Use of lactoperoxidase-catalyzed iodination of L-tyrosine to assess antithyroid activity of a factor present in certain tissues of mammals.

A factor, present in the kidneys of rats, has been shown to inhibit thyroid activity both in vivo (rat) and in vitro (porcine and bovine thyroid slices). The aim of the present study was to quantitate the inhibitory activity of this factor in renal and other tissues from rats by means of the lactoperoxidase-catalyzed iodination of L-tyrosine. Supernatants of renal homogenates (10,000, 27,000 and 35,000 g) obtained from rats inhibited this in vitro iodination system as well as the lactoperoxidase-catalyzed oxidation of guaiacol. The 35,000-g supernatant from both renal and hepatic tissues contained the greatest inhibitory activity per milligram of protein. Supernatants of homogenates (35,000-g fraction) from testes contained one eighth to one third the activities of liver and kidney, respectively. Kidneys of other species including fetal goat, mature goat, fetal lamb, mature monkey and mature dog, were also shown to contain the thyroid-depressing factor. These studies suggest that the thyroid-depressing factor is found mainly in liver and kidneys of the rat and that it is not unique to the rat but is present in the kidneys of other species as well.