Studies on the Membrane-Associated Nature of Human Thyroid Peroxidase: A Difference in the Solubility of the Enzyme from Benign and Malignant Thyroid Tissues*

Thyroid peroxidase (TPO) is an integral membrane protein. The intact protein can be solubilized from porcine thyroid 105,000 X g particles (“microsomal” membranes) by Triton X-100 and other nonionic detergents (J Biol Chem 251: 2525, 1976). The Triton X-100 solubilization procedure has now been applied to human thyroid microsomal membranes in an effort to investigate the membrane nature of TPO in thyroid disorders. We studied tissues from 6 patients with thyroid carcinoma (papillary, follicular, and oxyphilic cell), 10 with adenoma (macrofollicular, follicular, microfollicular, and trabecular), 1 with multinodular nontoxic goiter, and 6 with diffuse toxic goiters. Normal tissue was obtained from 4 of the carcinoma patients and 8 of the adenoma patients. TPO activity in thyroid homogenates was determined by the guaiacol assay method. icrosomes from each homogenate were prepared by centrifugation of the 8500 x g supernatant at 105,000 X g for 1 h, and the microsomes were washed hypotonically to release entrapped cytoplasmic proteins. The ability of the nonionic detergent, Triton X-100, to solubilize TPO and protein from the washed microsomes was investigated. We found a significant difference in the release of TPO from benign and malignant microsomal mem-branes. Although 42% of the TPO is solubilized from benign (adenomas and multinodular nontoxic goiter) microsomal membranes, only 6% is solubilized from thyroid cancer microsomal membranes. For normal and hyperthyroid microsomal membranes, 48% and 60% of the Tpo is solubilized. The decrease in solubility of cancer Tpo does not appear to depend on the concentration of TPO in the tissues; microsomal membrances from carcinoma tissues which contained low, normal or high levels of TPO activity (2.4–65.7 U/g tissue) released only a small amount (5–1.2%) of TPO activity when treated with Triton X-100, Examination of the protein levels in the Triton X-100 supernatants showed that although TPO cannot be released from the cancer microsomal membranes, other membrane proteins present in the same membranes can be solubilizded. This suggests that the low solubility of cancer TPO is not a reflection of a general structural defect common to all membrane proteins in the cancer microsomal membranes. The evidence obtained in this study indicates an abnormality in TPO from cancer microsomal membranes which is somehow associated with a specific hydrophobic alteration in the TPO structure or in the lipid environment surrounding the TPO. This abnormality may provide a biochemical tool to aid in the diagnosis of thyroid carcinoma. (J. Clin Endocrinol Metab46:791, 1978)