THE PROTEOLYTIC ACTIVITY OF THYROID AND OTHER TISSUES OF THE DOG

If the organic iodine (diiodotyrosine and thyroxine) in blood plasma is derived from the thyroglobulin in the colloid of the thyroid gland, a mechanismis necessary to explain whereby these organic iodine compounds are split from the thyroglobulin molecule. The existence of a proteolytic enzyme in the thyroid capable of liberating diiodotyrosine and thyroxine from thyroglobulin and thus allowing these smaller molecules toenter the blood stream would offer such an explanation. The level of organic (protein-bound) iodine inplasma is known to be increased in hyperthyroidism and decreased in hypothyroidism. Ifthese variations are to be attributed, at least in part, to alterations in the rate of enzymatic breakdown of thyroglobulin, it is necessary to have quantitative studies of proteolytic activityofthyroid tissue. Measurements of proteolytic activity in the thyroid tissue of dogs have been presented in the preceding paper. It should be noted that in these studies, the substrate was hemoglobin and not thyroglobulin. This proteolytic agent has not been isolated.