of the dimer than the reduced monomer when serologic reactivity was analyzed by Western blots. Eighty-three percent (40 of 48) of patients with Graves' disease and 76% (34 of 45) of Hashimoto's disease patients recognized the dimer form of TPO, while 48% (23 of 48) of Graves' and 60% (27 of 45) of Hashimoto's patients recognized reduced monomer TPO, even though both forms were denatured with SDS. Antibodies against different portions of the TPO chain all bound to the 105 kDa bands, indicating that the TPO chain is not bisected during posttranslational processing. To determine the exact structure of the membrane-bound TPO complex, immunoprecipitations of S-35-radiolabeled human thyroid and FRTL-5 membranes were performed using the polyclonal anti-TPO antiserum and human autoantibodies. Under nonreducing conditions, only a 220-kDa band was precipitated from human thyroid membranes while an additional band of 50 kDa was identified in the samples from the FRTL-5 cells. These results indicate that the membrane form of human TPO appears to be a disulfide-linked dimer, and this finding has implications for both the function and the antigenicity of the enzyme.