Stimulation of Thyroidal Thiol Endopeptidases by Thyrotropin*

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Abstract
Rabbit thyroids contain cathepsin D (CD) and several thiol endopeptidases including cathepsin B and 3 newly described enzymes (cathepsins 180K, 110K and 45K). The relative physiological importance of these enzymes in thyroglobulin degradation in rabbits was assessed. Thyroidal thiol endopeptidase [thiol thyroglobulin hydrolase (thiol TgH)] activity increased in the absence of changes in CD activity in animals treated with 10 U bovine TSH. Peak enzyme activity occurred 24 h after injection of hormone. After 20 U bovine TSH, thiol endopeptidase activity increased by approximately 100%; CD increased by 50%. The increase in thiol enzyme activity was attributed both to cathepsin B and to the other thiol endopeptidases. The lysosomal acid hydrolases, acid phosphatase and dipeptidyl peptidase II were unaffected by TSH at either dose level. Thiol TgG activity, but not CD activity, was decreased in thyroids of rabbits treated with T4 [thyroxine] (5 .mu.g/100 g BW [body weight] .cntdot. day) for 1 wk. All thyroidal acid hydrolases examined were suppressed in animals receiving T4 for 3 wk. Thiol TgH activity was localized primarily to a lysosome-enriched fraction of thyroid homogenates. Apparently, the thiol proteases probably are the most important endopeptidases in thyroglobulin hydrolysis in vivo, and their activities are influenced by TSH.