Characteristics of calcitonin degradation in vitro produced by incubation with procine thyroid tissue

The possibility of calcitonin (CT) degradation by thyroid tissue in vitro was investigated. Tissue slices or a thyroid supernatant solution was incubated with 131I-labeled porcine CT for 30--60 min at 37 degrees C. Calcitonin degradation and deiodination were determined by chromatoelectrophoresis and/or quso adsorption. Degradation of ]131I]CT occurred rapidly in the presence of 50--200 mg porcine thyroid tissues; however, little or no hormone deiodination was observed. Porcine liver, kidney and lung slices also degraded [131I]CT. The data indicate that a nonspecific proteolytic enzyme, capable of rapid CT degradation, is liberated from thyroid tissue slices in vitro. This substance, which is found in the cytosol fraction of thyroid tissue, is heat labile, has a molecular weight between 25,000 and 100,000, displays an acidic pH maximum for CT degradation and is inhibited by sulfhydryl enzyme inhibitors. The liberation of this substance from thyroid slices may explain the relative unresponsiveness of CT secretion observed in previous in vitro studies.