Effects of trypan blue on thyroid secretion. Inhibition of purified cathepsin D from bovine thyroid

Previous studies have demonstrated that trypan blue directly inhibits thyroid secretion when the dye is administered in vitro or in vivo. To further study the mechanisms of inhibition, cathepsin D (EC 3.4.23.5) (thyroidal acid proteinase) has been purified from bovine thyroid. Trypan blue inhibited the proteolysis of both 125I-labeled thyroglobulin and 125I-labeled hemoglobin in both crude lysosomal enzyme preparation and purified endopeptidase and the inhibition was competitive. Inhibition was also observed when the dye was allowed to prebind to either purified enzyme or purified substrate. Inhibition of cathepsin D is shown to account for part of the inhibition of thyroid secretion.