Reduced acid protease activity in vitro of lysosomes from thyroid gland of rats chronically treated with propylthiouracil and perchlorate.

We have recently demonstrated that proteolytic activity of lysosomal acid proteases from papillary carcinoma is significantly higher than in morphologically normal thyroid tissue. In the present study the activity of lysosomal acid proteases from parenchymatous proliferated thyroid epithelium, induced by action of antithyroid substances, has been examined in an in vitro system using 125I-labelled rat thyroglobulin as a substrate. Thyroid lysosomes were isolated from rats treated chronically for 3-4 weeks with propylthiouracil (PTU, 0.1% in drinking water) and perchlorate (NaClO4, 200 mg/rat/day) by centrifugation between 800 and 20,000 x g. It was observed that, in contrast to human malignant thyroid tissue, the proteolytic activity of lysosomal acid proteases from antithyroid substance-induced hyperplastic goitre was markedly reduced in comparison with control thyroid tissue (29-50%). Since reduced activity of total lysosomal proteases was found both per unit of wet weight thyroid tissue and per unit of lysosomal proteins, the results suggest that changes in lysosomal enzymes may probably have more quantitative than qualitative nature.